text:
Evolution by natural selection is a process where populations change over generations because some individuals leave more offspring than others due to heritable differences.

Break it into four key parts:

1. **Variation**
   - Individuals in a population differ in many traits (size, color, speed, disease resistance, behavior, etc.).
   - Some of this variation is **heritable**—it’s encoded in genes and can be passed from parents to offspring.

2. **Overproduction and competition**
   - More offspring are produced than can survive and reproduce, because resources (food, space, mates) are limited.
   - This creates **competition**: not all individuals will survive and reproduce.

3. **Differential survival and reproduction (“fitness”)**
   - Some heritable traits make individuals better at surviving and reproducing in a given environment (for example, better camouflage, more efficient metabolism, resistance to a local disease).
   - Individuals with those advantageous traits are, on average, more likely to:
     - survive to reproductive age
     - have more or healthier offspring
   - This is what “**natural selection**” means: the environment “selects” which traits are more likely to be passed on, simply because their carriers leave more descendants.

4. **Inheritance and change over generations**
   - Because the advantageous traits are heritable, they become more common in the next generation.
   - Over many generations, this can:
     - shift the average traits of the population (e.g., beaks get deeper, fur gets lighter)
     - produce new adaptations (e.g., echolocation in bats, antifreeze proteins in Antarctic fish)
     - eventually lead to **speciation**, where populations become so different they can no longer interbreed and form new species.

A simple example:
- Suppose a population of beetles varies in color: some are brown, some are green.
- Birds can see and eat the green beetles more easily against brown soil.
- Brown beetles are more likely to survive and reproduce.
- Brown-color genes become more common, and over time the population becomes mostly brown.

Important points:

- Natural selection acts on **existing variation**; it doesn’t “plan” or “aim” for perfection.
- Mutations and genetic recombination introduce new variation; selection then filters that variation.
- It’s not “survival of the strongest,” but “survival and reproduction of those best suited to the current environment.”

In essence: whenever you have (1) heritable variation, (2) more offspring than can survive, and (3) consistent differences in survival or reproduction linked to that variation, you automatically get evolution by natural selection.


prompt_id: sci-evolution

provider: openai

text:
Evolution by natural selection is a process where populations change over generations because some heritable traits make organisms more likely to survive and reproduce than others.

Here’s the core logic in steps:

1. **Variation**
   Individuals in a population differ in many traits (size, color, behavior, physiology, etc.).
   - Some variation is due to genes (heritable).
   - New genetic variation arises mainly by mutation, recombination, and gene flow.

2. **Overproduction and competition**
   Organisms tend to produce more offspring than the environment can support.
   - Food, space, mates, and other resources are limited.
   - Not all individuals survive and reproduce: there is a “struggle for existence.”

3. **Differential survival and reproduction (selection)**
   Because of their traits, some individuals are better at surviving and reproducing in a given environment.
   - For example, in a cold environment, individuals with thicker fur may survive winter more often.
   - “Fitness” in evolutionary biology means reproductive success: leaving more surviving offspring.

4. **Inheritance**
   Offspring tend to resemble their parents because traits are passed on genetically.
   - If parents with a certain trait (e.g., thicker fur) leave more offspring, that trait becomes more common in the next generation.

5. **Result: Change in populations over time**
   Over many generations:
   - Traits that increase reproductive success become more common (are “selected for”).
   - Traits that decrease reproductive success become less common (are “selected against”).
   This is evolution: a change in the genetic makeup (allele frequencies) of a population over generations.

6. **Adaptation**
   Natural selection can produce adaptations—features that are well-suited to a particular environment or way of life.
   - Example: long necks in giraffes (better access to food in tall trees)
   - Example: camouflage coloration in prey species (better avoidance of predators)

7. **Role of environment**
   What counts as “advantageous” depends on the environment.
   - A trait can be beneficial in one context and harmful in another (e.g., dark fur in a dark forest vs. in snow).
   - If the environment changes, which traits are favored can also change.

8. **Speciation (formation of new species)**
   Over long timescales, if populations become genetically different enough—often due to being separated geographically or ecologically—they may no longer interbreed successfully.
   - At that point, they are considered separate species.
   - This is evolution by natural selection leading to the diversity of life.

In short:
1) individuals vary,
2) some variations are heritable,
3) some of those variations affect reproductive success,
4) the environment “filters” which variations are passed on more often.
Over many generations, this filtering process produces evolutionary change and adaptations.

finish_reason: stop


text:
Evolution by natural selection is a process where heritable traits that help organisms survive and reproduce become more common in a population over generations. It rests on four key ideas:

1. **Variation**
   Individuals in a population are not identical. They differ in traits like size, color, behavior, disease resistance, etc.
   - These differences often come from genetic variation: mutations, reshuffling of genes during reproduction, and mixing of genes when parents reproduce sexually.

2. **Inheritance**
   Some of these differences are heritable: they are encoded in genes and passed from parents to offspring.
   - If a trait isn’t heritable (e.g., a scar), it can’t be shaped by natural selection, because it doesn’t consistently pass to the next generation.

3. **Differential survival and reproduction**
   More offspring are produced than can survive, and the environment is limited (food, space, mates, protection from predators, etc.).
   - Individuals with traits that fit the environment better tend to survive longer and/or have more offspring.
   - Individuals with less favorable traits are more likely to die earlier or leave fewer offspring.

4. **Accumulation of favorable traits over generations**
   Because advantageous traits are both helpful and heritable, they become more common in each generation.
   - Over many generations, this can:
     - Change average traits in the population (e.g., beak size in birds).
     - Produce adaptations: features that are well-suited to specific environments (e.g., camouflage, heat tolerance).
     - Eventually lead to new species if populations become different enough and no longer interbreed.

---

**Simple example: dark and light beetles**
- Start with a population of beetles: some dark, some light.
- Birds can see and eat the light beetles more easily on dark soil.
- Dark beetles are eaten less often, so they survive and reproduce more.
- Dark color is genetic and passed to offspring.
- Over generations, dark beetles become more common; the population “evolves” to be mostly dark.

---

**Key points and clarifications**
- Natural selection acts on individuals (who lives, who reproduces), but **evolution is a change in the genetic makeup of a population** over time.
- It is not goal-directed: there is no foresight or plan; traits spread only because they worked better *in past environments*.
- Other processes (mutation, genetic drift, gene flow) also influence evolution, but natural selection is the one that consistently produces adaptation to the environment.

If you’d like, I can go into more detail on any part (e.g., how mutations arise, examples from bacteria, or how new species form).


text:
Evolution by natural selection is a process where populations change over generations because some individuals leave more offspring than others, due to heritable differences between them.

Break it into four key ideas:

1. **Variation**
   Individuals in a population differ in many traits (size, color, speed, disease resistance, behavior, etc.).
   - Some variation is genetic (due to differences in DNA).
   - New genetic variation arises mainly through mutation and recombination (mixing of genes during reproduction).

2. **Inheritance**
   Some of these traits are heritable: parents pass them to offspring via genes.
   - If a trait has a genetic basis, offspring tend to resemble their parents in that trait.

3. **Differential survival and reproduction**
   In any environment, more offspring are produced than can survive and reproduce.
   - Individuals with certain traits are better at surviving, finding mates, and raising offspring in that specific environment.
   - Those individuals leave more descendants; others leave fewer or none.

4. **Change in trait frequencies over generations**
   Because the successful traits are heritable, they become more common in the next generation.
   - Over many generations, this can significantly change the average traits of the population.
   - If populations experience different conditions and become genetically distinct enough, they can form new species.

A simple example:

- In a population of beetles, some are green and some are brown due to genetic differences.
- Birds can see and eat green beetles more easily on brown soil.
- Brown beetles survive and reproduce more; green beetles are eaten more often.
- The “brown” genes are passed on more frequently.
- Over generations, the population becomes mostly brown: it has evolved by natural selection.

Important clarifications:

- Natural selection acts on **individuals** (who lives and reproduces), but evolution is a change in **populations** over time.
- Natural selection is not goal-directed; it doesn’t “try” to make organisms perfect. It simply favors traits that work better *here and now* in a given environment.
- Other processes (like genetic drift, migration, and mutation) also affect evolution, but natural selection is the process that systematically links traits to reproductive success.


text:
Evolution by natural selection is a process where populations change over generations because some individuals leave more offspring than others due to heritable differences.

Here’s the core logic, step by step:

1. **Variation exists**
   Individuals in a population differ in many traits (size, color, behavior, physiology, etc.).
   - Some variation is genetic (heritable), caused by mutations, recombination, and gene shuffling during reproduction.

2. **More offspring are produced than can survive**
   Environments have limited resources (food, space, mates).
   - Not all individuals survive and reproduce; there is a “struggle for existence.”

3. **Differential survival and reproduction (selection)**
   Some heritable traits make individuals better at surviving or reproducing in a given environment.
   - Those individuals are more likely to survive and have offspring.
   - Others, with less favorable traits, tend to leave fewer offspring.

4. **Inheritance of successful traits**
   Offspring resemble their parents genetically.
   - Traits that helped parents survive and reproduce tend to be passed on.
   - Over many generations, these traits become more common in the population.

5. **Population changes over time (evolution)**
   Because the genetic makeup of the population shifts, the population evolves.
   - This can lead to new adaptations (e.g., camouflage, resistance to disease).
   - Over long timescales, accumulated changes can produce new species.

---

**Example: Antibiotic resistance in bacteria**

- In a bacterial population, some cells randomly have a mutation that makes them slightly resistant to an antibiotic.
- When the antibiotic is applied, most bacteria die, but resistant ones survive.
- The survivors reproduce, passing on resistance genes.
- After many generations, most of the population is resistant: the population has evolved by natural selection.

Key points:
- Natural selection acts on **existing variation**; it doesn’t “plan” or “aim” for a goal.
- It favors traits that increase **reproductive success** in the current environment.
- Evolution is about **changes in populations over generations**, not individuals changing during their lifetimes.


text:
Evolution by natural selection is a process where populations change over generations because some individuals leave more offspring than others, due to heritable differences between them.

Here’s the core logic in steps:

1. **Variation exists**
   Individuals in a population differ in many traits (size, color, speed, disease resistance, behavior, etc.).
   - Some variation is genetic (heritable), caused by mutations, recombination, and mixing of genes during reproduction.

2. **More offspring are produced than can survive**
   Environments have limited resources (food, space, mates).
   - Not all individuals survive and reproduce; there is competition, predation, disease, and chance events.

3. **Differential survival and reproduction (selection)**
   Because of their traits, some individuals are better at surviving and reproducing in a given environment than others.
   - Example: In a cold climate, animals with thicker fur may survive winter more often and have more offspring.

4. **Inheritance of traits**
   Offspring tend to resemble their parents because traits are passed on via genes.
   - If a trait helps an individual leave more offspring, the genes underlying that trait become more common in the next generation.

5. **Change in the population over generations (evolution)**
   Over many generations, the frequencies of genes and traits in the population shift.
   - Traits that increase reproductive success tend to spread.
   - Traits that reduce it tend to become rare or disappear.

6. **Adaptation**
   As this process continues, populations become better suited (adapted) to their environments.
   - Example: In a dark cave, over many generations, a population of fish might lose pigmentation and eyesight, while enhancing other senses.

7. **Speciation (formation of new species)**
   If groups of the same species become isolated (geographically or reproductively) and experience different selection pressures, their traits and genes can diverge enough that they can no longer interbreed.
   - At that point, they are considered separate species.

Key points to keep in mind:
- Natural selection acts on **individuals** (who lives and reproduces), but **evolution** is the change in **populations** over time.
- Natural selection is not goal-directed; it doesn’t “try” to make organisms perfect. It just favors traits that work better *right now* in a given environment.
- Random mutation creates variation; natural selection is the non-random sorting of that variation.

If you’d like, I can give a concrete example (like antibiotic resistance in bacteria or Darwin’s finches) to make this more tangible.


text:
Evolution by natural selection is a process where populations change over generations because some individuals leave more offspring than others, and the traits that helped them do so become more common.

Here’s the core logic, step by step:

1. **Variation exists**
   Individuals in a population differ in many traits (size, color, speed, disease resistance, behavior, etc.).
   - Some variation is genetic (heritable), caused by mutations, recombination, and mixing of genes during reproduction.

2. **More offspring are produced than can survive**
   Environments have limited resources (food, space, mates).
   - Not all individuals survive and reproduce; there is competition, predation, disease, and chance events.

3. **Differential survival and reproduction (selection)**
   Because of their traits, some individuals are more likely to:
   - Survive to adulthood
   - Find mates
   - Produce offspring that survive
   These individuals are said to have higher **fitness** (in evolutionary terms, “fitness” = reproductive success, not strength or health alone).

4. **Inheritance of traits**
   Offspring tend to resemble their parents.
   - If a trait is heritable and helps an individual survive or reproduce, that trait is more likely to be passed on to the next generation.

5. **Change in trait frequencies over generations**
   Over many generations:
   - Helpful (advantageous) traits become more common.
   - Harmful (disadvantageous) traits become rarer.
   This change in the genetic makeup of the population over time **is evolution**.

---

### A simple example

Imagine a population of beetles:

- They vary in color: some are green, some are brown.
- Birds can see green beetles more easily on dark soil, so green beetles are eaten more often.
- Brown beetles survive and reproduce more, passing on genes for brown color.
- After many generations, most beetles in the population are brown.

Nothing “decides” to change; it’s just that brown beetles leave more descendants, so brownness spreads.

---

### Key points and clarifications

- **Natural selection acts on individuals; evolution is seen in populations.**
  Individuals don’t “evolve” in their lifetimes; populations evolve as trait frequencies shift.

- **Natural selection is not random, but variation arises partly by random mutation.**
  - Mutations occur without regard to whether they’re useful.
  - Selection is non-random: beneficial variants tend to spread.

- **Natural selection is one mechanism of evolution.**
  Others include genetic drift (random changes), gene flow (migration), and mutation. But natural selection is the mechanism that explains adaptation—how organisms become well-suited to their environments.

If you’d like, I can go into more detail on any part (e.g., mutations, examples in humans, antibiotic resistance, or common misconceptions).


text:
Evolution by natural selection is a process where populations change over generations because some individuals leave more offspring than others, due to heritable differences between them.

Here’s the core logic in steps:

1. **Variation exists**
   Individuals in a population differ in many traits (size, color, speed, disease resistance, behavior, etc.).
   - Some variation is genetic (heritable), caused by mutations, recombination, and mixing of genes during reproduction.

2. **More offspring are produced than can survive**
   Environments have limited resources (food, space, mates).
   - Not all individuals survive and reproduce; there is competition, predation, disease, and chance events.

3. **Differential survival and reproduction (selection)**
   Because of their traits, some individuals are more likely to survive and reproduce than others in that specific environment.
   - Example: In a cold climate, animals with thicker fur may survive winter better and have more offspring.

4. **Heritability of traits**
   Offspring tend to resemble their parents because traits are passed on genetically.
   - If thick fur is heritable, parents with thick fur tend to have thick-furred offspring.

5. **Change in trait frequencies over generations**
   Because individuals with certain traits leave more offspring, the genes for those traits become more common in the population over time.
   - Over many generations, this can significantly shift the average traits of the population.

6. **Adaptation**
   Traits that increase survival or reproduction in a given environment are called adaptations.
   - Natural selection “favors” adaptations, so populations become better suited to their environments—*not* because they “try” to adapt, but because individuals with beneficial traits leave more descendants.

7. **Speciation (over long timescales)**
   If populations of the same species become isolated (geographically or ecologically) and experience different selection pressures, their traits and genes can diverge.
   - Given enough time and genetic divergence, they may become separate species that can no longer interbreed.

Key clarifications:

- **Natural selection acts on individuals; evolution is seen in populations.**
  Individuals don’t “evolve” in their lifetimes; populations evolve as gene frequencies change.

- **Natural selection is not random, but variation is partly random.**
  Mutations occur largely at random; selection is non-random filtering: traits that help in a given environment tend to spread.

- **No foresight or goal.**
  Natural selection doesn’t plan ahead. It only “favors” what works *now* in the current environment.

If you’d like, I can walk through a concrete example (like antibiotic resistance in bacteria or the evolution of peppered moths) to make this more tangible.


prompt_id: sci-iq-heritability

text:
Human intelligence is substantially heritable, but not fixed, and the degree of heritability depends strongly on age and environment.

### 1. What “heritability” means (and doesn’t mean)

- **Heritability (h²)** = the proportion of *differences between people* in a given population that can be statistically attributed to genetic differences.
- It is **not**:
  - “The percent of your intelligence that comes from genes.”
  - A constant of human nature; it varies with environment, age, and population.
  - A measure of *immutability*; traits can be highly heritable yet still be changed by environment (e.g., height and nutrition).

So if a study says “intelligence is 60% heritable,” it means: within that group, at that time, about 60% of the variation in IQ scores is associated with genetic variation, under that mix of environments.

---

### 2. Twin, adoption, and family studies

These are the classic lines of evidence.

#### Twin studies

- **Monozygotic (MZ) twins** share ~100% of their genes; **dizygotic (DZ) twins** share ~50% on average.
- Meta-analyses of many twin studies typically find:
  - **Childhood**: heritability of IQ ≈ 0.3–0.4
  - **Adolescence**: ≈ 0.5
  - **Adulthood**: ≈ 0.6–0.8
- This pattern – **heritability rising with age** – is very robust. It suggests that as people choose and shape their environments, genetic influences increasingly express themselves.

MZ twins reared apart are especially informative:
- Correlations in IQ for MZ twins raised apart are often ~0.7–0.8, close to MZ twins raised together.
- That implies **shared family environment (home, parenting style, etc.) has a relatively modest effect on adult IQ variation within normal ranges in developed societies**, compared to genes and unique experiences.

#### Adoption studies

- Adopted children’s IQs correlate:
  - Moderately with **biological parents** (genetic influence).
  - Weakly, and declining with age, with **adoptive parents**.
- Early in childhood, shared environment matters more; by late adolescence, the correlation with adoptive parents’ IQ essentially disappears, while correlation with biological parents remains.

Across many such designs, the converging conclusion: **both genes and environment matter, but genetic differences account for a substantial fraction of IQ differences in typical Western samples, especially in adulthood.**

---

### 3. Molecular genetics (GWAS and polygenic scores)

Modern studies look directly at DNA.

#### GWAS (genome‑wide association studies)

- Very large GWAS (hundreds of thousands to >1 million people of mostly European ancestry) identify **thousands of variants** each with tiny effects associated with cognitive ability / educational attainment.
- Each variant explains only a minuscule fraction of variance; combined **polygenic scores** can now predict:
  - ~10–20% of the variance in educational attainment.
  - Roughly similar or somewhat lower percentages for cognitive test scores, depending on sample and measure.

This is often referred to as “**SNP heritability**” – the variance explained by common genetic variants captured on genotyping arrays. It tends to be lower than the total heritability inferred from twin studies, but in the same general ballpark, supporting the idea that intelligence is indeed substantially influenced by many genetic variants.

---

### 4. Role of environment and gene–environment interplay

Substantial heritability does **not** mean environment is unimportant.

#### Environmental effects

- **Severe deprivation** (e.g., extreme poverty, neglect, malnutrition, institutionalization) can markedly depress IQ and brain development.
- **Interventions**:
  - Early childhood education, better nutrition, reduced toxin exposure, and enriched learning environments can raise test performance, especially in disadvantaged kids.
  - Gains vary and often fade, but some interventions show lasting benefits for cognition and life outcomes.

#### Heritability varies with environment

- In more **equal and affluent** environments, people’s basic needs and learning opportunities are more similar, so **genetic differences explain more of the remaining variation** (heritability estimates go up).
- In harsh or unequal environments, **environmental variation can dominate**, and heritability can drop (especially seen in some low-SES samples in the US).

This means heritability estimates from, say, middle‑class Western twin samples don’t automatically apply to all societies or conditions.

#### Gene–environment correlation and interaction

- People often **select, modify, and evoke** environments partly based on their genetically influenced traits (e.g., a child who finds reading easy is more likely to read a lot, which further boosts ability).

finish_reason: length


text:
Human intelligence is substantially heritable, but not fixed, and its heritability depends strongly on age and environment.

Here’s what the evidence shows, in layers:

---

## 1. What “heritability” means (and doesn’t mean)

- **Heritability (h²)** in this context: the proportion of *differences between people* in a population that can be statistically attributed to genetic differences.
- It does **not** mean:
  - “X% of your intelligence comes from genes and Y% from environment.”
  - That intelligence is unchangeable.
  - That the same number applies to all populations and situations.

Heritability is a **population-level statistic**, and it depends on:
- The range of environments
- The range of genetic variation
- Age

---

## 2. Twin and family studies: the core evidence

Behavioral genetics (especially twin studies) has been the main source of data.

### Meta-analyses of IQ heritability

- Large reviews (e.g., by Ian Deary, Robert Plomin, and others) converge roughly on:

  - **Childhood:** heritability ~ **0.2–0.4** (20–40%)
  - **Adolescence:** ~ **0.4–0.6** (40–60%)
  - **Adulthood:** often **0.6–0.8** (60–80%)

Pattern: **heritability of intelligence tends to increase with age**.

### Why it increases with age

Several converging explanations:

1. **Gene–environment correlation**
   As people get older, they:
   - Select, modify, or create environments that fit their genetic tendencies (e.g., a child genetically inclined to enjoy reading is more likely to read a lot, seek cognitively demanding hobbies, choose more academic tracks, etc.).
   These self-chosen environments amplify genetic differences.

2. **Shared family environment matters more in childhood**
   Growing up in the same home (same socioeconomic status, parenting, etc.) has:
   - A noticeable effect on IQ in childhood.
   - A much smaller effect by adulthood, as siblings diverge into different lives.

3. **More stable expression of genes with maturation**
   Some genetic influences may only express strongly once the brain and body mature and educational/occupational choices accumulate.

---

## 3. Molecular genetics: what specific genes tell us

Genome-wide association studies (GWAS) look directly at DNA.

- Large GWAS have identified **thousands of variants** associated with cognitive ability or educational attainment.
- Each variant has a **tiny** effect; together they can explain a non-trivial but still partial fraction of IQ differences (often **10–20%** of variance with current methods in large European-ancestry samples).
- These “polygenic scores” predict:
  - Standardized test performance
  - Years of education
  - Some job and income outcomes

Key points:

- Molecular estimates of heritability are **lower than twin-study estimates**, because:
  - We only capture common variants
  - We imperfectly measure complex traits
- Nonetheless, they provide **independent support** that:
  - Intelligence is influenced by many genes of small effect (polygenic)
  - Genetic differences are genuinely associated with cognitive differences

---

## 4. Environment still matters a lot

High heritability ≠ weak environmental impact.

Evidence:

1. **Secular gains (Flynn effect)**
   Across much of the 20th century, many countries saw average IQ test scores rise by **~3 points per decade**, too fast to be explained by genetics. Likely contributors:
   - Better nutrition and health
   - More schooling and test familiarity
   - More cognitively complex environments

2. **Adoption and intervention studies**
   - Children adopted from deprived to more advantaged homes often show **substantial IQ increases** (commonly 5–15 points, sometimes more in extreme deprivation).
   - Early childhood interventions in highly disadvantaged groups can produce lasting improvements in education and life outcomes, and in some cases modest, partly sustained IQ gains.

3. **Socioeconomic status (SES) interactions**
   - Some (though not all) studies find that in **low-SES environments**, the heritability of IQ is **lower**, and environmental factors weigh more.
   - In **higher-SES environments**, where basic needs and schooling quality are more uniform, **genetic differences account for a larger fraction of remaining variation**.

So: when environments are very unequal, environment explains more of the differences; when environments become more equal and supportive, genetic differences loom larger in the remaining variation.

---

## 5. Group averages vs. individual differences

Heritability is about **variation within a group**, not about average differences between groups.

- High heritability within a population does not prove that **between-group** differences (e.g., ethnic


text:
Human intelligence is substantially heritable, but not fixed, and strongly shaped by environment. The best summary from current evidence is:

- In modern, reasonably affluent societies, genetic differences account for **about 50–80%** of the variation in general cognitive ability (often called “g” or IQ) among adults.
- In childhood, heritability is much lower (around **20–40%**) and **increases with age**.
- “Heritable” does *not* mean “unchangeable”: environments, education, nutrition, and social conditions still have large effects.

Below is what the main lines of evidence show and how to interpret them.

---

## 1. What “heritability” actually means

- **Heritability (h²)**: the proportion of *observed differences* between people in a specific population, at a specific time, that can be statistically attributed to genetic differences.
- It is:
  - **About differences, not individuals**: saying intelligence is 60% heritable in a population does *not* mean a person’s intelligence is 60% “from genes” and 40% “from environment.”
  - **Population- and context-specific**: change the environment (e.g., more inequality, better nutrition, better schools), and the heritability estimate can change.
  - **Not immutability**: traits can be highly heritable yet modifiable (e.g. height and nutrition).

So when we say “intelligence is 60–80% heritable in adults in rich countries,” it means: *within those societies, most of the differences in tested cognitive ability between people are statistically associated with genetic variation, given current environmental variation.*

---

## 2. Twin and adoption studies

These have been the core methods for decades.

### Twin studies

- **Monozygotic (MZ, identical) twins** share ~100% of genes;
  **Dizygotic (DZ, fraternal) twins** share ~50% on average.
- If MZ twins are more similar in IQ than DZ twins, this suggests genetic influence.

Findings (meta-analyses and large studies):

- Correlations in IQ:
  - MZ twins reared together: ~0.80–0.90
  - MZ twins reared apart: ~0.70–0.80
  - DZ twins reared together: ~0.40–0.60
- From this, heritability of general intelligence often comes out **around 50–80% in adulthood**.

Key point: MZ twins reared apart are **very similar** in cognitive ability despite differing environments, which strongly supports genetic influence. But they’re not identical, which shows environmental effects.

### Adoption studies

- Compare:
  - Adopted children to their **biological** parents/siblings (genes, little shared environment), and
  - Adopted children to their **adoptive** parents/siblings (shared environment, no shared genes).

Findings:

- In early childhood, adopted children often resemble their adoptive families somewhat in IQ.
- By adolescence and adulthood, their IQ correlates more strongly with their **biological** relatives, and the resemblance to adoptive relatives often approaches **near zero**.
- This pattern is consistent with:
  - **Genetic influence increasing with age**, and
  - Shared family environment having a **modest and mostly early** effect on IQ.

---

## 3. The “Wilson effect”: heritability rises with age

A repeated finding (sometimes called the Wilson effect):

- **Childhood**: IQ heritability ~0.20–0.40
- **Adolescence**: ~0.40–0.60
- **Adulthood**: ~0.60–0.80

Why might heritability increase?

- As people age, they gain more autonomy and can **select and shape environments** (friends, hobbies, education, jobs) that match their dispositions—so-called **gene–environment correlation**.
- Early environmental constraints (e.g., poor nutrition, deprivation) can suppress genetic potential; as constraints lift, genetic differences express more fully.
- Measurement reliability improves with age.

This does *not* mean environment stops mattering; it means that in many modern societies, the range of environments within families is relatively limited compared to the range of genetic differences.

---

## 4. Shared vs. non-shared environment

Behavioral genetic models usually split variance into:

- **A**: Additive genetic effects
- **C**: Shared (common) environment – things siblings growing up together tend to share (home, parents, neighborhood, some schooling).
- **E**: Non-shared environment – unique experiences, different peers, illnesses, accidents, random developmental variation, plus measurement error.

Typical pattern for IQ in affluent societies:

- Childhood:
  - A (genes


text:
Human intelligence is substantially heritable, but not fixed, and its heritability depends strongly on age and environment. Here’s what the evidence shows, in plain terms.

---

## 1. What “heritability” actually means

- **Heritability (h²)**: The proportion of differences between people in a given trait (e.g., IQ) that can be statistically attributed to genetic differences *within a specific population, at a specific time, in specific environments*.
- It is **not**:
  - The fraction of *your* intelligence “caused by genes”
  - A limit on how smart someone *can* become
  - Immune to environmental changes

If a trait has 50% heritability, that means: in that population, about half of the *variation* in that trait is associated with genetic variation, assuming the current range of environments.

---

## 2. Twin and adoption studies: the classical evidence

These are the bedrock of intelligence heritability estimates.

### Twin studies

- **Monozygotic (MZ/identical) twins** share ~100% of their genes.
- **Dizygotic (DZ/fraternal) twins** share on average ~50% of segregating genes.
- If MZ twins are more similar than DZ twins in IQ, that suggests genetic influence.

Meta-analyses of many studies show:

- IQ test score correlations (approximate midpoints):
  - MZ twins reared together: ~0.80–0.85
  - MZ twins reared apart: ~0.70
  - DZ twins reared together: ~0.50–0.60
- From these patterns, heritability of general intelligence (g/IQ) is typically estimated at:
  - **Childhood**: ~0.30–0.45
  - **Adolescence**: ~0.50
  - **Adulthood**: ~0.70–0.80

So by adulthood, **around 70–80% of the variation in IQ in many high‑income populations appears associated with genetic differences**.

### Adoption studies

- Compare adopted children to:
  - Their **biological** parents/siblings (genes)
  - Their **adoptive** parents/siblings (environment)

Key findings:

- In childhood, adopted children’s IQs correlate moderately with adoptive parents, but this correlation **fades by late adolescence**.
- Correlation with **biological parents** and biological siblings **grows over time**, even if they never lived together.
- This again suggests increasing genetic influence and decreasing shared family environment influence as people age.

---

## 3. How heritable is intelligence? A summary by age

Across many studies in mostly Western, industrialized populations:

- **Early childhood** (3–7 years):
  - Heritability: ~20–40%
  - Shared family environment: often 30–40% or more.
- **Late childhood / adolescence**:
  - Heritability: ~40–60%
  - Shared environment: shrinking, maybe ~20% or less.
- **Adulthood**:
  - Heritability: ~60–80%
  - Shared environment: often close to zero (nonshared experiences + measurement error explain the rest).

This pattern—that genetic influence appears to **increase with age**—is robust and sometimes called the “Wilson effect.”

Interpretation: as people grow up, they increasingly select, shape, and move into environments that *match* their dispositions (gene–environment correlation). That magnifies genetic influence over time.

---

## 4. The role of environment: big, but uneven

High heritability does **not** imply that environment is unimportant.

Key points:

- **Heritability is population-specific.** In a population where basic needs are met and schooling is widespread, environmental variation in key aspects might be relatively narrow. In that context, genetic differences explain more of what’s left.
- **Severe deprivation massively lowers IQ**, regardless of heritability estimates in advantaged samples:
  - Early malnutrition, extreme neglect, lack of stimulation, and heavy exposure to toxins (e.g., lead) can significantly impair cognitive development.
  - Early interventions for children from very deprived backgrounds can produce large IQ gains, even if such effects are smaller or harder to sustain in more typical environments.
- **Flynn effect**: Across many countries, average IQ scores rose roughly 2–3 points per decade through much of the 20th century.
  - Genes didn’t change that fast; the gains must be largely environmental (education, health, complexity of modern life, test familiarity, etc.).
  - This shows **high heritability does not mean immutability**: the mean can change greatly even if relative differences are partly genetic.

---

## 5. Socioeconomic status


text:
Human intelligence is substantially heritable, but not “genetically fixed,” and environment still matters a lot. The picture is nuanced and depends on age, what you mean by “intelligence,” and what level of explanation you’re interested in.

I’ll break it into key points:

---

## 1. What “heritability” actually means

- **Heritability (h²)** in this context = the proportion of **differences between people** in a given population that can be statistically attributed to genetic differences.
- It does **not** mean “the percent of your intelligence that comes from your genes.”
- It depends on:
  - The population (e.g., country, social conditions).
  - The time and environment (e.g., nutrition, schooling).
  - The age group being measured.

You can have high heritability of a trait **even if** environment is crucial (e.g., height: good nutrition is required, but variation in modern rich countries is mostly genetic).

---

## 2. Twin and adoption studies: the core evidence

These are the backbone of the literature.

### Twin studies

Look at:
- **Monozygotic (MZ) twins**: share ~100% of segregating genes.
- **Dizygotic (DZ) twins**: share ~50% on average.

If MZ twins are more similar in intelligence than DZ twins, that suggests a genetic contribution.

**Findings (broadly consistent across dozens of large studies):**

- In **childhood**: heritability of IQ ≈ **0.3–0.4** (30–40%).
- In **adolescence**: ≈ **0.4–0.6**.
- In **adulthood**: ≈ **0.6–0.8** (many meta-analyses converge around ~0.7).

So, by adulthood, most of the **variation in IQ within a given developed-country population** appears to be associated with genetic differences.

MZ twins reared apart are especially informative:
- They show **high correlations** in IQ (around 0.7–0.8 in classic data), often similar to MZ twins reared together.
- This supports a substantial genetic role and relatively smaller shared-environment influence in adulthood.

### Adoption studies

Compare:
- Adopted children vs. their **biological** parents/siblings.
- Adopted children vs. their **adoptive** parents/siblings.

Findings:
- In **childhood**, adopted kids’ IQ is moderately correlated with both:
  - Biological parents (genetic influence).
  - Adoptive parents (shared environment influence).
- By **late adolescence and adulthood**, resemblance to **biological relatives** is stronger; resemblance to **adoptive parents** typically falls close to zero.

This suggests:
- Shared family environment does matter early in life.
- Genetic differences increasingly account for who ends up cognitively similar in adulthood.

---

## 3. The role of environment

High heritability is **not** the same as environmental irrelevance.

Key points:

1. **Heritability is population-specific**
   In a setting with:
   - Uniform good nutrition, universal schooling, low disease and deprivation,
   there’s **less variation** in environment, so **a larger fraction** of what’s left is genetic.

   In contrast, in populations with big environmental inequalities (e.g., severe poverty vs. affluence), **environmental effects often loom larger**, and measured heritability can be lower.

2. **Socioeconomic status (SES) modifies heritability**
   Several studies (especially in the U.S.) show:
   - In **low-SES families**, IQ heritability appears **lower**, and shared environment accounts for more variance.
   - In **high-SES families**, IQ heritability appears **higher**, and shared environment is smaller.

   Interpretation: In deprived environments, **environmental constraints** limit potential, so genes have less opportunity to “show” their effects.

3. **Shared vs. nonshared environment**
   Behavioral genetics breaks “environment” into:
   - **Shared environment (C)**: factors making siblings more similar (same home, parents, neighborhood).
   - **Nonshared environment (E)**: factors making them different (peer groups, teachers, illnesses, random life events, measurement error).

   For adult IQ in rich countries, a typical decomposition looks like:
   - Genes (A): ~50–70%
   - Shared environment (C): ~0–20% (often near zero in late adulthood)
   - Nonshared environment + error (E): the remainder

   This doesn’t mean families don’t matter; it means **differences among families**, *within that dataset*, explain less of the **remaining differences** in adult IQ than genes and idiosyncratic experiences do.

4. **Secular (Flynn


text:
Human intelligence is substantially heritable, but far from genetically “fixed,” and it’s strongly shaped by environment. Here’s what the evidence shows in a compact overview.

---

## 1. What “heritability” actually means

In this context:

- **Heritability (h²)** = the proportion of differences in a trait **within a particular population at a particular time** that can be statistically attributed to genetic differences.
- It **does not** mean “X% of your intelligence comes from genes and Y% from environment.” It’s about differences among people, not the composition of an individual.

Example:
If intelligence has a heritability of 0.6 (60%) in a given population, that means 60% of the **variation between people’s scores** can be linked to genetic variation in that population, given its current environments.

---

## 2. Twin, family, and adoption studies

This is the classic evidence base.

**Key findings:**

- **Identical (MZ) twins** raised together show IQ correlations around **0.8–0.9**.
- **Fraternal (DZ) twins** raised together: correlations around **0.5–0.6**.
- **Adopted siblings** raised together: correlations that **fade toward zero** by adulthood.
- **Identical twins raised apart** still show high similarity in IQ (~0.7 correlation).

From such patterns, behavioral geneticists estimate:

- **Childhood IQ heritability:** around **0.3–0.5**
- **Adult IQ heritability:** around **0.6–0.8**

So, in reasonably well-fed, schooled, stable societies, **genes account for most of the variation in adult intelligence**, but not all.

---

## 3. Heritability changes with age

A robust result is that **heritability of intelligence increases from childhood to adulthood**:

- Early childhood: ~20–30%
- Late childhood/teen years: ~40–50%
- Adulthood: often ~60–80%

Why?

- As people age, they **increasingly select and shape their own environments** (friends, education, hobbies, jobs) in ways that fit their genetic tendencies — a process called **gene–environment correlation**.
- Early on, **family environment** and random developmental factors matter more; later, self-selected niches amplify genetic differences.

---

## 4. Molecular genetics (SNPs and GWAS)

More recent work uses actual DNA data:

- **GWAS (Genome-Wide Association Studies)** have identified **thousands of SNPs** each making tiny contributions to intelligence-related traits.
- **SNP-based heritability** (using measured genetic variants) for intelligence is around **20–40%**, depending on sample and method.
  - This is **lower than twin-based estimates**, because:
    - We don’t capture all kinds of genetic variation (e.g., rare variants, structural variants).
    - SNP-based methods are more conservative and limited by measurement noise.

These DNA-based findings **converge with** twin/adoption data in showing:

- Intelligence is **highly polygenic** (shaped by many genes, each with small effect).
- Genetic scores (polygenic scores) can predict some variation in cognitive ability and educational attainment, but **far from perfectly**—they typically explain **a modest portion of the variance** (often under 20% for IQ in out-of-sample predictions, depending on ancestry and data).

---

## 5. Environment still matters a lot

High heritability is **not** the same as inevitability or immutability.

Evidence of strong environmental effects:

- **Socioeconomic status (SES):**
  - In some studies, particularly in the US, **heritability is lower in low-SES families and higher in high-SES families**.
  - Under deprivation, **environmental constraints mask genetic potential**; under better conditions, genetic differences can express more fully.
- **Massive secular IQ gains (Flynn effect):**
  - Average IQ scores have risen dramatically over the 20th century in many countries (often 10–30 points).
  - Such changes in a few generations **cannot be genetic**; they must be environmental (education, nutrition, complexity of life, health, etc.).
- **Early life interventions & health:**
  - Severe malnutrition, toxin exposure (e.g., lead), and extreme neglect **can substantially depress cognitive development**.
  - Interventions (preschool quality, nutrition, health care, schooling quality) can **raise cognitive performance** and/or associated life outcomes, even if mean IQ proves somewhat resilient.

So: even with high heritability, **environmental improvements can shift the whole distribution** and strongly affect individual outcomes.

---

## 6. Group differences and heritability

A crucial point:
**High


text:
Human intelligence is substantially heritable, but not fixed, and strongly shaped by environment. The key is to separate three questions:

1. How much of *current differences* in intelligence within a given population are due to genetic differences?
2. How does that change across the lifespan and across environments?
3. What does this *not* imply (e.g., about groups, determinism, or immutability)?

Below is a concise overview of what the evidence shows.

---

## 1. Heritability: what the numbers actually mean

**Heritability** is a statistic (h²) that describes, in a specific population at a specific time, the proportion of *variation in a trait* that is associated with genetic differences.

- If h² = 0.5 for IQ, that means: within that population, about 50% of *the observed differences between people* in IQ scores are associated with genetic differences.
- It does *not* mean “50% of *your* intelligence is genetic.”
- It does *not* mean the trait is fixed or unaffected by environment.
- Heritability can change if the environment changes.

---

## 2. Twin, family, and adoption studies

The most robust evidence comes from twin and adoption studies, which compare:

- Identical twins (share ~100% of genes)
- Fraternal twins / siblings (share ~50% of segregating genes)
- People raised together vs. apart

Across many large studies and meta-analyses:

- **Childhood**:
  - Heritability of general intelligence (g or IQ) is often around **0.2–0.4**.
  - Shared family environment plays a substantial role.

- **Adolescence to adulthood**:
  - Heritability estimates rise to roughly **0.5–0.8** in many high‑income countries.
  - Shared family environment effects shrink; non‑shared environment and measurement error account for much of the rest.

This pattern—heritability increasing with age—is one of the most replicated findings. A common interpretation: as people grow, they seek, select, and shape environments partly in line with their genetic predispositions (gene–environment correlation), amplifying genetic effects.

**Key large-scale results (high-level):**

- Adult twin studies in Western populations typically find intelligence heritability around **0.6–0.8**.
- Adoption studies show:
  - Adopted children’s IQs are more similar to their *biological* relatives than to adoptive relatives by adulthood.
  - The *short-term* boosts from enriched adoptive homes are real but tend to attenuate over time, though they can still raise average outcomes meaningfully.

---

## 3. Molecular genetics (GWAS and polygenic scores)

Genome-wide association studies (GWAS) scan the genome in very large samples:

- They find **thousands** of genetic variants each with tiny effects on educational attainment and cognitive performance.
- **Polygenic scores** (aggregating many variants) can currently explain:
  - Roughly **10–20%** of the variance in educational attainment in European-ancestry samples.
  - A smaller but meaningful fraction of variance in cognitive test performance (lower than twin-based heritability, because we don’t yet capture all relevant genetic variation and because of measurement/noise).

These molecular results:

- Independently confirm that genetic differences contribute substantially to cognitive differences.
- Also show large environmental and stochastic components: even the best current polygenic scores leave most of the variance unexplained.

---

## 4. Environment still matters a lot

High heritability does **not** mean environmental factors are unimportant.

Key environmental influences:

- **Early life conditions**: nutrition, toxins (e.g., lead), illness, prenatal factors.
- **Socioeconomic status (SES)**: poverty, stress, access to learning resources, schooling quality.
- **Education**: years of schooling, schooling quality, cognitively demanding curricula.
- **Macro-environment**: public health, infectious disease burden, general safety and stability.

Evidence:

- Large gains in average IQ over generations (the **Flynn effect**) coinciding with improved nutrition, education, and health show that the *mean* level of intelligence is highly environmentally responsive.
- Programs that reduce lead exposure, improve nutrition, or extend schooling consistently raise cognitive test scores at the group level.
- Randomized or quasi‑randomized studies (e.g., school reforms, extra years of education) show causal effects of schooling on IQ and related outcomes.

Crucially, **heritability is higher in more equal / supportive environments**:
- When severe deprivation is reduced, variability due to environment shrinks, so genetic differences account for a larger share of what’s left.
- In very harsh or unequal environments, environmental factors can dominate (and heritability estimates can be lower).

---

## 5. Group differences vs. individual differences

Heritability is **about


text:
Human intelligence is substantially heritable, but not fixed, and its heritability depends on age, environment, and how it’s measured.

### 1. What “heritability” means (and doesn’t mean)

- **Heritability (h²)**: The proportion of variation in a trait *within a population* that can be statistically attributed to genetic differences *in that population and environment*.
- It does **not** mean:
  - “X% of your intelligence comes from genes and Y% from environment.”
  - “Intelligence is unchangeable.”
  - “Genes determine individual destiny.”

Heritability is about **differences between people**, not about the development of a single person.

---

### 2. Twin, adoption, and family studies

These have been the core evidence for decades:

- **Identical twins reared together**: IQ correlations ~0.75–0.85
- **Identical twins reared apart**: ~0.65–0.75
- **Fraternal twins reared together**: ~0.40–0.60
- **Biological siblings**: ~0.40–0.50
- **Adoptive siblings (no shared genes)**: ~0.00–0.20, often close to 0 in adulthood

When modeled, these data typically yield:

- **Childhood IQ heritability**: ~0.30–0.45
- **Adolescence**: ~0.50–0.60
- **Adulthood**: often ~0.60–0.80 in Western samples

So, in many adult Western populations, **most of the variation in general intelligence (g) is statistically attributable to genetic differences**, with a smaller but still meaningful role for shared family environment and a substantial role for unique (nonshared) environment and measurement error.

---

### 3. Age-related changes

Heritability of intelligence tends to **increase with age**:

- Early childhood: environment (especially family and early schooling) explains more variance.
- By adulthood: genetic influences become more expressed (gene–environment correlation and interaction), so estimated heritability rises.

This doesn’t mean genes “switch on” intelligence later. It means that as people choose environments and activities partly based on their dispositions, their genetic differences become more strongly reflected in test scores.

---

### 4. Molecular genetic evidence (GWAS, SNP-based heritability)

Modern genomics confirms and refines the twin-study picture, though it still doesn’t capture all the heritability:

- Genome-wide association studies (GWAS) with millions of people find **thousands of variants**, each with a tiny effect.
- Polygenic scores built from these variants:
  - Currently explain ~10–20% of IQ variance in European ancestry samples, less in other ancestries due to data limitations and portability issues.
- SNP-based heritability estimates (using measured common variants) usually find:
  - Roughly **20–40% of variance** in intelligence explained by common SNPs captured in current arrays.

This “missing heritability” gap between twin-study estimates (~60–80%) and SNP-based estimates (~20–40%) is typical for complex traits and is likely due to:
- Rare variants, structural variation, gene–gene interactions, etc., that are hard to measure.
- Imperfect measurement of both genotypes and phenotypes.

Still, the fact that polygenic scores predict educational and cognitive outcomes, and that these predictions replicate across datasets, strongly supports real genetic influence.

---

### 5. Role of environment

High heritability does **not** imply environment is unimportant.

Evidence for environmental effects includes:

- **Secular trends (Flynn effect)**: Large population-wide IQ gains across much of the 20th century (now plateauing or reversing in some places). Gene pools didn’t change that fast; environment did (nutrition, schooling, complexity of daily life, testing familiarity, etc.).
- **Severe deprivation** (e.g., institutional neglect, extreme poverty, malnutrition, uncorrected health issues) can substantially reduce cognitive development, sometimes with lasting effects.
- **Education and training**:
  - Schooling reliably boosts performance on many tests, especially ones tapping learned content and some reasoning skills.
  - Effects on “g” itself are debated, but schooling clearly matters for real-world cognitive functioning.
- **Environmental inequalities**: Heritability can vary with socioeconomic status (SES). Some studies (especially in the U.S.) find:
  - **Lower heritability and stronger shared environmental effects at low SES** (because basic needs and cognitive stimulation differ more), and
  - **Higher heritability at high SES**, where environmental conditions are more uniformly conducive to development and genetic differences have freer expression.
  - This SES‑by‑heritability interaction is weaker or inconsistent in some European samples, likely because of differing social and educational


text:
Human intelligence is substantially heritable, but not fixed, and not determined by genes alone. Here’s what the evidence shows, as clearly and cautiously as possible.

---

## 1. What “heritability” actually means

- In behavior genetics, **heritability (h²)** = the proportion of **variation in a trait within a particular population** that’s statistically attributable to genetic differences.
- It is **not**:
  - “How much of your intelligence comes from your genes” (that’s a misleading phrasing).
  - A fixed property of the trait across all places/times.
- Heritability is **population-specific** and **environment-dependent**.

Example: If everyone had identical environments (nutrition, schooling, health), differences in intelligence would be relatively more due to genetics, so heritability would be high. If environments differ a lot, environmental factors account for more variance, so heritability is lower.

---

## 2. Twin and adoption studies: the main evidence

Most classic evidence comes from:

- **Twin studies**:
  - Compare:
    - Identical (MZ) twins: share ~100% of segregating genes.
    - Fraternal (DZ) twins: share on average ~50%.
  - If MZ twins are more similar in IQ than DZ twins, that implies genetic influence.
- **Adoption studies**:
  - Compare adopted children to:
    - Their biological relatives (genes, not shared environment).
    - Their adoptive relatives (shared environment, not genes).

Across many studies and meta-analyses:

### Childhood
- Heritability estimates for IQ: roughly **20–40%** in early childhood.
- Shared environment (family-level influences: home, parenting, SES) accounts for a **large portion** of variance.
- Genetic influence is present but relatively modest.

### Adolescence to adulthood
- Heritability estimates for IQ typically rise to around **50–80%** by late adolescence and adulthood in affluent, stable societies.
- Shared environmental influence **declines**, often approaching zero in some samples.
- Non-shared environment (experiences that differ between siblings—unique friends, illnesses, random life events, etc.) accounts for the remainder.

This **increase in heritability with age** is a robust finding. It’s usually interpreted as “gene–environment correlation”: as people grow older, they select, create, and adapt environments that fit their genetic predispositions (e.g., gravitating toward cognitively demanding activities).

---

## 3. Molecular genetics (GWAS and polygenic scores)

Beyond twin/adoption studies, we now have:

- **Genome-Wide Association Studies (GWAS)**:
  - Look for associations between millions of genetic variants (SNPs) and measures of cognitive ability or educational attainment.
  - Each variant has a **tiny** effect. Only by aggregating thousands of them can we predict anything meaningful.
- **Polygenic scores (PGS)**:
  - Sum of many small-effect variants associated with cognition/education.
  - In large samples of European ancestry, current scores explain:
    - Roughly **10–20%** of the variance in years of education.
    - Typically **5–15%** of the variance in cognitive test scores (varies by dataset and measure).

This is **less** than twin-study heritability, a gap known as “missing heritability,” but the molecular evidence **confirms that many genetic variants distributed across the genome are associated with cognitive performance**.

Important caveats:
- Current PGS work best in the populations they were derived from (often Europeans), and transfer poorly to other ancestries.
- They are **probabilistic**, not deterministic: people with low scores can achieve high IQ/education and vice versa.

---

## 4. Environment still matters a lot

High heritability does **not** mean environment is unimportant.

Key points:

- **Mean level vs. variance**:
  - Heritability is about **relative differences** within a group, not average levels between groups.
  - Environmental shifts (e.g., improved nutrition, schooling, disease control) can raise or lower **average IQ** substantially even when heritability is high.
  - The **Flynn effect** (substantial IQ gains over generations during the 20th century in many countries) is a classic example of large environmental impact.

- **Context-dependence**:
  - In **highly unequal or deprived environments**, environmental differences can loom larger; heritability estimates can be lower.
  - In **more equal, affluent environments**, heritability estimates tend to be higher because basic environmental needs are more uniformly met.

- **Interventions**:
  - Early childhood programs, deworming, lead removal, iodization, better education, and general health and safety improvements can increase cognitive performance.
  - Effects sometimes fade, sometimes persist; the literature is mixed, but it’s clear that conditions


text:
Human intelligence is substantially heritable, but that does **not** mean it is fixed, determined at birth, or unaffected by environment. It means that, in a given population and set of environments, **genetic differences explain a large fraction of the *variation* in intelligence between people**.

Here’s what the evidence shows, in layers:

---

## 1. What “heritability” means (and doesn’t)

- **Heritability (h²)**: The proportion of *observed differences* in a trait within a population that can be statistically attributed to genetic differences.
- It is **not**:
  - the fraction of *your* intelligence that “comes from genes”
  - a statement about *immutability*
  - a universal constant

Heritability is:
- **Population-specific** (depends on who you study)
- **Environment-specific** (depends on how varied or equal environments are)

If environments become more equal (e.g., universal good nutrition, schooling), then **relative genetic differences explain more of what is left**, so heritability can go up even while everyone improves.

---

## 2. Twin, adoption, and family studies

These classic designs are the backbone of the evidence.

### Twin studies

- Identical (monozygotic, MZ) twins share ~100% of segregating genes.
- Fraternal (dizygotic, DZ) twins share ~50% on average.
- Compare similarity in IQ between MZ and DZ twins raised together or apart.

Findings (approximate ranges for adults in modern Western samples):

- **MZ twins reared together**: IQ correlations ~0.80–0.90
- **MZ twins reared apart**: ~0.70
- **DZ twins reared together**: ~0.40–0.50

From these patterns, meta-analyses estimate:

- **Adult IQ heritability**: roughly **0.50–0.80** (50–80%) in high-income countries.

Notably:
- Heritability of IQ **increases with age**:
  - childhood: often ~20–40%
  - adolescence: ~40–60%
  - adulthood: ~60–80%

This “genetic amplification” likely reflects people increasingly shaping and selecting their environments in line with their genetic dispositions (gene–environment correlation).

### Adoption studies

Adoption studies compare:

- adopted children vs. their **biological** parents/siblings (genes, different environment)
- adopted children vs. their **adoptive** parents/siblings (environment, not genes)

Findings:

- As children grow up, their IQ becomes **more similar to biological relatives** and **less similar to adoptive relatives**.
- Yet, adoption from deprived backgrounds into more advantaged homes can raise IQ by **~10–20 points** on average, especially in early life.

This supports **both** a substantial genetic component **and** large environmental effects, especially at the low end of environmental quality.

---

## 3. Molecular genetics: what specific genes show

With large genome-wide association studies (GWAS) and polygenic scores, we’ve moved from statistical inference to identified DNA variants.

### GWAS findings

- Intelligence and related traits (educational attainment) are **highly polygenic**: thousands of variants, each with tiny effects.
- Modern GWAS sample sizes: >1 million individuals for education; hundreds of thousands for cognitive tests.
- Polygenic scores for education / cognition now explain:
  - ~10–20% of the variance in educational attainment
  - ~5–15% of the variance in measured cognitive ability, depending on sample, ancestry, and measure.

This is **lower** than twin-based heritability (50–80%), for reasons including:
- rare variants not well captured
- imperfect measurement
- gene–gene and gene–environment interactions
- current statistical limits

But the fact that:
- we can **predict some variance from DNA alone**, and
- the effect sizes scale with sample size as theory predicts,

strongly corroborates that **intelligence differences are substantially influenced by common genetic variation**.

---

## 4. How environment matters

High heritability does not mean environment is unimportant. Environments can:

- Shift the **average level** of intelligence (mean) substantially while heritability (a ratio of *variation*) remains high.
- Affect how much genetic differences show up.

Key environmental influences supported by evidence:

1. **Severe deprivation and malnutrition**
   - Early-life malnutrition, toxic exposures (e.g., lead), and profound neglect can depress IQ markedly.
   - Improvements in these factors raise population IQ averages (part of the “Flynn effect”).

2. **Education and cognitive stimulation**
   - More years of schooling causally increase IQ and related abilities, not just the reverse.
   - Cognitive interventions show smaller, more context-dependent


text:
Human intelligence is substantially heritable, but not fixed, and strongly shaped by environment. The best-studied case is general cognitive ability (often called “g” or IQ).

Here’s what the evidence shows, in layers:

---

## 1. What “heritability” actually means

- **Heritability (h²)** in this context = the proportion of **differences between people** in a population that can be statistically attributed to **genetic differences**, given a particular time and environment.
- It **does not** mean:
  - “X% of your intelligence comes from genes and the rest from environment.”
  - “Your IQ is X% fixed from birth.”
- Heritability is **population- and context-specific**. It can go up or down depending on social conditions, inequality, education, nutrition, etc.

---

## 2. Twin, family, and adoption studies

These were the first major lines of evidence:

### Classic findings

Across many samples in developed countries:

- **Monozygotic (identical) twins** raised together: IQ correlations ≈ **0.75–0.85**
- **Monozygotic twins raised apart**: ≈ **0.65–0.75**
- **Dizygotic (fraternal) twins** raised together: ≈ **0.40–0.50**
- **Biological siblings**: ≈ **0.40–0.50**
- **Adoptive siblings** (no shared genes): near **0** in adulthood

Using these and related designs, most reviews converge on:

- **Childhood IQ heritability**: ~**0.30–0.50**
- **Adult IQ heritability**: ~**0.60–0.80**

So in typical Western samples, **60–80% of the variation in adult general intelligence is statistically associated with genetic differences**.

### Age pattern

- **Genetic influence increases with age**:
  - Childhood: lower heritability, stronger family environment effects
  - Adolescence: heritability rises
  - Adulthood: heritability of g often ~0.7 or higher
- Shared family environment (“being raised in the same home”) matters more early in life and often **fades by adulthood** for g, while **non-shared environment** (individual experiences, accidents, peers, idiosyncratic events, measurement error) remains important.

Key references (consensus-level):
- Plomin & Deary, 2015, *Molecular Psychiatry*
- Polderman et al., 2015, meta-analysis of twin studies (∼2,700 traits)

---

## 3. Molecular genetics (GWAS, SNP-based heritability)

Genome-wide association studies (GWAS) and related methods provide an independent line of evidence:

- **SNP-based heritability** (what common genetic variants can explain) for g / educational attainment is typically:
  - **~15–30%** for measured cognitive performance
  - Higher when using broad educational-outcome proxies (completed years of schooling, etc.), sometimes up to ~40% in large datasets.
- Polygenic scores (PGS) built from large GWAS:
  - Can predict around **10–20% of the variance** in educational attainment in some European-ancestry samples.
  - Typically **7–15%** for measured IQ/g, depending on sample and measure.

These molecular results:
- **Undershoot** twin-based heritabilities (due to imperfect tagging of all variants, rare variants, gene–gene and gene–environment interactions, measurement noise).
- Still strongly support that **genetic differences systematically relate to cognitive differences**.

---

## 4. Gene–environment interplay

High heritability **does not** mean environment is unimportant:

1. **Gene–environment interaction (G×E)**
   Genetic effects can be **stronger or weaker depending on environment**.
   - In some studies, **heritability is lower in deprived environments**, because adverse conditions swamp genetic potentials.
   - When basic needs and education are fairly uniform, **genetic variation explains more of what’s left**.

2. **Gene–environment correlation (rGE)**
   People’s genetic dispositions influence the environments they experience:
   - Child who is slightly better at reading (partly genetically influenced) may seek more books, get more encouragement, leading to higher eventual ability.
   - Parents pass on genes and also create correlated environments.

3. **Massive environmental interventions work**
   - **Fetal health, micronutrient deficiencies, infectious disease, lead exposure, iodine deficiency**, and early-life deprivation reliably affect cognitive outcomes.
   - Adoption out of extremely deprived environments produces **large IQ gains** relative to siblings left behind.
   - Historical “Flynn effect”: IQ test scores rose dramatically (∼2–3 points/decade in


text:
Intelligence (as measured by IQ and related cognitive tests) is substantially heritable, but not fixed by genes, and its heritability depends strongly on age and environment.

Here’s what the evidence shows, in layers:

---

## 1. What “heritable” means

- **Heritability (h²)** = proportion of *differences between people* in a trait that can be statistically attributed to genetic differences in a given population at a given time.
- It **does not** mean:
  - “X% of your intelligence comes from genes.”
  - “Your IQ is X% determined and can’t change.”
- It’s about **variation between people**, not about any individual.

---

## 2. Classic estimates from twin and family studies

Large meta‑analyses of twin, adoption, and family studies show:

- **Childhood:** heritability of IQ ≈ **0.20–0.40**
- **Adolescence:** ≈ **0.40–0.60**
- **Adulthood:** ≈ **0.60–0.80**

So by adulthood, **most of the variation in cognitive ability within typical Western populations appears to be influenced by genetic differences**, but that still leaves a substantial environmental component.

Key findings:

- **Identical (MZ) twins** reared apart are highly similar in IQ, almost as similar as identical twins reared together.
- **Adopted children** resemble their adoptive parents in IQ in early childhood, but this similarity fades; by adulthood they resemble their **biological parents and siblings** more than their adoptive family.
- The apparent heritability of intelligence **increases with age**; environmental influences in childhood can either mask or amplify genetic differences that become more visible later.

---

## 3. Gene–environment interplay

Three main processes explain the age increase in heritability:

1. **Gene–environment correlation**
   - As people get older, they **select, modify, and create environments** that fit their dispositions and abilities.
   - A child with a genetic predisposition toward high verbal ability may:
     - Be more drawn to reading.
     - Seek academic challenges.
     - Receive more encouragement and advanced instruction.
   - This makes genetic differences show up more strongly over time.

2. **Gene–environment interaction**
   - The effect of genes can **depend on environment**.
   - Example: A child with high genetic potential for cognitive ability may not reach it under conditions of extreme deprivation; in supportive environments, genetic differences express more.

3. **Shared vs non‑shared environment**
   - **Shared environment** (family SES, parenting style, home) has a **moderate influence in childhood** on IQ, but typically **shrinks toward zero by adulthood** in many studies.
   - **Non‑shared environment** (different peers, teachers, illnesses, random life events, measurement error) plus chance factors continue to matter throughout life.

---

## 4. Socioeconomic status and context effects

Heritability is **not the same in all environments**.

- In some U.S. samples, particularly in childhood, **heritability is lower in low‑SES families and higher in high‑SES families**:
  - In harsher or more deprived environments, variation in intelligence is more constrained by environmental factors (nutrition, stress, school quality).
  - In more advantaged contexts, basic needs are met for nearly everyone, so **genetic differences explain more of the remaining variation**.
- This moderation by SES is **not always found** in European and other samples, likely due to different social policies and inequality levels.

This shows that **heritability is context‑dependent**: change the environment, and the estimated heritability changes.

---

## 5. Molecular genetics: what do specific genes tell us?

Genome‑wide association studies (GWAS) on very large samples (hundreds of thousands to millions) find:

- Intelligence is **highly polygenic**:
  - Thousands of genetic variants each have **tiny** effects.
  - No single “intelligence gene”.
- Polygenic scores (PGS) built from these variants:
  - Currently explain about **10–20%** of IQ variance in European-ancestry samples, depending on sample and measure.
  - This is **less than** twin-based heritability estimates (the gap is called “missing heritability”), but the gap is shrinking as samples grow and methods improve.

Crucially:

- These scores are **probabilistic, not deterministic**; they shift *average* outcomes, with huge overlap between people at different score levels.
- They are **less predictive across ancestries** because GWAS samples have been heavily biased toward European ancestries.

---

## 6. Intelligence is not purely “genetic” or purely “environmental”

Multiple lines of evidence show **strong environmental effects**:

- **Secular rise in IQ (Flynn


text:
Human intelligence is substantially heritable, but not fixed, and its heritability depends on age, environment, and how it’s measured.

**1. What “heritability” means (and doesn’t mean)**
- Heritability (h²) is the proportion of variation in a trait *within a particular population, at a particular time, in particular environments* that is due to genetic differences.
- If h² for IQ = 0.6, that means about 60% of *differences between people* in that group are statistically associated with genetic differences.
- It does **not** mean:
  - “60% of *your* intelligence is genetic.”
  - “Environment doesn’t matter.”
  - “The trait can’t be changed.”

**2. Twin and family studies: the classic evidence**

Most of what we know comes from:
- **Monozygotic (MZ) twins**: identical, ~100% shared genes.
- **Dizygotic (DZ) twins**: fraternal, ~50% shared genes (like typical siblings).

Key findings from large meta-analyses and long-running cohorts (e.g., Minnesota Study of Twins Reared Apart, Swedish Adoption/Twin studies):

- **Childhood (roughly age 5–12):**
  - Typical heritability estimates: **0.3–0.5** (30–50%).
  - Shared family environment (same home, parents, etc.) often explains a sizable portion of the rest.

- **Adolescence to adulthood (around 16–40+):**
  - Heritability estimates often rise to **0.6–0.8** (60–80%).
  - Shared family environment effects usually **shrink** markedly by adulthood. Non-shared environment (different peers, teachers, illnesses, random life events) remains important.

- **Twins reared apart:**
  - Identical twins raised in different families show IQ correlations ~0.7–0.8 in adulthood, almost as strong as twins raised together.
  - This strongly supports a big genetic component.

Patterns consistently replicated across countries and cohorts:
- IQ correlations:
  - MZ twins together: ~0.8–0.9
  - MZ twins apart: ~0.7–0.8
  - DZ twins together: ~0.4–0.6
  - Biological siblings: ~0.4–0.5
  - Adoptive siblings (no shared genes, same home): close to 0 by adulthood

**3. Why heritability of IQ increases with age**

Several likely reasons:
- As people age, they **select and shape environments** that fit their traits (gene–environment correlation). For example, a child who learns quickly may choose more challenging courses, hobbies, and careers, amplifying initial genetic differences.
- Early in life, **family-level factors** (nutrition, early education, home stability) play a bigger role. Later, individuals’ differentiated paths dominate.
- Measurement reliability and test variety improve in adolescence/adulthood.

**4. Molecular genetics: what specific genes show**

With large genome-wide association studies (GWAS):

- Current GWAS of educational attainment and cognitive performance involve >1 million people. They find:
  - Thousands of genetic variants, each with **tiny** effect sizes.
  - “Polygenic scores” built from these variants can predict ~10–20% of variance in educational attainment in European-ancestry samples, and generally a bit less for IQ directly (often in the ~5–15% range, depending on sample and measure).
- This is **lower** than twin-based heritability (the gap is called “missing heritability”), but:
  - As sample sizes grow and methods improve, the predictive power gradually increases.
  - It confirms that many genes of small effect contribute.

Importantly:
- The same variants often influence multiple traits (pleiotropy): IQ, years of schooling, some psychiatric risks, etc.
- Polygenic scores are **not** destiny; people with low scores can still achieve very high cognitive performance, and vice versa.

**5. Environment still matters a lot**

High heritability does not mean the environment is unimportant:

- **Severe deprivation and toxins:**
  - Extreme poverty, malnutrition, neglect, lead exposure, and some infections can substantially lower cognitive development, regardless of genes.
- **Educational quality:**
  - Schooling raises measured IQ; extra years of education and cognitively demanding schooling improve test performance.
  - Adoption from deprived to enriched homes often yields large IQ gains in childhood and adolescence.
- **Secular (Flynn) effect:**
  - IQ scores rose substantially (often 2–3 points per decade) in many countries during the 20th century.
  - This change is far too rapid to be genetic;


text:
Human intelligence is moderately to highly heritable, but that does **not** mean it is fixed or determined by genes. The evidence shows a complex, age‑dependent picture where both genes and environment matter, and they interact.

### 1. What “heritability” means (and doesn’t mean)

- **Heritability (h²)** in this context = the proportion of **differences between people** in a population that can be statistically attributed to genetic differences.
- It is **not**:
  - How “genetic” intelligence is for an individual.
  - A measure of immutability or “hardwiring.”
  - A universal constant: it depends on the population and environment studied.

If a trait has 50% heritability, that means: in that specific group, under those conditions, about half of the variation between people is associated with genetic differences.

---

### 2. Typical estimates for intelligence (IQ / general cognitive ability)

Most of the evidence comes from:
- Twin studies (identical vs fraternal)
- Adoption studies
- Genome-wide association studies (GWAS) and SNP-based heritability

**Broad picture across life:**

- **Early childhood:** h² ≈ 20–40%
- **Adolescence:** h² ≈ 40–60%
- **Adulthood:** h² ≈ 60–80%
- **Old age:** may stay high, though disease and differential aging complicate estimates

Meta-analyses of twin studies (e.g., by Plomin, Deary, Polderman et al., 2015) consistently find:
- In adulthood, genetic factors account for **roughly two‑thirds** of the variance in general intelligence in Western populations with reasonably similar schooling and nutrition.

---

### 3. Why does heritability increase with age?

Several likely reasons:

1. **Gene–environment correlation**
   - As people get older, they increasingly choose, modify, and create environments that fit their dispositions (e.g., choosing more intellectually demanding hobbies, education, jobs), which amplifies genetic influences.

2. **Cumulative small effects**
   - Many small genetic differences may have larger cumulative effects over time (e.g., slight advantages making school a bit easier, which leads to more learning, which leads to greater advantage, etc.).

3. **Shared family environment declines**
   - Siblings raised together become adults who live different lives; shared home effects on IQ are strongest in childhood and mostly fade by adulthood in many studies.

---

### 4. Genetic architecture: many genes, tiny effects

- Intelligence is **polygenic**: influenced by **thousands of genetic variants**, each with very small effects.
- GWAS have found:
  - Individual variants usually explain **far less than 1%** of IQ variance.
  - Aggregated into polygenic scores, currently they can explain on the order of **10–20%** of the variance in large European-ancestry samples.
- These scores:
  - Have **predictive value in groups**, not precise prediction for individuals.
  - Perform less well across ancestries because of differences in genetic backgrounds and study biases.

This genomic evidence broadly **agrees** with twin/adoption estimates, though it currently captures only part of the “twin-study” heritability.

---

### 5. Environmental influences are still crucial

Even with high heritability, environments can have large effects:

- **Severe deprivation vs normal environments**
  - Extreme poverty, malnutrition, neglect, and lack of stimulation can markedly reduce cognitive development.
- **Adoption studies**
  - Children adopted from adverse backgrounds into more enriched homes often show **substantial IQ gains** compared to non‑adopted siblings or peers who stayed in poorer conditions.
  - Shared environment: strongest in childhood (e.g., adoption can raise IQ by ~6–12 points on average, depending on study and context).

- **Education**
  - Additional years of schooling are consistently associated with higher IQ test scores and better cognitive performance.
  - Natural experiments (e.g., compulsory schooling age changes) suggest **causal** effects: roughly **1–5 IQ points per extra year** of schooling in some studies.

- **Population-level changes**
  - The **Flynn effect**: average IQ scores rose dramatically (roughly 2–3 points per decade) over much of the 20th century in many countries, too fast to be genetic—this is strong evidence that **environmental changes** (education, health, complexity of life, test familiarity) can shift cognitive performance for entire populations.

Crucially, **high heritability within a relatively equal environment says little about what would happen if the environment changed dramatically** (e.g., major improvements or deteriorations in nutrition, pollution, schooling, or childhood adversity).

---

### 6. Group differences vs individual heritability

A core distinction:

- **Heritability describes


text:
Human intelligence is substantially heritable, but not fixed, and strongly shaped by environment. Here’s what the evidence shows.

---

## 1. What “heritable” means (and doesn’t)

- **Heritability (h²)**: the proportion of *differences between people* in a trait that can be statistically attributed to genetic differences *in a given population, at a given time*.
- It does **not** mean:
  - “X% of your intelligence comes from genes and Y% from environment.”
  - “You can’t change it.”
  - “Genes matter equally in all places and times.”

Heritability is about **variation across people**, not about an individual’s potential.

---

## 2. Twin and family studies: the classic evidence

Studies comparing identical (MZ) and fraternal (DZ) twins, adopted siblings, and families have been remarkably consistent.

### Broad patterns

- **Childhood (~5–12 years)**
  - Typical heritability estimates: **0.30–0.45** (30–45%).
  - Shared family environment (home, parenting, etc.) has a substantial effect.

- **Adolescence (~13–18)**
  - Heritability: **0.50–0.60**.
  - Shared environment effects start to shrink.

- **Adulthood (~20+)**
  - Heritability: often **0.60–0.80** in high-income, relatively equal societies.
  - Shared family environment becomes small or even near-zero in many studies.
  - Non‑shared environment (unique experiences, chance, measurement error) still matters a lot.

A major meta-analysis (Plomin & colleagues and subsequent work) finds that **heritability of intelligence tends to increase with age**, a robust and somewhat counter‑intuitive result.

### Why does heritability increase with age?

Leading explanations:

- **Genetic amplification**: as people grow up, they increasingly choose, seek out, and shape environments that fit their genetically influenced traits (e.g., curious kids read more, which boosts their abilities further).
- **Diminishing power of shared home environment**: siblings diverge as they age and have more distinct experiences.

---

## 3. Molecular genetics: GWAS and SNP heritability

Genome-wide association studies (GWAS) examine millions of genetic variants (SNPs) across very large samples.

### Key findings

- Intelligence and related traits (like years of education) are **highly polygenic**:
  - Thousands of variants of very small effect.
  - No “intelligence gene”; each variant nudges risk or propensity slightly.
- **SNP heritability** (the fraction of variance explained by common SNPs in GWAS):
  - Often around **20–30%** for cognitive ability in large European-ancestry cohorts.
  - This is lower than twin-based estimates because:
    - GWAS don’t capture all genetic variation (rare variants, structural variants, etc.).
    - Twin estimates include all genetic effects (including non-additive ones).

### Polygenic scores

- A polygenic score for intelligence or education is a composite index from many SNPs.
- In large samples, polygenic scores for education/ IQ can explain:
  - Roughly **10–20%** of variance in years of education.
  - Roughly **5–15%** of variance in measured cognitive ability (depending on sample, ancestry, measure).
- Predictive power is much weaker:
  - In non-European ancestry groups (due to biased GWAS training data and population structure).
  - At the individual level (they are probabilistic, not deterministic).

---

## 4. Environmental and societal influences

High heritability does **not** mean environment doesn’t matter. It means **within a given environment, genetic differences explain much of the variation**.

Key environmental effects:

- **Severe deprivation and adversity** (e.g., malnutrition, extreme neglect, chronic lead exposure) reliably lower cognitive development.
- **Education**:
  - More years of schooling and higher-quality education raise performance on intelligence tests and related abilities.
- **Socioeconomic status (SES)**:
  - In some U.S. samples, heritability of IQ is **lower in low‑SES children** (environmental constraints limit expression of genetic differences) and higher in high‑SES children.
  - In more egalitarian countries (e.g., Nordic nations), this SES–heritability interaction is weaker or absent, likely because basic needs and educational quality are more uniform.
- **Secular trends (Flynn effect)**:
  - Average IQ scores rose dramatically (roughly 2–3 points per decade in many countries during the 20th century) due to environmental changes (education, health, test familiarity, complexity of modern life).
  - This happened too fast to be genetic, demonstrating that environments can shift population mean


text:
Heritability of intelligence is substantial but not absolute, and it depends strongly on age and environment. The best current evidence:

## 1. What “heritability of intelligence” actually means

- In behavior genetics, **heritability (h²)** = the proportion of **variation in a trait within a population** that is statistically attributable to genetic differences.
- It is **not**:
  - A fixed property of “intelligence”
  - The percent of an individual’s IQ “caused by genes”
  - Immune to environmental change (heritability can change when environments change)

Heritability is about **differences among people in a given time/place**, not about whether genes matter at all (they obviously do for brain development).

## 2. Overall estimates for adults

Converging evidence from twin, family, and genomic studies shows:

- **Adult IQ heritability** is typically estimated around **50–80%** in economically developed, relatively stable environments.
  - Meta-analyses of twin studies often give values in the **~0.6–0.8** range for general intelligence (g) in adults.
  - SNP-based methods (using measured DNA rather than family resemblance) give more conservative lower bounds, often **~0.2–0.4**, but they capture only common variants and not all genetic effects.

This means that, in such populations, **most of the variation in test-measured cognitive ability is statistically associated with genetic differences**.

## 3. Heritability changes with age

A strong, replicated pattern:

- **Early childhood**: heritability ~**20–40%**
- **Adolescence**: ~**40–60%**
- **Adulthood**: ~**60–80%**

And correspondingly:

- **Shared environment** (things that make siblings similar: family SES, parenting style, neighborhood, etc.) tends to:
  - Be **stronger in childhood** (sometimes 20–30% of variance)
  - **Decline toward near-zero in adulthood** in many twin models
- **Nonshared environment** (experiences that differ between siblings, measurement error, random life events) accounts for the rest.

Interpretation: as people age, they increasingly select, shape, and seek out environments correlated with their genetic dispositions (gene–environment correlation), so **genetic differences get amplified over development**.

## 4. Evidence sources

### Twin and adoption studies

These form the classic evidence base:

- **Monozygotic (MZ) twins** share ~100% of their segregating genes; **dizygotic (DZ) twins** share ~50% on average.
- If MZ twins are more similar in IQ than DZ twins, this suggests a genetic contribution.
- Large twin meta-analyses (tens of thousands of pairs) consistently show:
  - MZ correlations for IQ ~0.7–0.8 in adults
  - DZ correlations ~0.3–0.4 in adults

**Adoption studies**:
- Children adopted into higher-SES families show:
  - **Higher mean IQ** than they likely would have had in their birth environments (strong environmental effect on average level).
  - But within such adoptive families, **resemblance to biological relatives > adoptive relatives** for IQ by adolescence, consistent with substantial heritability.

### Molecular genetics (GWAS, polygenic scores)

- Genome-wide association studies (GWAS) have identified **thousands of genetic variants** each making tiny contributions to educational attainment and cognitive performance.
- **Polygenic scores (PGSs)** built from these variants:
  - Can explain **~10–20%** of the variance in educational attainment in some European-ancestry samples, and a smaller but nontrivial share of IQ variance.
  - Are still incomplete: they capture only common additive variation in studied populations and are less predictive in non-European ancestries due to biases in discovery datasets.
- SNP-heritability (from DNA alone) for cognitive traits is around **20–40%**, which is a lower bound on total heritability.

This molecular evidence **matches the direction and approximate magnitude** suggested by twin studies, lending strong support to real genetic influence rather than purely methodological artifacts.

## 5. Environment still matters a lot

High heritability does **not** imply that environment is unimportant or that traits can’t change.

Key points:

- **Mean levels can change quickly even if heritability is high**.
  - The **Flynn effect**: over the 20th century, many countries saw IQ test scores rise by ~2–3 points per decade—too fast to be genetic, so driven by environmental changes (education, nutrition, complexity of life, etc.).
- **Severe deprivation** (malnutrition, extreme poverty, institutional neglect, lack of schooling) can dramatically lower cognitive development regardless of genetic potential.
-


text:
Heritability of human intelligence is high, but what that means—and doesn’t mean—is often misunderstood. Here’s a concise overview of what the evidence shows.

---

## 1. What “heritability” actually means

- **Heritability (h²)**: The proportion of *differences between people* in a population that can be statistically attributed to genetic differences.
- It:
  - Refers to **variation within a specific population at a specific time**.
  - Does **not** say “X% of *your* intelligence is genetic and Y% is environment.”
  - Can change if environments change (e.g., less inequality in education → heritability can go up because environmental differences shrink).

---

## 2. Twin and family studies: core evidence

Most classical evidence comes from:

- **Identical (MZ) twins**: share ~100% of their segregating genes.
- **Fraternal (DZ) twins**: share ~50% on average.
- **Adopted siblings**: share environment but not genes.

Key findings (broad strokes; exact values vary by study and age):

- In **childhood**, heritability of IQ is often around **0.3–0.4** (30–40%).
- In **adolescence**, it typically rises to **0.5–0.6** (50–60%).
- In **adulthood**, large meta-analyses tend to find **0.6–0.8** (60–80%).

These estimates come from:
- Very high resemblance of **MZ twins raised apart** vs **DZ twins**.
- Weak resemblance between **adoptive siblings**, which fades with age, suggesting shared family environment explains relatively little of adult IQ variance.

A widely cited meta-analysis:
- **Plomin & colleagues** and later reviews: adulthood IQ heritability ≈ **0.7** is a common midpoint estimate in high-income countries.

---

## 3. Developmental pattern: heritability increases with age

One of the most robust observations:

- Heritability of intelligence **increases from childhood to adulthood**.
- Shared environmental influence (things that make siblings similar: same home, same parents) **decreases** with age.
- Non-shared environment (peers, injuries, random events, idiosyncratic experiences) remains important.

Possible reasons:
- As people age, they **select and shape environments** that fit their genetic predispositions (gene–environment correlation).
- Schooling and opportunities allow genetically influenced differences in ability and motivation to express more fully.

---

## 4. Role of environment

High heritability does *not* mean environment is unimportant.

Evidence:

- **Massive environmental effects**:
  - **Flynn effect**: IQ scores rose by ~2–3 points per decade in many countries over the 20th century—far too fast to be genetic.
  - **Education**: Additional schooling raises IQ and achievement test scores; school quality matters.
  - **Severe deprivation** (e.g., extreme neglect, malnutrition) can drastically reduce cognitive development.

- **Socioeconomic status (SES)** differences:
  - In some studies from the U.S., **heritability of IQ is lower in low-SES groups** and higher in high-SES groups: when basic needs and stimulation are missing, environmental variation plays a bigger role.
  - In some European and Scandinavian samples with stronger social safety nets, this SES moderation is weaker or absent.

So:
- Heritability is a **statement about sources of variation**, *given* the current range of environments.
- Change the environmental range or quality substantially, and **average intelligence levels and heritability estimates can both change**.

---

## 5. Molecular genetics: what specific genes show

Genome-wide association studies (GWAS) use hundreds of thousands to millions of people:

- Thousands of genetic variants each have **tiny effects**.
- **Polygenic scores** (combining effects of many variants) can:
  - Explain currently around **10–20%** of the variance in IQ or educational attainment, depending on sample and method.
  - Predict outcomes like years of education, job complexity, and some health outcomes, though **far from deterministically**.

This lower percentage (vs 60–80% from twin studies) is called the **“missing heritability”** problem:
- We know from twin studies there is substantial genetic influence.
- We can only directly measure part of it with current GWAS (many rare variants, gene–gene and gene–environment interactions are not fully captured).

---

## 6. Group differences and heritability

A common confusion:

- **High heritability within a group does not imply genetic explanations for average differences between groups.**

Example:
- Height is highly heritable within modern populations, but **average height differences across countries** can be strongly shaped