text:
Evolution by natural selection is a process that explains how populations change over generations and how new species can arise. It rests on a few key ideas:

1. **Variation**
   Individuals in a population are not identical. They differ in traits such as size, color, behavior, metabolism, etc.
   - Some variation is genetic (heritable), caused by differences in DNA (mutations, recombination, etc.).

2. **Inheritance**
   Offspring tend to resemble their parents.
   - Traits influenced by genes can be passed from one generation to the next.

3. **Overproduction and competition**
   More offspring are produced than can survive and reproduce, because resources (food, space, mates) are limited.
   - This creates competition, directly or indirectly, for survival and reproduction.

4. **Differential survival and reproduction (“fitness”)**
   Some heritable variants make it more likely that an organism will survive and reproduce in its environment.
   - “Fitness” in evolution just means reproductive success: leaving more viable offspring.
   - Individuals with advantageous traits tend, on average, to have more offspring than those without them.

5. **Change in allele frequencies over generations**
   Because the advantageous traits are heritable, the genes (alleles) underlying those traits become more common in the population over generations.
   - This change in allele frequencies is evolution.
   - If the environment changes, which traits are advantageous can change as well.

---

### A concrete example

Consider 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 more often eaten.
- Brown beetles survive longer and, on average, have more offspring.
- The brown color is heritable.

Over many generations:

- Brown beetles become more common.
- Eventually, the population might be almost entirely brown.

No individual beetle “decides” to change; instead, **the proportion of types** in the population changes because of differential survival and reproduction.

---

### Key points and clarifications

- **Natural selection is not random**:
  Mutations that create variation are largely random, but natural selection is a non-random process that consistently favors traits better suited to the current environment.

- **No foresight or goal**:
  Natural selection doesn’t “plan ahead” or strive for perfection. It only favors what works *better than alternatives* in the present environment.

- **Adaptation**:
  An adaptation is a trait shaped by natural selection because it increased fitness in past environments (e.g., camouflage, efficient lungs, social behaviors).

- **Speciation**:
  Over very long timescales, accumulated differences between populations (often separated geographically or ecologically) can become large enough that they can no longer interbreed successfully. At that point, they are considered different species.

If you’d like, I can go into more detail on any part—e.g., mutations and genetic drift, how we measure selection, or examples from humans or specific animals/plants.


prompt_id: sci-evolution

provider: openai

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 ingredients:

1. **Variation**
   Individuals in a population differ in many traits: size, speed, coloration, disease resistance, behavior, etc.
   - Some variation is genetic (due to differences in DNA).
   - New genetic variants arise by mutation, recombination, and gene flow (migration).

2. **Inheritance**
   Some of these differences are heritable: parents pass genetic traits to their offspring.
   - If a trait has a genetic basis, relatives tend to resemble each other.

3. **Differential survival and reproduction**
   In a given environment, not all individuals survive and reproduce equally.
   - Resources (food, mates, shelter) are limited.
   - Predators, diseases, climate, and competition mean some individuals die earlier or have fewer offspring.
   - If certain heritable traits make survival or reproduction more likely (e.g., better camouflage, more efficient metabolism), their bearers tend to leave more offspring.

4. **Change in trait frequencies over generations**
   Because individuals with certain heritable traits produce more offspring, those traits become more common in the next generation.
   - Over many generations, this can:
     - Adapt populations to their environments (e.g., thicker fur in cold climates)
     - Create new features (e.g., complex eyes from simpler light-sensitive patches)
     - Eventually produce new species when populations diverge enough.

A simple example:

- In a population of beetles, some are green and some are brown, and color is genetically determined.
- Birds see and eat green beetles more easily on brown soil.
- Brown beetles survive and reproduce better.
- Each generation, brown alleles (versions of genes) become more common.
- Over time, the population may become almost entirely brown—an adaptation to predation in that environment.

Important clarifications:

- **Natural selection acts on individuals** (who lives/reproduces), but **evolution is a change in populations** (gene frequencies) over time.
- Natural selection is not “goal-directed” or “trying” to improve organisms; it just favors traits that work better *right now* in a given environment.
- Evolution includes other processes too (genetic drift, migration, mutation), but **natural selection** is the one that consistently produces adaptation.

If you’d like, I can walk through a specific trait (like antibiotic resistance in bacteria or beak shapes in finches) as a concrete case study.

finish_reason: stop


text:
Evolution by natural selection is a process where populations change over generations because some individuals leave more offspring than others, due to inherited differences that affect survival or reproduction.

Here’s the core logic, step by step:

1. **Variation**
   - Individuals in a population differ from one another (in size, color, behavior, disease resistance, etc.).
   - Much of this variation is genetic—encoded in DNA.

2. **Inheritance**
   - Offspring tend to resemble their parents.
   - Traits are passed from one generation to the next via genes.

3. **Overproduction and competition**
   - More offspring are produced than can survive and reproduce given limited resources (food, space, mates, etc.).
   - This creates competition, directly or indirectly.

4. **Differential survival and reproduction (“fitness”)**
   - Because of their traits, some individuals are slightly better at surviving and reproducing in a given environment.
   - Those individuals leave more offspring on average.
   - “Fitness” in this context means reproductive success, not strength or health in a general sense.

5. **Change in trait frequencies over generations**
   - If a trait helps individuals leave more offspring, the genes underlying that trait become more common in the population.
   - Over many generations, this can significantly change what the average organism in the population is like.

6. **Adaptation**
   - Traits that consistently increase reproductive success in a particular environment become more common; the population becomes better suited—or “adapted”—to that environment.
   - If the environment changes, different traits may become advantageous, so evolution can change direction.

---

### Simple example: dark vs. light moths

- Suppose a population of moths has both light and dark individuals due to genetic variation.
- On light-colored tree bark, birds more easily see and eat dark moths. Light moths survive and reproduce more.
- Over generations, alleles (gene variants) for light color increase; the population becomes mostly light-colored.
- If air pollution darkens the bark, the advantage flips: now dark moths are better camouflaged, so dark alleles increase.

---

### What natural selection is *not*

- It is not goal-directed or foresightful; it does not “try” to make organisms perfect.
- It works only with existing variation (and new mutations), favoring what works *now* in the current environment.
- Evolution is not always slow and gradual; rates can vary, but the mechanism is the same: differences in reproductive success based on heritable traits.

In summary: evolution by natural selection is the cumulative result, generation after generation, of some genetic variants producing more descendants than others in a given environment.


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

Here’s the core idea in steps:

1. **Variation**
   Individuals in a population differ in many traits (size, color, behavior, disease resistance, etc.).
   - Some of this variation is genetic (heritable), caused by mutations, gene shuffling in sexual reproduction, etc.

2. **Overproduction and competition**
   Organisms tend to produce more offspring than can survive because resources (food, mates, shelter) are limited.
   - This creates competition: not all individuals will survive and reproduce.

3. **Differential survival and reproduction (“fitness”)**
   Some trait variants make it more likely that an individual will survive and/or reproduce in its environment.
   - Example: In a cold climate, animals with thicker fur may survive winter better and have more offspring.
   - “Fitness” in evolution means reproductive success, not strength or health per se.

4. **Inheritance**
   Offspring tend to resemble their parents.
   - If a trait that improves survival/reproduction is heritable, individuals with that trait will, on average, leave more offspring who also have it.

5. **Change in trait frequencies over generations**
   Because the helpful traits are more often passed on, they become more common in the population over many generations.
   - Harmful traits become rarer (unless they are linked to other benefits, only harmful in certain environments, or kept in the population by other processes).

Put together, this is **evolution by natural selection**:
- **Evolution** = change in the genetic makeup (allele frequencies) of a population over generations.
- **Natural selection** = the non-random process where certain heritable traits become more or less common because they affect survival and reproduction.

A concrete example:

- Suppose a population of beetles has two colors: green and brown.
- Birds can see green beetles more easily on brown soil, so green beetles get eaten more often.
- Brown beetles survive longer and have more offspring.
- Color is heritable, so more of the next generation is brown.
- Over many generations, the population becomes mostly brown: the population has evolved, and natural selection favored brown coloration.

Key points to keep in mind:

- Natural selection acts on **individuals’ traits**, but evolution is observed as changes in **populations** over time.
- It is **not** goal-directed; it just reflects what works better under current conditions.
- Environments change, so what is “fit” can also change.
- Natural selection is one mechanism of evolution (others include genetic drift, gene flow, and mutation), but it is the one that explains adaptation—how organisms become well-suited to their environments.


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 are not all the same. They differ in traits like 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 differences are heritable: parents pass their genes—and thus many of their traits—to their offspring.
   - If a trait has a genetic basis, offspring tend to resemble their parents in that trait.

3. **Differential survival and reproduction (fitness)**
   In a given environment, some traits help individuals survive and reproduce better than others.
   - More offspring are produced than can survive (limited food, space, predators, disease, etc.).
   - Individuals with traits better suited to the current environment tend, on average, to:
     - survive longer,
     - find more mates,
     - produce more or healthier offspring.
   This “success” is called **fitness** (in evolutionary terms, it means reproductive success, not strength or health alone).

4. **Change in trait frequencies over generations**
   Because individuals with advantageous traits leave more offspring, the genes underlying those traits become more common in the next generation.
   - Over many generations, this can:
     - shift the average traits in the population (e.g., birds gradually having longer beaks),
     - produce new adaptations (e.g., camouflage, antibiotic resistance),
     - eventually lead to new species if populations diverge enough.

Put simply:

- Traits vary.
- Some of that variation is heritable.
- Some variants help individuals leave more offspring in a given environment.
- Those variants become more common over time.

Natural selection is not goal-directed or conscious. It’s a statistical outcome: given variation, heredity, and differences in reproductive success, populations will evolve.


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 ingredients:

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

2. **Inheritance (Heritability)**
   Some of these differences are passed from parents to offspring.
   - If a trait has a genetic basis, offspring tend to resemble their parents in that trait.

3. **Differential survival and reproduction**
   In a given environment, not all individuals survive and reproduce equally.
   - Some traits make it more likely an individual will survive, find mates, and have offspring (e.g., better camouflage, more efficient metabolism, better immune system).
   - Others make it less likely (e.g., being too slow to escape predators).

4. **Accumulation of favorable traits over generations**
   Individuals with traits that increase survival and reproduction tend to leave more offspring.
   - Those offspring inherit the advantageous traits.
   - Over many generations, these traits become more common in the population.
   - Traits that reduce survival or reproduction become rarer.

This process—non-random differences in reproductive success due to heritable traits—is **natural selection**.

Over long timescales, natural selection (plus other evolutionary processes like genetic drift, migration, and mutation) can:
- Adapt populations to their environments (e.g., polar bears’ thick fur and fat for cold climates).
- Split one lineage into multiple species when populations become isolated and adapt to different conditions.
- Produce complex structures (like eyes) through many small, cumulative changes, each step being useful in its own context.

In short:
1) Individuals vary.
2) Some variation is heritable.
3) Some heritable variants help individuals leave more offspring.
4) Those variants spread, changing the population over time.


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 ingredients:

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

2. **Inheritance (Heritability)**
   Some of these differences are passed from parents to offspring.
   - If a trait has a genetic basis, offspring tend to resemble their parents in that trait.
   - Non-heritable differences (like a scar) don’t matter for evolution.

3. **Differential survival and reproduction**
   In a given environment, not all individuals survive and reproduce equally.
   - Some traits make it more likely an individual will survive, find mates, and have offspring (e.g., better camouflage, more efficient metabolism, better immune system).
   - Others make it less likely (e.g., being too slow to escape predators).

4. **Change in trait frequencies over generations**
   Because individuals with certain heritable traits leave more offspring, the genes underlying those traits become more common in the next generation.
   - Over many generations, this can significantly shift the average traits of the population.
   - This is evolution: a change in the genetic makeup (and thus traits) of a population over time.

---

### A simple example

Imagine a population of beetles:

- They vary in color: some are green, some are brown.
- Color is heritable (offspring tend to have the same color as their parents).
- Birds can see green beetles more easily on dark soil, so green beetles are eaten more often.
- Brown beetles survive and reproduce more, so more brown-beetle genes are passed on.

After many generations, most beetles in the population are brown. The population has evolved by natural selection.

---

### Important points

- **Natural selection is not random.**
  Mutations are random with respect to usefulness, but selection consistently favors traits that increase reproductive success in a given environment.

- **“Fittest” means best at leaving offspring, not strongest or smartest.**
  Fitness is about reproductive success in a specific environment.

- **Environments change.**
  A trait that is beneficial in one environment may be neutral or harmful in another, so what selection favors can change over time.

- **Natural selection can produce complex adaptations.**
  Step by step, small advantages accumulate, leading to complex structures (like eyes) and behaviors, as long as each step provides some benefit.

If you’d like, I can go into more detail on any part (e.g., mutations, genetic drift vs. natural selection, 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 ingredients:

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, recombination (mixing of genes during sex), and gene flow (migration).

2. **Inheritance (Heritability)**
   Some of these differences are passed from parents to offspring.
   - If a trait has a genetic basis, offspring tend to resemble their parents in that trait.
   - Non-heritable differences (like a scar) don’t matter for evolution.

3. **Differential survival and reproduction**
   In a given environment, not all individuals survive and reproduce equally.
   - Some traits make it more likely an individual will survive, find mates, and have offspring (e.g., better camouflage, more efficient metabolism, better immune system).
   - Others make it less likely (e.g., being too slow to escape predators).

4. **Change in trait frequencies over generations**
   Because individuals with certain heritable traits leave more offspring, the genes underlying those traits become more common in the next generation.
   - Over many generations, this can significantly shift the average traits of the population.
   - If environments change, which traits are favored can also change.

Put simply:
- Individuals don’t “evolve” in the biological sense; **populations evolve**.
- Natural selection is **non-random**: it consistently favors traits that improve reproductive success in a given environment.
- Evolution by natural selection can produce complex adaptations (like eyes, wings, or behaviors) through many small, cumulative changes over long periods.

If you’d like, I can walk through a concrete example (like antibiotic resistance in bacteria or beak size in finches) to make this more tangible.


prompt_id: sci-iq-heritability

text:
Human intelligence is substantially heritable, but not fixed, and it’s strongly shaped by environments. The evidence is clearest for *variation within* modern populations, not for comparing across groups or predicting individual potential.

Below is what the data show, and what it *doesn’t* justify.

---

## 1. What does “heritability” actually mean?

- **Heritability (h²)**: the proportion of *variation in a trait within a specific population, at a specific time, in a specific range of environments* that can be statistically attributed to genetic differences.
- It does **not** mean:
  - “X% of your intelligence comes from genes.”
  - “Your intelligence is X% determined and Y% changeable.”
- It’s a *population* statistic, not an individual one.

---

## 2. Twin, adoption, and family studies

These are the core evidence.

### Twin studies
Meta‑analyses of many twin studies (e.g., Thomas Bouchard, Robert Plomin, and others) find:

- **Monozygotic (MZ, identical) twins** share ~100% of their genes.
- **Dizygotic (DZ, fraternal) twins** share ~50% on average.

Comparing IQ similarity:

- MZ twins raised together: very high correlation in IQ.
- MZ twins raised apart: almost as similar as MZ together.
- DZ twins raised together: clearly less similar than MZ twins.

From these:

- **Adult IQ heritability is typically estimated around 0.5–0.8** (50–80%) in high‑income, relatively stable environments.
- Shared environment (family upbringing) effects on *adult IQ* are modest or near zero in many studies, though they matter more in childhood (see below).

### Adoption studies

- Adopted children resemble their **biological** parents more than their adoptive parents in IQ as they reach adolescence and adulthood.
- IQ correlations with **adoptive** parents are often small and decline with age.
- But adoption can still raise IQ relative to what it would have been: children adopted from deprived backgrounds into more advantaged homes often show IQ gains of ~5–15 points in childhood, though some fade over time.

Taken together:

- **Genes: strong contributions to differences in IQ within a population.**
- **Environment: can still make large absolute differences**, especially at lower socioeconomic levels or in extreme environments.

---

## 3. Heritability changes with age

A robust finding:

- **Childhood**: heritability of IQ ~20–40%.
- **Adolescence**: ~40–60%.
- **Adulthood**: ~60–80% in many high‑income samples.

This “rising heritability” likely reflects:

- As people age, they select, shape, and move into environments that match their genetically influenced abilities and interests (“gene–environment correlation”).
- Early childhood environments are more constrained and more shared; later environments diverge more based on individual tendencies.

Implication: high heritability in adults does **not** mean environment is unimportant; it often means environments are more unequal and more *responsive* to individual differences.

---

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

Beyond family resemblance, we now look at actual DNA.

### GWAS (genome‑wide association studies)

- Very large GWAS (hundreds of thousands to millions of people) identify many genetic variants each having *tiny* effects on cognitive test performance or educational attainment.
- Each variant’s effect is minuscule; together, thousands account for a nontrivial share of variance.

### Polygenic scores (PGS)

- A polygenic score sums effects of many variants.
- Currently, for European-ancestry samples:
  - PGS for educational attainment can explain ~10–15% of variance in years of education.
  - PGS for cognitive performance explains less (a single‑digit to low‑teens percentage), but still meaningfully predicts test scores.

Caveats:

- Predictive power is much weaker in non‑European ancestries because GWAS have been Euro‑biased.
- These scores are **correlational**, not destiny: people with “low” scores can achieve very high levels; people with “high” scores may not.

Nonetheless, molecular data broadly **confirm** what twin/adoption studies suggested: many genes of small effect contribute to cognitive differences.

---

## 5. Role of environment

High heritability does not mean environment is trivial. Some key points:

### Socioeconomic status (SES) moderation

- In several U.S. samples, heritability of IQ is **lower in low‑SES families** and higher in high‑SES families:
  - Poorer environments may suppress genetic potential (e.g., worse schools, more stress, poorer health).
  - In higher-SES environments,

finish_reason: length


text:
Human intelligence is moderately to highly heritable, but not in a simple or fixed way. The evidence comes from several lines of research that mostly converge on a similar picture.

## 1. What “heritability” actually means

- **Heritability (h²)** is the proportion of variation in a trait *within a particular population at a particular time* that can be statistically attributed to genetic differences.
- It does **not** mean “what percent of your intelligence comes from your genes.” It’s about differences between people, not about how a single person’s ability is caused.

So if we say “heritability of intelligence is 60%,” we mean: in that population, about 60% of the *differences* in IQ scores between individuals can be linked to genetic variation, given that population’s environment.

## 2. Twin and family studies

### Classic estimates

Using identical (MZ) vs fraternal (DZ) twins, adoption studies, and family correlations:

- **Childhood:** heritability of IQ ~ **0.3–0.4**
- **Adolescence/young adulthood:** ~ **0.5–0.6**
- **Adulthood:** commonly **0.6–0.8** in well-studied Western samples

This “rising heritability” pattern is very robust:

- Genetic influences on intelligence appear to **increase with age**.
- Shared family environment (being raised in the same home) has a **clear effect in childhood**, but its apparent influence declines into adulthood as people select and shape their own environments (friends, education, jobs) in line with their tendencies—a process called **gene–environment correlation**.

### Examples of findings

- Identical twins reared apart are much more similar in IQ than fraternal twins reared together, which strongly implicates genetics.
- Adopted children’s IQs tend to become **more like their biological parents** and **less like their adoptive parents** as they grow older.

## 3. Socioeconomic status (SES) and environment

Heritability is **not constant across environments**.

- In some U.S. samples, in low-SES families:
  - Genetic influences on IQ appear weaker; environmental constraints dominate.
  - In high-SES families, heritability estimates can be quite high (e.g., >0.7).
- In several European and Scandinavian samples with stronger social safety nets, this SES moderation is **smaller or absent**.

Interpretation: when basic needs and educational opportunities are unevenly met, environment plays a larger role in who can reach their potential; when environments are more equal, **genetic differences explain more of the remaining variation**.

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

With large-scale genome-wide association studies (GWAS):

- Intelligence and educational attainment are **highly polygenic**: thousands of genetic variants each with tiny effects.
- Current **polygenic scores** (aggregated genetic indices) can explain:
  - Roughly **10–20%** of variance in educational attainment
  - Typically **10–15%** of variance in measured cognitive ability in large European-ancestry samples

This is **less than twin-based heritability estimates**, due to:
- Incomplete tagging of all causal variants
- Limited understanding of regulatory and rare variants
- Ancestry biases in GWAS (scores don’t transfer well across populations)

But the pattern from molecular data supports:
- Substantial genetic influence
- Many genes of small effect
- Genetic overlap between intelligence, education, and some brain-structure traits

## 5. Gene–environment interplay

Two critical points:

1. **Gene–environment correlation**: People with certain genetic dispositions tend to:
   - Seek out more cognitively stimulating environments
   - Evoke different responses from others (e.g., more encouragement)
   These processes magnify early small genetic differences over time.

2. **Gene–environment interaction**: The **effect of genes depends on environment**.
   - Certain genetic predispositions only manifest in cognitively rich settings.
   - Environmental disadvantages can suppress genetic potential.

So heritability is not “genes versus environment” but “genes operating within environments.”

## 6. Group differences vs individual differences

A crucial distinction:

- Heritability estimates describe **variation within a specific population**, not reasons for **differences between populations or groups**.
- High heritability within a group **does not imply** that differences between groups are genetic.
  Example: height is highly heritable, but population average differences in height can be strongly shaped by nutrition, disease burden, and other environmental factors.

For intelligence, historical and contemporary changes (e.g., **Flynn effect**—rising IQ scores over the 20th century) clearly show environment can substantially shift average cognitive performance over generations, even when genes in that population have not changed much.

##


text:
Human intelligence is substantially heritable, but not fixed, and strongly shaped by environment. The best-supported answer from modern behavioral genetics is:

- In wealthy, stable environments: **heritability of adult intelligence (IQ) is around 50–80%.**
- In childhood: **heritability is lower, roughly 20–40%,** then rises with age.
- In poorer or highly unequal environments: **heritability can be much lower**, because environmental disadvantages matter more.

Below is what “heritability” means, what the major lines of evidence show, and what we can and cannot conclude.

---

## 1. What “heritability” actually means

In this context:

- **Heritability (h²)** = the proportion of *differences between people* in a trait that can be statistically attributed to genetic differences, *in a specific population and environment*.
- It does *not* mean “X% of your intelligence comes from genes” or “X% fixed at birth.” It’s about variation across people, not the absolute level in any one person.

So, if adult IQ heritability is 60% in a particular country:

- That means: 60% of the *variation* in IQ scores in that population is associated with genetic variation, *given their current environment* (schooling, nutrition, inequality, etc.).
- If the environment changes (better schools, less poverty), this number can change.

---

## 2. Twin, family, and adoption studies

### Twin studies

Comparisons of:

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

If MZ twins are more similar in IQ than DZ twins, the difference is attributed to genetic influence.

Meta-analyses over many decades (thousands of twin pairs) show roughly:

- **Childhood IQ heritability**: ~0.2–0.4 (20–40%)
- **Adolescent IQ**: ~0.4–0.6
- **Adult IQ**: ~0.6–0.8 (60–80%)

This “rising heritability with age” is one of the best-replicated findings. As people grow up, they have more freedom to choose environments (friends, activities, education) that fit their genetically influenced traits—so genes increasingly channel which environments they experience.

### Adoption and family studies

- **Adopted children**’s IQs are:
  - More correlated with their **biological parents** as they age.
  - Less correlated with their **adoptive parents** in adulthood, even though the adoptive parents provide the childhood environment.
- Adopted siblings raised together but genetically unrelated show IQ correlations that **decrease toward zero** by late adolescence.

Taken together, these consistently support a substantial genetic contribution, plus a meaningful (but sometimes time-limited) family environmental effect.

---

## 3. Molecular genetics and GWAS

Earlier studies inferred “genetic influence” indirectly via family resemblance. Modern genetics can now look at the actual DNA.

### GWAS (Genome-Wide Association Studies)

- Large studies (hundreds of thousands to >1 million people) identify **many thousands of genetic variants** each contributing a *tiny* effect to IQ or educational attainment.
- When combined into **polygenic scores**, these variants:
  - Predict a modest fraction of variance in IQ directly (currently ~10–20% in European-ancestry datasets).
  - Predict more for educational attainment (~10–25%), which is a noisy proxy for cognitive traits plus environment.

### SNP-based heritability

Methods using measured SNPs (like GREML, LD score regression):

- Typically find **20–30% of IQ variance** captured by common genetic variants directly measured on DNA chips.
- This is lower than twin-based estimates, likely because:
  - Not all genetic variation (rare variants, structural variants, etc.) is measured.
  - Twin studies include both direct genetic effects and some indirect genetic/environmental correlations.

Still, they converge on: **intelligence is polygenic and substantially influenced by many genetic differences.**

---

## 4. Environment still matters a lot

High heritability does not mean environment is unimportant. Some key points:

- **Height is ~80–90% heritable**, yet populations have grown several inches taller in just a century because of nutrition and health improvements. Intelligence shows similar environmental sensitivity (e.g., the Flynn effect).
- **Severe deprivation** (malnutrition, lead exposure, no schooling, extreme neglect) can dramatically lower IQ regardless of genetic potential.
- **Enriched conditions** (quality early education, good nutrition, cognitively stimulating home, safe neighborhoods) can raise average performance, especially for disadvantaged children.
- Social and institutional factors (school quality, segregation, systemic inequality) shape who can realize


text:
Human intelligence is substantially heritable, but *not* fixed, and genes are only part of the story. The evidence is strong, but often misunderstood. Here’s the picture from mainstream behavioral genetics and cognitive science.

---

## 1. What “heritability” actually means

- **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”
  - “Intelligence is predetermined”
  - “Environment doesn’t matter”

If IQ had heritability of 0.50 in some group, that means: about half of the *variation* in IQ scores *within that group* is associated with genetic differences, assuming their current environments.

---

## 2. Classical twin and adoption findings

Across many decades and countries, using mostly twins and adoption studies:

- **Childhood IQ**
  - Heritability: often around **0.30–0.50**
  - Shared environment (home, parenting style, SES): relatively large effect in early years
- **Adolescence and adulthood IQ**
  - Heritability: typically **0.60–0.80**
  - Shared environmental effects shrink; non-shared environment (experiences unique to each person, randomness, measurement error) remains important

This “**heritability increases with age**” pattern is a robust and surprising finding: as people grow up and choose or create their own environments (friends, education, jobs), genetic tendencies are reinforced. This is called **gene–environment correlation**.

**Adoption studies** add to this:

- Children adopted into higher-SES, more stimulating homes gain **several IQ points** on average relative to similar non-adopted children.
- Correlation in IQ between adopted children and their *biological* parents or siblings is clearly higher than with their *adoptive* parents or siblings, especially by adolescence.
- So: both genes and enriched upbringing matter, but genetic resemblance becomes clearer over time.

---

## 3. Molecular genetics: what DNA actually shows

With modern DNA-based methods (GWAS, SNP-heritability, polygenic scores):

- **SNP-heritability** (using measured variants) for IQ and educational attainment is roughly **0.20–0.40**.
  This is lower than twin-based estimates, because:
  - Current methods only capture common measured variants
  - They miss rare variants, structural changes, and complex interactions.

- **Many thousands of genetic variants** each make extremely tiny contributions to cognitive ability. No “intelligence gene” exists.

- **Polygenic scores** (aggregate of many variants):
  - Predict a modest share of the variance in IQ or educational attainment (often **10–20%** of variance in education, somewhat less for IQ, depending on sample and ancestry).
  - Prediction improves in large, well-matched samples, but they are far from determinative and much weaker across different ancestral groups.

Overall, molecular data **support** the idea that intelligence is strongly influenced by genetics, but also show how **polygenic and probabilistic** it is.

---

## 4. Environmental effects are real and large

Several lines of evidence show that environment can shift intelligence measurably:

- **The Flynn effect**: 20th-century populations in many countries gained about **2–3 IQ points per decade** on standardized tests.
  Genes didn’t change that fast; improvements likely came from:
  - Nutrition, health, and reduced infections
  - More schooling and cognitively demanding work
  - Increased familiarity with abstract, test-like problems

- **Early life factors**:
  - Severe malnutrition, chronic disease, lead exposure, extreme deprivation, and lack of schooling can significantly reduce cognitive development.
  - High-quality early interventions (e.g., enriched preschool, better early education) show modest but real long-term effects on cognitive and academic outcomes, even if raw IQ gains partly fade.

- **Socioeconomic status (SES)**:
  - In some studies (especially in the U.S.), heritability of IQ appears **higher in high-SES families and lower in low-SES families**.
    Interpretation: in more advantaged contexts, kids can more fully express genetic differences; in harsher contexts, environmental constraints cap potential.
  - This “SES × heritability” pattern is not equally strong in all countries; it likely depends on how unequal and resource-constrained the society is.

So, heritability is **not a ceiling** on environmental impact. If you equalize or change environments, both average levels and the heritability estimate can change.

---

## 5. Within-group vs. between-group differences

A common confusion:

- From heritability *within* a group, you **cannot**


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

---

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

- **Heritability** is the proportion of differences between people in a given population that can be statistically attributed to genetic differences.
- It is **not**:
  - A measure of how “genetic” an individual’s intelligence is.
  - A fixed property of a trait across time or places.
  - A statement that environment doesn’t matter.

Heritability is always **about variation within a specific population, in a specific environment, at a specific time**.

---

## 2. Twin and adoption studies

These are the classic sources of evidence.

### Twin studies

Comparing:
- **Identical (MZ) twins**: share ~100% of segregating genes.
- **Fraternal (DZ) twins**: share ~50% on average.

Key findings:
- In modern, mostly Western samples, **heritability estimates for general intelligence (g)** are typically in the range:
  - **~20–40% in early childhood**
  - **~50–60% in adolescence**
  - **~60–80% in adulthood**

A widely cited meta-analysis (Plomin & colleagues; Tucker-Drob & others) finds that heritability of IQ often climbs into the **~0.7–0.8 range** (70–80%) by late adulthood in relatively affluent, stable environments.

### Adoption studies

Comparing:
- Adopted children with **biological** vs **adoptive** parents.

Key findings:
- In early childhood, adopted children’s IQs correlate modestly with both biological and adoptive parents.
- By late adolescence and adulthood, their IQ **resembles biological relatives much more than adoptive relatives**, suggesting a growing role of genetic factors over time and a declining direct influence of shared home environment.

---

## 3. Age effects: heritability rises with age

A robust pattern:
- Genetic influence on intelligence **increases with age**.
- Shared family environment influence (things siblings share: home, parents, income, etc.) **decreases with age**.
- Non-shared environment (different peers, teachers, illnesses, random life events, measurement error) remains important at all ages.

Interpretation:
- As people grow up, they increasingly select, seek out, and shape environments (schools, hobbies, jobs, friends) that are **correlated with their genetic dispositions**. This is called **gene–environment correlation**.
- Thus, genes partly “amplify” themselves over time via the environments people gravitate toward.

---

## 4. Role of environment and socioeconomic status (SES)

Heritability is **context-dependent**.

### SES moderation

- In some U.S. samples, heritability of IQ is:
  - **Lower in low-SES environments** (e.g., 0.1–0.3).
  - **Higher in high-SES environments** (e.g., 0.7–0.8).
- Interpretation: when basic needs, schooling, nutrition, and safety are inadequate or highly variable, **environmental constraints dominate**; genetic influences have less room to manifest.
- In more egalitarian, high-income countries (e.g., several Western European nations), this SES effect appears **weaker or absent**, likely because environments are more equalized in childhood.

### Environmental effects that clearly matter

Despite high heritability in some groups, several environmental factors have demonstrable effects on cognitive development and test scores:

- Severe **malnutrition**, chronic illness, or lead exposure: reduces IQ.
- **Iodine deficiency**: historically large IQ reductions; iodization raised scores.
- **Education**: more years of schooling and higher-quality schooling boost IQ and specific cognitive skills.
- **Early childhood interventions** (some high-quality programs): small-to-moderate, often lasting effects.
- Broader improvements in health, schooling, and complexity of life (industrialization, technology, literacy) are linked to historical rises in IQ scores (**Flynn effect**), which are far too fast to be genetic.

This shows that **substantial environmental leverage exists**, especially at the population and policy level.

---

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

Modern genetic studies look at actual DNA variants:

### GWAS (genome-wide association studies)

- Large studies (n > 1 million in some cases) have identified **thousands of SNPs** associated with educational attainment and cognitive performance.
- Each variant has a **tiny effect**; combined, they begin to explain:
  - Roughly **10–20%** of variance in educational attainment.
  - Lower but growing percentages for cognitive test performance (polygenic scores for cognitive ability often account for **~5–15%** currently


text:
Human intelligence is substantially heritable, but not fixed, and it arises from many genes interacting with environments over time. Here’s what the evidence shows, in structured form.

---

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

- **Heritability (h²)** in this context: the proportion of *differences between people* in a trait (e.g., IQ) that can be statistically attributed to genetic differences *in a given population at a given time*.
- It is **not**:
  - “How genetic intelligence is for an individual”
  - “How changeable it is” (high heritability ≠ unchangeable)
  - A fixed universal number; it depends on environment and population.

Example: If everyone has very similar environments (schooling, nutrition), then more of the remaining variation in IQ will appear “genetic,” so heritability estimates go up—even though environment still matters a lot in absolute terms.

---

## 2. Twin and family studies

These are the classic source of estimates.

### Key designs

- **MZ twins (identical)** share ~100% of segregating genes.
- **DZ twins (fraternal)** share on average ~50%.
- If MZ twins resemble each other more than DZ twins in IQ, that difference is attributed to genetic factors (under certain assumptions).

### Main findings

Across large meta-analyses of twin and family studies:

- **Childhood IQ**: heritability often in the range of **0.30–0.50**
- **Adolescent IQ**: typically **0.50–0.60**
- **Adult IQ**: often around **0.60–0.80**

A well‑cited review (e.g., Plomin & Deary, 2015 and later work) suggests:
- **Adult intelligence heritability ~ 0.50–0.80** in industrialized countries.

So in many contemporary, relatively advantaged societies, **roughly half or more of the observed variation in IQ is associated with genetic differences**.

### Shared vs nonshared environment

Twin models often separate environmental effects into:

- **Shared environment (C)**: factors that make siblings more similar (e.g., family SES, parenting style, neighborhood, schools—though “sharedness” is a statistical not literal definition).
- **Nonshared environment (E)**: factors that make siblings different (different teachers, peer groups, illnesses, random developmental events, measurement error).

Results:
- **Shared environment**: moderate for IQ in early childhood (maybe ~20–30% of variance), but **shrinks in adulthood**, often approaching near zero in classic twin models.
- **Nonshared environment + measurement error**: accounts for the remainder (often ~20–30% in adults).

Interpretation:
- As people age, they select and create environments partly based on their genetically influenced traits (gene–environment correlation), amplifying genetic effects and reducing the estimated role of shared family environment.

---

## 3. Adoption studies

Adoption studies are powerful because they separate genetic from rearing influences.

Findings:

- Young adopted children resemble their **adoptive** (rearing) parents in IQ more than their biological parents.
- By adolescence and adulthood, this flips: their IQ correlates more with their **biological parents’** IQ and much less with adoptive parents’.

Typical patterns:
- Correlation between IQ of adopted children and:
  - **Biological parents**: increases with age, often ~0.3–0.4 or higher in adolescence/adulthood.
  - **Adoptive parents**: declines toward zero in adulthood.

This suggests:
- Family environment (as provided by adoptive parents) has **real, noticeable effects early on**, but **genetic influence becomes more apparent as individuals age**.

---

## 4. Molecular genetics (GWAS, polygenic scores)

Twin/adoption studies are *indirect*. Molecular genetics looks directly at DNA.

### Genome-wide association studies (GWAS)

- Very large GWAS (hundreds of thousands to over a million participants) show:
  - Intelligence and educational attainment are **highly polygenic**: thousands of genetic variants across the genome each make tiny contributions.
  - No single “intelligence gene”; effects per variant are very small.

### SNP-based heritability

- Using measured common variants (SNPs), researchers estimate how much variance these collectively explain.
- SNP-based heritability for intelligence is often around **20–30%** of variance—lower than twin-based estimates because:
  - GWAS typically capture only common variants and not rare variants, structural variants, or all gene–gene interactions.
- This is consistent with the idea that:
  - **Total narrow-sense heritability is higher than what current SNP data capture**, but the molecular evidence aligns in direction and magnitude with twin studies.

### Polygenic scores


text:
Human intelligence is substantially heritable, but not fixed, and how heritable it appears depends strongly on age and environment.

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

- **Heritability (h²)** is the proportion of *differences between people* in a particular trait (in a particular population, at a particular time) that can be statistically attributed to genetic differences.
- It is **not**:
  - The percent of *your* intelligence that “comes from genes”
  - A measure of immutability
  - A universal constant applying to all groups and environments

If everyone had identical environments, measured heritability would rise; if environments differed wildly, heritability would fall—even with the same underlying biology.

---

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

Most of what we know comes from:

- **Twin studies**: comparing identical (MZ) twins who share ~100% of their genes with fraternal (DZ) twins who share on average ~50%.
- **Adoption studies**: comparing adopted children with their biological and adoptive parents/siblings.

Across large meta-analyses:

- **Childhood (~5–12 years)**
  - Heritability estimates of IQ: ~0.3–0.4
  - Shared family environment (home, parenting, etc.) accounts for a sizable portion of variance.

- **Adolescence (~13–18 years)**
  - Heritability: ~0.5–0.6
  - Shared environment effects begin to shrink.

- **Adulthood**
  - Heritability: commonly ~0.6–0.8 in high-income, relatively homogeneous societies.
  - Shared family environment: often close to 0 in many studies; differences in intelligence among adults are mostly explained by genes and nonshared environment (experiences not shared even by siblings).

Key findings:

- **MZ twins reared apart** are nearly as similar in IQ as MZ twins reared together, pointing to a strong genetic component.
- **Adopted children** resemble their biological parents more than their adoptive parents in adult IQ, even though they grow up in the adoptive family.

---

### 3. Molecular genetics: GWAS and polygenic scores

With genome-wide association studies (GWAS):

- Intelligence is **highly polygenic**: thousands of genetic variants each contribute tiny effects.
- Genome-wide studies have:
  - Identified many associated variants, each explaining <0.1% of variance.
  - Shown that **common SNPs collectively account for ~20–40% of IQ variance** (this is “SNP heritability” – a lower bound on total heritability).

**Polygenic scores (PGS)**:

- Combining many SNPs into a single genetic score predicts cognitive performance and educational attainment.
- In European-ancestry samples:
  - PGS for educational attainment can explain ~10–15% of variance in years of schooling.
  - PGS for cognitive performance explain a smaller but growing share of IQ variance (commonly ~5–10% in current large datasets).
- These are **predictive probabilities**, not destinies; people with low scores can and do perform very well, and vice versa.

---

### 4. Gene–environment interplay

Heritability is not “genes vs. environment”; they interact:

1. **Gene–environment correlation (rGE)**
   - People with certain genotypes are more likely to select, modify, or evoke particular environments.
   - Example: a child with a genetic propensity for higher verbal ability may seek more reading, teachers may respond differently, parents may offer more intellectual resources.

2. **Gene–environment interaction (G×E)**
   - The effect of genes can depend on environment, and vice versa.
   - Example: a genetic propensity for high IQ may only fully express in environments with adequate nutrition, schooling, and safety.

3. **Socioeconomic differences**
   - In some studies (especially in the US), heritability of IQ is **lower in low-SES families** and higher in high-SES families:
     - In harsh or deprived conditions, environmental constraints suppress genetic potential.
     - In supportive conditions, genetic differences have more room to manifest.
   - This pattern is not universal; some European studies find weaker or absent SES moderation, likely due to differences in inequality and social policy.

---

### 5. Stability and change across the lifespan

- **Rank-order stability** of IQ from childhood into adulthood is high (often correlations ~0.6–0.8 over long spans), meaning relative standing among peers tends to be fairly consistent.
- But **individual scores can change**:
  - Health events, trauma, education, training, and other experiences can shift measured ability.
  - Measured IQ is also sensitive to test conditions, motivation, and practice,


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

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

- **Heritability (h²)**: the proportion of *differences between people* in a trait that can be statistically attributed to genetic differences *within a given population and environment*.
- It **does not** mean:
  - “X% of *your* intelligence comes from genes.”
  - That the trait can’t change.
  - That environment doesn’t matter.

Heritability is about **variation among people**, not about how malleable the trait is in an individual.

---

### 2. Twin and family studies: broad-sense heritability

The classical evidence comes from comparing:

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

By comparing how similar MZ vs. DZ twins are in IQ, we can estimate genetic influence.

**Key findings:**

- In affluent, Western populations:
  - **Childhood (early)**: heritability of IQ often in the range **~0.3–0.4** (30–40%).
  - **Adolescence**: rises to about **0.5–0.6**.
  - **Adulthood**: typically **0.6–0.8** (60–80%).

Large meta-analyses (e.g., by Ian Deary, Robert Plomin, and others) support this pattern: **genetic influence on intelligence tends to increase with age**, while shared family environment effects often decrease.

Why it increases:
- As people age, they increasingly select, modify, and create environments consistent with their genetic predispositions (so-called **gene–environment correlation**).
- Early in life, family and early schooling constrain variation more strongly; later, people can “express” innate differences more fully.

---

### 3. Shared vs. non-shared environment

Twin models typically decompose variance into:

- **A**: Additive genetic influences.
- **C**: Shared environment (family, neighborhood, schooling features siblings share).
- **E**: Non-shared environment (idiosyncratic experiences, measurement error).

Approximate pattern across development:

- **Early childhood**:
  - A: ~30–40%
  - C: ~30–40%
  - E: ~20–30%
- **Adolescence**:
  - A: ~50–60%
  - C: ~10–20%
  - E: ~20–30%
- **Adulthood**:
  - A: ~60–80%
  - C: ~0–10% (often estimated near zero in many studies)
  - E: remainder

This doesn’t mean family background is unimportant; it is crucial for things like nutrition, health, education, safety, and thus the **range** within which genes can act. But by adulthood, most *within-population* IQ differences in affluent countries are statistically more associated with genetic and non-shared environmental factors than with shared family environment.

---

### 4. Socioeconomic status (SES) effects

Heritability estimates are **not uniform across SES**:

- In several U.S. samples, **heritability of IQ is higher in higher-SES families** and lower in low-SES families.
  - In very deprived conditions, environmental constraints can “mask” genetic potential, so genetic differences explain less variance.
- Some European studies show weaker SES moderation, possibly because basic needs and schooling are more equalized.

Implication: in better environments, **genetic differences account for more of the remaining variation**, because the worst environmental limitations are removed.

---

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

More recent work looks directly at DNA rather than just family resemblance.

- Large **genome-wide association studies (GWAS)** have identified thousands of genetic variants, each with tiny effects, associated with intelligence and educational attainment.
- **Polygenic scores (PGS)** that combine many variants:
  - Explain roughly **10–20%** of the variance in cognitive test scores in recent large European-ancestry samples (lower in more diverse or non-European samples due to current data limitations).
- These molecular estimates are **lower** than twin-study heritability (the “missing heritability” problem), partly because:
  - Many variants with ultra-small effects are still undetected.
  - Current methods capture mainly **common additive variants** and miss rare variants, structural variants, non-additive effects, and some gene–environment interplay.

Nonetheless, molecular genetics **converges** with twin/family evidence: intelligence is **polygenic** (influenced by many genes),


text:
Human intelligence is substantially heritable, but not fixed, and it is strongly shaped by environment. The details are subtle, and a lot of common interpretations are wrong.

Below is a summary of what the evidence shows, with the clearest points first.

---

## 1. What “heritability” actually means

- **Heritability (h²)** in this context is the proportion of *variation in a trait within a particular population* that can be statistically attributed to genetic differences.
- It does **not** mean “what percent of an individual’s intelligence comes from genes.”
- It also does not mean the trait is unchangeable: a trait can be highly heritable and still very responsive to environment (e.g., height).

So when we say “intelligence is X% heritable,” we mean:
> In this population, under these conditions, X% of *differences between people* in intelligence can be linked to genetic differences.

---

## 2. What is meant by “intelligence” here?

Most evidence is about **general cognitive ability (“g” or IQ-type measures)**:
- Performance on a range of cognitive tasks (reasoning, memory, vocabulary, spatial skills, processing speed) that tend to correlate.
- These measures are not perfect, but they are:
  - Highly reliable (repeatable over time),
  - Predictive of education, job performance, income, health, and some life outcomes.

So when we talk about heritability of “intelligence,” we’re usually talking about these general cognitive test scores.

---

## 3. Twin and family studies: the main findings

Across many large studies:

- **In childhood**: heritability of g is typically around **30–50%**.
- **In adolescence**: estimates are often around **50–60%**.
- **In adulthood**: estimates often rise to **60–80%** in Western, relatively affluent populations.

The increase with age is one of the most robust findings.

Why does heritability increase over the lifespan?

- As people grow up, they:
  - Have more freedom to select and shape their environments (friends, hobbies, education, jobs),
  - Often choose environments that “match” their genetically influenced preferences and abilities.
- This “gene–environment correlation” makes genetic differences show up more strongly as people age.

Important: this does not mean environment stops mattering in adulthood. It means *among adults in that population, under current conditions*, more of the *remaining variation* in intelligence is explainable by genetics.

---

## 4. Shared vs non-shared environment

Twin studies usually break variance into:
- **Genetic influences (A)**
- **Shared environment (C)**: family factors that make siblings more similar (e.g., family SES, parenting style, home culture).
- **Non-shared environment (E)**: experiences that differ between siblings (different peers, teachers, illnesses, random life events) + measurement error.

Patterns often found:

- **Shared environment**:
  - More important in **childhood**, sometimes explaining 20–30% (or more) of variance in IQ.
  - Often shrinks toward **near-zero in adulthood** in Western samples.
- **Non-shared environment**:
  - Consistently explains a substantial portion of variance (often 20–30%+), at all ages.

This means:
- Growing up in the same family is clearly important early on, but
- By adulthood, most environmental differences that remain are *individual-specific* rather than “family-level.”

---

## 5. Molecular genetics: what DNA data show

### SNP heritability (from genome-wide data)

Using measured DNA (SNPs), we can estimate how much variance is explained by common genetic variants.

- For IQ and educational attainment, SNP-based heritability estimates are typically around:
  - ~20–30% for g,
  - A bit higher for educational attainment.
- These are **lower than twin-based estimates** (60–80%) because:
  - They only capture common variants measured on genotyping arrays,
  - They miss rare variants and some complex genetic effects.

The fact that SNP-based estimates are substantial and consistent strongly supports *real* genetic influence, not just twin-study artifacts.

### GWAS (genome-wide association studies)

- Very large GWAS (hundreds of thousands to millions of people) have found **thousands** of SNPs associated with IQ/educational attainment.
- Each variant has a **tiny effect**; together, polygenic scores can:
  - Predict maybe **10–20%** of variance in educational attainment,
  - Less (often 5–10% in many cohorts) for IQ directly, though these numbers are slowly increasing as samples grow.
- **Polygenic scores**:
  - Are somewhat predictive on average,
  - Have much lower accuracy across different ancestral groups (due to linkage disequilibrium patterns, allele frequency


text:
Human intelligence is substantially heritable, but not “fixed,” and it depends heavily on environment. The best summary from modern behavioral genetics is:

- Heritability of IQ in adults: ~50–80%
- Heritability of IQ in children: ~20–40%
- Environment matters strongly, especially early in life and in disadvantaged contexts.
- “Heritable” does **not** mean “unchangeable,” and it does **not** apply to differences between groups the way it does within a group.

Below is what the evidence actually shows and how to interpret it.

---

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

**Heritability (h²)** in this context is a population statistic:
> The proportion of variation in IQ **within a given population at a given time** that is attributable to genetic differences.

Key points:

- It’s a **percentage of variance**, not “how genetic IQ is for a person.”
- It says nothing directly about **what’s possible** for an individual with better or worse environments.
- It depends on **environmental variation**. In a very equal environment, heritability rises because fewer differences are due to environment.

So saying “IQ heritability is 0.7 in adults” means: in that sample, about 70% of the **differences among people** in IQ scores can be statistically linked to genetic differences, not that a person’s IQ is 70% determined by genes.

---

## 2. Evidence from twin and adoption studies

### Twin studies

Comparisons:

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

Findings from large meta-analyses:

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

From this, standard models estimate:

- **Childhood IQ heritability:** ~0.2–0.4
- **Adult IQ heritability:** increasing to ~0.5–0.8

This age trend—known as the **“Wilson effect”**—is one of the most robust findings: genes appear to matter **more** as people age, partly because:

- People increasingly choose environments that match their traits (gene–environment correlation).
- Shared family influences (home, parenting) fade as independent life starts.

### Adoption studies

Typical findings:

- Adopted children’s IQs are **closer to their biological parents** than to their adoptive parents in adulthood, even though they grow up with the latter.
- In childhood, adoptive family environment has a noticeable effect; by adulthood, the correlation between adoptive parent and adopted child IQ often approaches zero.

This supports:

- **Genetic influence** on long-term individual differences in intelligence.
- **Substantial early environmental effects**, especially in disadvantaged vs enriched homes.

---

## 3. Evidence from molecular genetics (DNA-based studies)

### GWAS (Genome-Wide Association Studies)

Large GWAS of educational attainment and cognitive performance (N > 1 million in some cases) show:

- Intelligence is **polygenic**: thousands of genetic variants each have tiny effects.
- “Polygenic scores” built from these variants can predict:
  - A modest portion of IQ differences (currently ~10–20% of the variance in some European-ancestry samples).
  - Some aspects of educational outcomes.

DNA-based heritability (from SNPs alone) typically explains:

- ~20–30% of the variance in IQ, which is **lower than twin-based estimates**. This gap likely reflects:
  - Rare variants not well captured in GWAS.
  - Gene–gene and gene–environment interactions.
  - Measurement and modeling limitations.

Nevertheless, these results confirm that **many specific genetic variants are associated with cognitive ability**, consistent with twin/adoption findings.

---

## 4. Role of environment

Even with high heritability, environment is powerful.

### Shared vs nonshared environment

Behavioral genetics decomposes variance into:

- **A (additive genes)**
- **C (shared/common environment)** – family, home, socioeconomic status aspects that siblings share.
- **E (nonshared environment)** – experiences unique to each individual, plus measurement error.

Rough pattern for IQ:

- In childhood: A ~0.2–0.4, C ~0.3–0.4, E ~0.3–0.4
- In adulthood: A ~0.5–0.8, C ~0.0–0.1, E ~0.2–0.5

So:

- **Shared


text:
Human intelligence is substantially heritable, but that does *not* mean it’s fixed, purely genetic, or unaffected by environment. The evidence is rich and nuanced.

I’ll organize this into:

1. What “heritability” actually means
2. Core findings from twin, adoption, and family studies
3. What modern DNA studies show
4. How heritability changes across the lifespan
5. Gene–environment interplay (why “genes vs. environment” is misleading)
6. Limits and caveats (including group differences)

---

## 1. What heritability actually means

- **Heritability (h²)** is the proportion of *variation within a population* that can be statistically attributed to genetic differences, under current environmental conditions.
- It does **not** mean:
  - “X% of your intelligence is genetic” (that’s a misunderstanding)
  - That the trait is unchangeable
  - That it’s the same in all populations or contexts

Heritability is **about differences among people**, not about the absolute level of the trait, and it depends on the environment.

---

## 2. Classic behavioral genetics: twin, family, and adoption studies

Most of the foundational data come from comparing:

- **Monozygotic (MZ) twins**: share ~100% of segregating genes
- **Dizygotic (DZ) twins**: share ~50% on average
- **Adopted children**: genetically unrelated to their adoptive parents

### Key findings:

1. **MZ twins are more similar in IQ than DZ twins.**
   - In large meta-analyses, correlations for IQ:
     - MZ twins raised together: ~0.80–0.85
     - MZ twins raised apart: ~0.70–0.80
     - DZ twins raised together: ~0.50–0.60
   - The high similarity of MZ twins reared apart is a strong signal of genetic influence.

2. **Adoption studies**:
   - Young adopted children resemble their *adoptive* parents in IQ somewhat, but this similarity **declines with age**.
   - By adolescence and adulthood, their IQ correlates much more with their *biological* parents and siblings than with adoptive family, even though they never lived with their biological relatives.

3. **Heritability estimates** from these methods:
   - In reasonably well-off modern societies:
     - Childhood IQ heritability: often ~0.30–0.50
     - Adulthood IQ heritability: often ~0.60–0.80

So in adult twins in such environments, around 60–80% of the *differences* in intelligence test scores can be attributed to genetic variation.

---

## 3. Molecular genetics: what DNA-level data show

Behavioral studies show *that* genes matter; molecular studies probe *which* and *how much*.

### Genome-wide association studies (GWAS)

- Hundreds of thousands to millions of people are genotyped.
- Each variant’s small effect on cognitive test performance or educational attainment is estimated.
- **Findings**:
  - Intelligence is **highly polygenic**: thousands of variants each have tiny effects.
  - The sum of these effects (polygenic scores) can statistically explain a modest share of IQ variance, but this share is growing as studies get larger.

### SNP-based heritability

Instead of comparing relatives, SNP-based methods use measured common genetic variants to estimate heritability.

- For IQ and closely related traits:
  - SNP-based heritability is typically ~0.20–0.30 (or more with larger samples).
  - This is a **lower bound** on total heritability, because:
    - It only captures common variants that are measured.
    - It doesn’t fully capture rare variants, structural variants, or gene–gene interactions.

The convergence between **twin-based** (0.6–0.8 in adults) and **SNP-based** (~0.2–0.3 or higher with better data) methods strongly supports substantial genetic influence.

---

## 4. Heritability changes with age

One of the most robust and striking findings:

- **Heritability of IQ increases from childhood to adulthood.**
  - Early childhood: ~0.20–0.40
  - Adolescence: ~0.40–0.60
  - Adulthood: ~0.60–0.80 in many studies

Possible reasons:

- **Active gene–environment correlation**: as people get older, they choose, modify, and seek out environments that fit their genetically influenced traits (e.g., reading more if they find it easy and rewarding).
- **Shared family environment matters more early** (e.g., nutrition, early stimulation), then rapidly declines in importance after adolescence


text:
Human intelligence is substantially heritable, but not fixed, and environment still matters a lot. The clearest picture comes from combining twin, adoption, and DNA-based studies.

### 1. What “heritability” actually means

- **Heritability (h²)** = the proportion of **differences between people in a given population** that can be statistically attributed to genetic differences.
- It:
  - Does **not** mean “X% of your IQ is due to genes.”
  - Depends on context (age, era, socioeconomic conditions, etc.).
  - Says nothing about what could be changed by interventions (education, health, etc.).

We’re talking about variation *between people*, not destiny *within a person*.

---

### 2. Twin and family studies

These are the foundation.

- **Identical twins (MZ)** share ≈100% of segregating genes.
- **Fraternal twins (DZ)** share ≈50% on average.
- If MZ twins resemble each other more than DZ twins, that points to genetic influence.

**Key findings:**

- In **childhood**, heritability of IQ is often around **0.2–0.4** (20–40%).
- In **adolescence**, it rises to around **0.4–0.6** (40–60%).
- In **adulthood**, it often reaches **0.6–0.8** (60–80%).

This “rising heritability with age” is one of the most robust findings (shown in large meta-analyses such as Plomin & Deary, 2015).

Why it rises:
- As people age, they increasingly choose and shape environments that match their genetic predispositions (“gene–environment correlation”).
- Shared-family influences (home, parenting) fade; nonshared experiences and genetics dominate.

---

### 3. Adoption studies

Adoption designs separate genes from home environment:

- Adopted children share genes with **biological** parents but environment with **adoptive** parents.
- Correlations of IQ:
  - Child–biological parent: moderate and growing with age → genetic effect.
  - Child–adoptive parent: small and often declining by adulthood → limited long-term effect of shared family environment on adult IQ.

However:
- Adoption usually moves children from lower to higher-quality environments.
- That often **raises average IQ and educational outcomes**, even if it doesn't create a large long-term *correlation* with the adoptive parents’ IQ.

So: adoption studies show both **genetic influence on individual differences** and **environmental influence on average levels**.

---

### 4. DNA-based (molecular) evidence

Genome-wide association studies (GWAS) look at millions of SNPs in hundreds of thousands to millions of people.

Main points:

- Many thousands of variants each have **tiny** effects on intelligence and educational attainment.
- Polygenic scores built from these variants:
  - Currently explain around **10–20%** of variation in standardized IQ test scores in European-ancestry samples (for educational attainment, up to ~15–20%, sometimes more with very large samples).
  - They’re *lower* than twin-based estimates because:
    - Current methods only capture common variants.
    - Effect sizes are tiny and incompletely mapped.
    - There’s measurement error and population limits.

- SNP-based “chip heritability” (using methods like GREML or LD-score regression) generally finds that **common SNPs** explain **20–40%** of IQ variance—again consistent with a substantial genetic component.

This molecular work **confirms** that:
- Intelligence is influenced by many genes (highly polygenic).
- No single “intelligence gene” exists.
- The effect of any single variant is minuscule.

---

### 5. Environment: big impacts despite high heritability

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

Key environmental factors:

- **Early nutrition and health:** Severe malnutrition, iodine deficiency, and exposure to lead can markedly reduce cognitive ability.
- **Education quality and duration:**
  - Additional years of schooling raise IQ test scores (often by several points per year).
  - Schooling also boosts specific abilities (literacy, numeracy) and performance on “fluid” tests.
- **SES (socioeconomic status):**
  - In high-poverty, high-inequality contexts, environmental constraints can strongly limit expression of genetic differences.
  - Some US studies find **lower heritability** of IQ in low-SES children and higher in high-SES children (though this pattern is weaker or absent in some European countries).

Population-level changes:

- **The Flynn effect:** Across much of the 20th century, average IQ scores rose dramatically (e.g., 2–3 points per decade in many countries). Genes did not change that fast; environment did


text:
Human intelligence is substantially heritable, but not fixed, and influenced by both genes and environment. The best evidence comes from large twin, adoption, and genomic studies. Here’s what those show, in compact form.

---

## 1. What “heritability” means

- Heritability (h²) = proportion of **differences between people** in a given population that can be statistically attributed to genetic differences.
- It does **not** mean “what percent of *your* intelligence comes from genes.”
- Heritability is **population-specific** and **context-dependent**: it can change with age, social conditions, and how environments vary.

---

## 2. How heritable is intelligence?

Most researchers look at “general intelligence” (g), the common factor underlying performance across diverse cognitive tasks.

### Twin and family studies (classical behavioral genetics)

Across many Western samples:

- **Childhood (≈4–12 years)**:
  - Heritability of g: ~0.30–0.45
  - Shared environment (things that make siblings similar: home, parenting, SES, etc.): ~0.20–0.30
  - Non-shared environment (experiences unique to each individual, plus measurement error): remainder (~0.30–0.50)

- **Adolescence–early adulthood**:
  - Heritability: ~0.50–0.65
  - Shared environment: falls toward 0.10 or less
  - Non-shared environment: ~0.30–0.40

- **Middle and later adulthood**:
  - Heritability: often ~0.70–0.80 in well-studied cohorts
  - Shared environment: near zero
  - Non-shared environment: remainder

A widely cited summary: **heritability of adult g in modern, relatively affluent populations is roughly 50–80%.**

---

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

Several converging explanations:

1. **Active gene–environment correlation:**
   As people age, they increasingly choose, shape, and seek environments that fit their genetic predispositions (e.g., choosing more cognitively demanding hobbies, courses, jobs).

2. **Diminishing impact of shared childhood environment:**
   Effects of being raised in the same household weaken as individuals diverge in life trajectories.

3. **Cumulative effects:**
   Small genetic advantages in attention, processing speed, or memory can compound over many years of learning.

Meta-analyses of longitudinal twin samples (e.g., the Colorado Adoption Project, Swedish Twin Registry) consistently show rising heritability from childhood into adulthood.

---

## 4. Adoption studies

Adoption studies help separate genes and shared family environment:

- **Biological vs. adoptive relatives:**
  - IQ correlates **more strongly** between biological relatives (who share genes, but not necessarily rearing) than between adoptive relatives (who share household, but not genes).
- **Findings:**
  - In childhood, adoptive siblings and adoptive parents–children show **modest IQ similarity**, indicating real shared-environment effects.
  - By late adolescence/early adulthood, that similarity largely **disappears**, while resemblance to biological parents increases, even when reared apart.

Result: adoption studies align with twin studies: environmental effects are clearly present, especially early, but **genetic resemblance predicts adult cognitive similarity more strongly**.

---

## 5. Molecular genetic evidence (GWAS, SNP heritability)

Past 15 years: genome-wide association studies (GWAS) have examined hundreds of thousands to millions of people.

Key points:

- **SNP heritability** for intelligence (using common genetic variants captured on genotyping arrays) is typically **~20–35%**, lower than twin-based estimates because:
  - It only measures common variants, not all genetic variation.
  - It assumes simple additive effects.
- **Polygenic scores (PGS)**:
  - Aggregate the effects of many associated SNPs into a single score.
  - In large European ancestry samples, PGS for educational attainment or cognitive performance can explain **~10–20%** of variance in test scores or years of schooling.
  - Predictive power drops in non-European ancestries due to differences in linkage disequilibrium, allele frequencies, sample composition, and bias in training data.

These molecular findings strongly support that **intelligence differences are partly caused by many genetic variants of very small effect**, consistent with twin/adoption evidence, but they also show we currently capture only part of the genetic signal.

---

## 6. Gene–environment interaction and context

Heritability is **not a ceiling on environmental influence**.

Examples and patterns:

- **Socioeconomic status (SES) moderation (G×E):**
  - In some U.S. samples, heritability of IQ is **lower in


text:
Human intelligence is substantially heritable, but not fixed, and environment still matters a lot. Here’s what the evidence shows, in layers.

---

## 1. What “heritable” means

- **Heritability (h²)** = the proportion of *variation between people* in a trait, in a given population and environment, that can be attributed to genetic differences.
- It does **not** mean “X% of your intelligence comes from genes and Y% from environment.”
- It is **population- and context-specific**: if environments become more equal, heritability can rise, even though genes themselves haven’t changed.

---

## 2. Classic twin and family studies

Most of what we know comes from:

- **Monozygotic (MZ) twins**: share ~100% of segregating genes
- **Dizygotic (DZ) twins**: share ~50% on average
- **Adoption studies**: compare adopted children to biological vs adoptive parents

Typical findings for general cognitive ability (often “g”):

- In **childhood**: heritability estimates ~0.30–0.45
- In **adolescence**: ~0.50–0.60
- In **adulthood**: ~0.60–0.80
  - A widely cited meta-analysis (Plomin & colleagues) finds heritability of adult intelligence around **0.70** in Western samples.

This means that, in those populations, about 70% of *differences between individuals* in IQ are statistically associated with genetic differences, with the remainder due to environment and measurement error.

Adoption and twin results:

- MZ twins reared apart are **almost as similar** in IQ as MZ twins reared together.
- DZ twins and ordinary siblings are less similar than MZ twins but more similar than unrelated people.
- Adoptive siblings raised in the same family have IQ correlations that **start positive in childhood but often approach zero in adulthood**, indicating that shared family environment effects fade with age.

---

## 3. Shared vs non-shared environment

Behavioral genetic models often break variance into:

- **Genetic (A)** – additive genetic effects
- **Shared environment (C)** – factors that make siblings more similar (same home, schooling quality, etc.)
- **Non-shared environment (E)** – factors that make siblings different (different peers, teachers, illnesses, random events) + measurement error

For intelligence:

- **Childhood**: A moderate A, substantial C, and E
- **Adulthood**: A large A, C close to zero, E moderate

Interpretation:

- Early on, **family environment and schooling** have noticeable effects on IQ differences.
- As people grow up, they select, shape, and move into environments that correlate with their genetic predispositions (gene–environment correlation), so **genetic differences explain more of the remaining variation**.
- The non-shared environment (idiosyncratic life experiences, random developmental noise) remains important at all ages.

---

## 4. Molecular genetics: SNP-based heritability and GWAS

Modern studies use measured DNA:

- **SNP-based heritability (h²_SNP)**: how much variance can be explained by all common SNPs measured on genotyping arrays (a lower bound on total genetic influence).
  - For intelligence and educational attainment, typical h²_SNP estimates: **~0.20–0.30**.
- **GWAS (Genome-Wide Association Studies)** with hundreds of thousands to millions of participants:
  - Identify thousands of variants, each with **tiny effects**.
  - Combined into **polygenic scores**, which can explain:
    - ~10–20% of variance for educational attainment
    - typically lower but growing percentages for IQ or related cognitive measures (often ~5–15% in recent large studies, depending on phenotype and sample).

These molecular estimates are lower than twin-based ones because:

- GWAS mainly captures **common variants with additive effects**.
- Rare variants, structural variants, and non-additive effects are only partially captured.
- Measurement and modeling limitations remain.

Still, the molecular data strongly **confirm a real genetic influence** and broadly align with twin-study magnitudes, once limitations are considered.

---

## 5. Developmental pattern: heritability increases with age

A robust finding:

- **Heritability of intelligence increases from childhood to adulthood.**
- Environment matters a lot early, but genetic effects become more expressed over time.

Likely mechanisms:

- **Gene–environment correlation**: People with certain genetic tendencies:
  - seek out more cognitively demanding environments,
  - get tracked into different educational paths,
  - choose different hobbies and careers.
- **Gene expression changes** and brain maturation across development.

So the older the sample, the more of the remaining variance tends to map onto genetic differences


text:
Heritability of intelligence is substantial but not total, and it depends strongly on age, environment, and how you measure it. Most modern estimates put the heritability of general cognitive ability (“g”) in adults somewhere around 50–80%. That means genetic differences explain a bit more than half of the variation in intelligence between people in a given population, under current conditions.

Below is a structured summary of the main evidence and nuances.

---

## 1. What “heritability” actually means

- Heritability (h²) = the proportion of variation in a trait in a specific population, at a specific time, that is attributable to genetic differences between individuals.
- It does **not** mean:
  - “X% of your intelligence is genetic and the rest is environment.”
  - “The trait can’t be changed.”
- It is **population-specific** and **context-dependent**:
  - In more equal environments (e.g., universal schooling), relative genetic influence looks larger.
  - In highly unequal environments, environmental effects loom larger and heritability can be lower.

---

## 2. Twin and adoption studies

These are the classic methods.

### Twin studies

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

Key findings from large meta-analyses and reviews (e.g., Bouchard, Plomin, Polderman et al. 2015):

- In **childhood**, heritability of IQ: ~30–40%.
- In **adolescence**, ~50–60%.
- In **adulthood**, commonly ~60–80%.
- The shared family environment (things that make siblings similar: home, school, parenting style) tends to:
  - Be important in childhood (often ~20–30% of variance).
  - Decline to a smaller role in adulthood, sometimes approaching zero in twin models.
- Non-shared environment (experiences that differ between siblings, plus measurement error) accounts for the remaining variance.

This pattern—heritability increasing with age—is robust and somewhat counterintuitive: as people grow up and gain autonomy, they select and shape environments that align with their genetic predispositions (gene–environment correlation), amplifying genetic effects.

### Adoption studies

- Compare:
  - Adopted children vs their biological vs adoptive parents/siblings.
- Findings:
  - In **childhood**, adopted children resemble both their adoptive and biological families to some degree.
  - By **late adolescence/adulthood**, their IQ correlates much more with their biological parents and biological siblings than with their adoptive family.
  - Correlations in adulthood with biological relatives can be around 0.3–0.5; with adoptive family, often near zero for IQ.

These results support a strong role for genes and a diminishing shared-environment effect on stable adult intelligence.

---

## 3. Molecular genetics and GWAS

Historically, twin/adoption studies couldn’t say *which* genes were involved. Modern genome-wide association studies (GWAS) look at hundreds of thousands or millions of genetic variants across many people.

Key points:

- Large GWAS (n > 1 million in some meta-analyses) have identified thousands of variants associated with intelligence, educational attainment, or related outcomes.
- Each individual variant has a **tiny** effect; intelligence is highly polygenic.
- Polygenic scores (summaries of many variants) can:
  - Explain ~10–20% of variance in educational attainment;
  - ~5–15% of variance in cognitive test performance, depending on sample, ancestry, and measure.
- These molecular estimates are **lower than twin-study heritability** because:
  - GWAS capture only common SNPs (not rare variants, structural variants, etc.).
  - There’s incomplete tagging of causal variants.
  - Polygenic scores are imperfect predictors even of the SNP-based heritability.

Using methods like GREML (GCTA), which estimate how much variance is explained by all measured SNPs collectively (even without “discovering” each SNP), SNP-based heritability for intelligence in European-ancestry samples is often ~20–40%. That’s a **lower bound** on total genetic influence, consistent with the higher estimates from twin studies.

---

## 4. Environment still matters a lot

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

Examples:

- **Secular gains (Flynn effect)**:
  - Average IQ scores rose ~2–3 points per decade for much of the 20th century in many countries—too fast to be genetic.
  - Likely contributors: better nutrition, education, health care, smaller families, more cognitively


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

- In modern, reasonably well‑nourished populations, **general cognitive ability (g)** shows **heritability of roughly 50–80% in adulthood**.
- In childhood it’s lower—often **20–40%**—and **heritability tends to increase with age**.
- High heritability does **not** mean “genetically determined,” nor that environments don’t matter. It means that, given the environments in the studied population, **most *differences* in IQ between individuals** are statistically associated with genetic differences.

Below is what the evidence shows and how to interpret it.

---

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

- **Heritability (h²)** = proportion of **variation in a trait within a specific population, at a specific time, in a specific range of environments** that can be statistically attributed to genetic differences.
- If intelligence has **h² = 0.6**, that means: in that population, about 60% of the *differences* in cognitive test scores between people are related to genetic differences; 40% to environmental and random factors.
- It **does not** mean:
  - 60% of *your* intelligence is “from genes” and 40% from environment.
  - The trait cannot change with environment.
  - Heritability is universal across all groups and conditions.

Change the environment enough (e.g., widespread malnutrition, toxin exposure, massive improvements in schooling) and heritability estimates can shift.

---

## 2. Main lines of evidence

### A. Twin studies

Twin studies compare:

- **Monozygotic (MZ, “identical”) twins**: share ~100% of segregating genes.
- **Dizygotic (DZ, “fraternal”) twins**: share on average ~50% of segregating genes.

If MZ twins are consistently more similar in intelligence than DZ twins, genetics is implicated.

Key findings from large meta‑analyses (e.g., T. J. Bouchard, N. J. Mackintosh, and others):

- **Childhood IQ**:
  - Heritability often estimated around **0.2–0.4**.
- **Adolescence / early adulthood**:
  - Heritability around **0.5–0.6**.
- **Middle adulthood**:
  - Heritability often rises to **0.7–0.8**.

Important nuance:
- The **shared environment** (family-wide factors like SES, parenting style, neighborhood) has a **noticeable effect in childhood**.
- By adulthood, shared environment estimates often **shrink toward zero** in twin models; differences are then more attributable to **genes + non‑shared environment** (idiosyncratic experiences, peer groups, accidents, illnesses, etc.).

### B. Adoption studies

Adoption studies compare:

- Correlation of IQ between biological relatives raised apart.
- Correlation of IQ between adoptive parents/siblings and the adopted child.

Findings:

- **Adopted children’s IQs** tend to correlate more strongly with their **biological parents** than with adoptive parents by adolescence/adulthood.
- In early childhood, **adoptive family environment has a positive impact**:
  - Children adopted into higher-SES, more cognitively stimulating homes usually show **higher average IQ** than if they remained in deprived settings.
  - But as they age, their IQ scores become more similar to biological relatives and less to adoptive relatives—consistent with substantial genetic influence plus early environmental gains.

### C. Twins reared apart

Studies such as the Minnesota Study of Twins Reared Apart:

- MZ twins raised in different households still show **high correlations in IQ** (often ~0.7 or more).
- They typically resemble each other in many cognitive, personality, and attitudinal traits despite separate upbringings.
- This is strong evidence that a large share of variation in intelligence is related to genetic differences.

---

## 3. Molecular genetics: GWAS and polygenic scores

Older studies infer genes from family resemblance; newer work looks at DNA directly.

### Genome-wide association studies (GWAS)

- Large studies test millions of SNPs (common genetic variants) across tens/hundreds of thousands of people.
- For intelligence and related traits (e.g., educational attainment), GWAS have:
  - Identified **thousands of SNPs** individually having **tiny effects**.
  - Shown that intelligence is **highly polygenic**: influenced by **many genes of very small effect**, not by a few “intelligence genes.”

### Polygenic scores (PGS)

- A PGS sums up the effects of many SNPs, weighted by their GWAS-est


text:
Human intelligence is substantially heritable, but not fixed, and it is strongly shaped by both genes and environment. The evidence is consistent and quite nuanced.

Below is a structured summary of what we know.

---

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

- **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 is due to genes.”
  - “Intelligence is predetermined and unchangeable.”
- Heritability is **population-specific and context-dependent**. If environments become more equal (e.g., universal schooling), heritability often rises because relative differences are more due to genes.

The trait usually studied is **general intelligence (g)** — the common factor underlying performance across many cognitive tasks.

---

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

These are the classic tools for estimating heritability.

### Twin studies

- **Identical (MZ) twins** share ~100% of their segregating genes.
- **Fraternal (DZ) twins** share ~50% on average.
- If MZ twins are more similar in IQ than DZ twins, that difference is attributable primarily to genetic influence.

Findings (meta-analytic and large-study ranges):

- In adulthood, IQ correlations:
  - 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 these:

- **Adult heritability estimates**: typically **50–80%**.
- **Shared environmental influence** (factors making siblings similar, such as family SES, parenting style, etc.): often **close to zero or small in adulthood**, though higher in childhood.
- **Nonshared environment** (experiences unique to an individual, plus measurement error): accounts for much of the remaining variance.

### Adoption studies

They compare:

- Adopted children with their **biological** parents/siblings (genetic similarity, different rearing).
- Adopted children with their **adoptive** parents/siblings (environmental similarity, little/no genetic resemblance).

Findings:

- In childhood, adoptees’ IQs correlate modestly with both:
  - Biological relatives (genetic influence)
  - Adoptive relatives (shared environmental influence)
- By adulthood, **correlations with biological relatives remain**, but **correlations with adoptive parents typically shrink toward zero**.

Conclusion: genetic factors have a strong, persistent influence; family-wide environmental influences are important earlier in life but tend to fade in relative importance by late adolescence/adulthood.

---

## 3. Heritability changes with age

One of the most robust findings:

- **Childhood**: heritability of IQ often around **20–40%**.
- **Adolescence**: rises to around **40–60%**.
- **Adulthood**: often **60–80%**.

At the same time:

- **Shared environment** (family-wide factors) is stronger early (sometimes 20–30% of variance in childhood).
- It declines and is often negligible by middle adulthood in many Western samples.

Why does heritability rise?

- **Gene–environment correlation**: as people grow older, they increasingly:
  - Select, modify, and create environments that fit their genetic predispositions (e.g., seeking cognitively demanding jobs, educational paths).
  - This makes genetic potentials more fully expressed over time.
- **Autonomy and differentiation**: with age, siblings’ experiences diverge, magnifying nonshared environmental and genetic effects.

---

## 4. Molecular genetics: what specific genes show

Twin/adoption studies show *that* genes matter; molecular genetics asks *which* genes and *how*.

### Early candidate-gene era

- Many single-gene “findings” failed to replicate.
- No single gene with large effects on normal-range IQ in the general population has been robustly confirmed.

### Genome-wide association studies (GWAS)

- Very large GWAS (hundreds of thousands to >1 million individuals) find **thousands of variants**, each with tiny effects.
- Polygenic scores (PGS) aggregate these variants:
  - Current PGS can explain around **10–20%** of variation in cognitive performance and education in large European-ancestry samples.
  - This is **less than twin-based heritability** (“missing heritability”) but steadily improving as sample sizes grow and methods refine.
- GWAS also show:
  - Strong **genetic correlations** between IQ, educational attainment, and some brain traits.
  - Shared genetic influences with some mental health outcomes (e.g., negative