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

Here’s the core logic, step by step:

1. **Variation exists**
   Individuals in a population differ in many traits (size, color, behavior, physiology, etc.).
   - Some variation is genetic (due to different alleles, mutations, recombination).
   - Some is environmental (diet, injuries, etc.).
   Only the **genetic** part can be passed on.

2. **More offspring are produced than can survive**
   Organisms tend to produce more young than the environment can support (limited food, space, mates, etc.).
   - This creates **competition** (direct or indirect) for resources and reproductive opportunities.

3. **Differential survival and reproduction (“fitness”)**
   Because of variation, some individuals are better suited to current conditions than others.
   - Those with traits that help them survive and reproduce in that environment tend, on average, to leave more offspring.
   - “Fitness” in evolution means **reproductive success**, not strength or health in a general sense.

4. **Inheritance of traits**
   Offspring resemble their parents because they inherit their genes.
   - If a trait that increases fitness is heritable, it will be more common in the next generation, because more of the offspring carry the genes for it.

5. **Change in the population over generations**
   Over many generations, the frequencies of genes (and thus traits) in the population shift.
   - Traits that increase fitness become more common.
   - Traits that decrease fitness become rarer or disappear.
   This change in gene frequencies over time **is evolution**.

---

### A simple example: antibiotic resistance in bacteria

- In a bacterial population, random mutations create some cells that are slightly more resistant to an antibiotic.
- When the antibiotic is applied, most bacteria die, but resistant ones survive better.
- The survivors reproduce, passing on resistance genes.
- After many generations, the population is dominated by resistant bacteria.

The key points:
- Variation arises mostly by **mutation and recombination**, not because organisms “need” a trait.
- Natural selection is **non-random** in its outcome: traits that help survival/reproduction tend to spread.
- Evolution has **no foresight or goal**; it just reflects which variants worked better in past environments.

Over long timescales, this process can:
- Adapt populations to their environments,
- Split lineages into new species (speciation),
- Produce complex structures (eyes, wings) through many small, cumulative changes.


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.

Here’s the core logic, step by step:

1. **Variation exists**
   Individuals in a population differ in many traits (size, color, behavior, physiology, etc.).
   - Some variation is genetic (due to different alleles, mutations, recombination).

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

3. **Differential survival and reproduction (selection)**
   Some heritable traits make individuals better at surviving or reproducing in a given environment.
   - Those individuals are more likely to survive and have offspring.
   - This is “natural selection”: the environment “filters” which traits are more successful.

4. **Inheritance of traits**
   Offspring tend to resemble their parents because they inherit their genes.
   - Traits that helped parents survive and reproduce are passed on more often.

5. **Change in the population over generations (evolution)**
   Because individuals with certain traits leave more descendants, the genetic makeup of the population shifts.
   - Beneficial traits become more common.
   - Harmful traits become rarer (or disappear).
   - Over long timescales, this can produce new adaptations, new body plans, and eventually new species.

---

### Simple example

Imagine a population of beetles:

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

Nothing “decides” this; it’s just the statistical outcome of who survives and reproduces more, given the environment.

---

### Key points and clarifications

- **Natural selection acts on individuals; evolution is seen in populations.**
- **Selection is not random**, but the mutations that create new variation are largely random.
- **No foresight or goal:** evolution doesn’t “try” to make organisms better; it just favors traits that work *now* in the current environment.
- **Different types of selection:**
  - Directional (shifts traits in one direction, e.g., larger body size)
  - Stabilizing (favors the average, reduces extremes)
  - Disruptive (favors extremes, can split populations)

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).

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 heritable differences.

Here’s the core logic, step by step:

1. **Variation exists**
   Individuals in a population differ in many traits (size, color, behavior, physiology, etc.).
   - Some variation is genetic (due to different alleles, mutations, recombination).
   - Some is environmental (diet, injuries, etc.).
   Only the **genetic** part can be passed on.

2. **More offspring are produced than can survive**
   Resources (food, space, mates) are limited.
   - Not all individuals survive to reproduce.
   - This creates a “struggle for existence” (not necessarily violent, but competitive).

3. **Differential survival and reproduction (fitness)**
   Because of their traits, some individuals are **better at surviving and reproducing** in a given environment.
   - “Fitness” in evolution means **reproductive success**, not strength or health alone.
   - Even small advantages (e.g., slightly better camouflage, slightly more efficient metabolism) can matter over many generations.

4. **Heritability of advantageous traits**
   If the traits that help individuals survive and reproduce are **heritable**, their offspring are more likely to have those traits too.
   - Over time, alleles (gene variants) that increase fitness become more common in the population.
   - Alleles that reduce fitness become rarer.

5. **Population changes over generations (evolution)**
   As advantageous alleles spread and disadvantageous ones decline, the **genetic makeup of the population shifts**.
   - This is evolution: a change in allele frequencies in a population over generations.
   - Given enough time, these changes can accumulate into large differences, leading to new species.

---

### Simple example: dark vs. light moths

- Start with a population of moths: some light, some dark. Color is heritable.
- On light-colored trees, birds see dark moths more easily, so dark moths are eaten more often.
- Light moths survive and reproduce more, so the “light” alleles increase in frequency.
- The population becomes mostly light-colored: it has evolved by natural selection.
- If pollution darkens the trees, the advantage flips: now dark moths are better camouflaged, and the population may evolve toward darker coloration.

---

### Key points and clarifications

- **Natural selection acts on individuals; evolution is seen in populations.**
- **Selection is not goal-directed.** It doesn’t “try” to make organisms better; it just filters what works *here and now* in a given environment.
- **Environments change.** A trait that’s beneficial in one context can be neutral or harmful in another.
- **Natural selection is one mechanism of evolution**, alongside others like genetic drift, gene flow, and mutation. But natural selection is the one that systematically favors traits that improve reproductive success.

If you’d like, I can go into specific types of selection (stabilizing, directional, disruptive) or connect this to genetics in more detail.


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

Here’s the core logic, step by step:

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

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

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

4. **Traits are inherited**
   Offspring tend to resemble their parents.
   - If a trait that helps survival/reproduction is heritable, individuals with that trait will, on average, leave more descendants.

5. **Change in trait frequencies over generations**
   Over many generations, traits that increase reproductive success become more common; traits that reduce it become rarer.
   - This change in the genetic makeup of the population over time is **evolution**.

6. **Adaptation**
   Natural selection can produce **adaptations**: features that are well-suited to a particular function in a particular environment.
   - Example: The streamlined bodies of dolphins and fish are adaptations for efficient swimming.

7. **Speciation (formation of new species)**
   If populations of the same species become isolated (geographically or ecologically) and experience different selection pressures, their traits and genes can diverge.
   - Over long timescales, they may become so different that they can no longer interbreed: they are now separate species.

Key points to keep in mind:

- Natural selection acts on **individuals** (who lives and reproduces), but evolution is observed in **populations** (changes in gene frequencies).
- Natural selection is not goal-directed; it doesn’t “try” to make organisms perfect. It just favors traits that work better *right now* in a given environment.
- Random processes (mutation, genetic drift) create and shuffle variation; natural selection is the non-random sorting of that variation by differential reproductive success.

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


prompt_id: sci-iq-heritability

text:
Human intelligence is **moderately to highly heritable**, but *not* genetically fixed, and environment still matters a lot. Here’s what the evidence shows.

---

## 1. What “heritability of intelligence” 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 intelligence comes from your genes” (that’s wrong).
  - “Intelligence can’t be changed” (also wrong).

Heritability is:
- **Population-specific** (depends on environment, inequality, education, nutrition).
- **Time- and place-dependent**.
- About *variation*, not about any single person’s potential.

---

## 2. Twin, adoption, and family studies: core evidence

These classic designs are the backbone of what we know.

### Twin studies

Meta-analyses of many twin studies find:

- **Identical (MZ) twins** reared together: IQ correlations ≈ 0.80–0.85
- **Fraternal (DZ) twins** reared together: IQ correlations ≈ 0.50–0.60

From this, heritability estimates:

- **Childhood**: h² ≈ 0.20–0.40
- **Adolescence**: h² ≈ 0.40–0.60
- **Adulthood**: h² ≈ 0.60–0.80

So among adults in relatively developed countries, **60–80% of IQ differences are statistically linked to genetic differences**.

### Adoption studies

- Adopted children’s IQs correlate:
  - Moderately with **biological parents**.
  - Weakly or not at all with **adoptive parents** by late adolescence/adulthood.
- In childhood, shared family environment does matter more, but its effect **wanes with age**.

These findings are broadly consistent with twin data.

---

## 3. Shared vs non-shared environment

Behavioral genetic models usually split variance into:

- **Genetic (A)**
- **Shared environment (C)** – aspects that make siblings more similar (home, SES, parenting style).
- **Non-shared environment (E)** – things that make siblings different (different peers, illnesses, teachers, random life events, measurement error).

Findings:

- **Shared environment**:
  - Larger in early childhood, sometimes 20–30% of variance.
  - Drops toward near-zero in many adult samples.
- **Non-shared environment**:
  - Remains important across life: often 20–30% of variance.

So even when heritability is high, **environmental factors that differ between individuals remain substantial**.

---

## 4. Age pattern: heritability increases with age

Well-replicated pattern:

- IQ heritability is **lower in preschool years**, and **increases into adulthood**.
- Often described as going from ~30% in early childhood to ~70%+ in mature adulthood.

Main explanation: **gene–environment correlation**:
- As people grow up, they:
  - Select, modify, and create environments that fit their genetic predispositions (e.g., choosing more cognitively demanding hobbies, jobs, friends).
- This amplifies genetic influences over time.

---

## 5. Molecular genetics: GWAS and polygenic scores

### Genome-wide association studies (GWAS)

Large GWAS (hundreds of thousands to millions of people) identify many tiny-effect variants associated with cognitive performance and educational attainment.

Findings:

- Single genetic variants have **minuscule effects**.
- But thousands together can explain:
  - Roughly **10–20%** of variance in educational attainment.
  - Less (often **5–10%**) of variance in measured cognitive ability, depending on sample and measure.

This is **lower than twin-based heritability** (the “missing heritability” issue), but consistent with intelligence being:
- **Highly polygenic** (many genes, each tiny effect),
- Influenced by **environment and gene–environment interplay**.

Polygenic scores:
- Can modestly predict group-level differences (e.g., average IQ/education in samples).
- Are **poor predictors for any individual** and do **not** deterministically fix outcomes.

---

## 6. Role of environment and social context

Heritability depends heavily on **environmental variation**.

### Socioeconomic status (SES) effects

Some studies (especially in the US) suggest:

- In **low-SES** environments:
  - Heritability of IQ is **lower**, and shared environment explains more variance.
- In **high-SES** environments:
  - Heritability is **higher**, because basic needs and educational inputs are more equal,

finish_reason: length


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 “heritability” means (and doesn’t mean)

- **Heritability (h²)**: The proportion of variation *between people in a given population* that can be statistically attributed to genetic differences.
- It does **not** mean:
  - “X% of *your* intelligence comes from genes.”
  - “X% of intelligence is predetermined and unchangeable.”
  - “Environment doesn’t matter.”

Heritability is a **population-level statistic** that depends on:
- that population,
- that time and place,
- that range of environments.

If environments become more equal (e.g., universal schooling, nutrition), then *relative* genetic differences can explain a larger share of the remaining variation.

---

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

Most classic work comes from:
- **Identical (MZ) twins**: share ~100% of their segregating genes.
- **Fraternal (DZ) twins**: share ~50% on average.
- **Adoption studies**: compare adopted children to biological vs adoptive parents/siblings.

### Main findings

Across large meta-analyses:

- **Childhood**: Heritability of IQ ~ **0.2–0.4**
  (20–40% of differences attributable to genetic variation)
- **Adolescence / early adulthood**: ~ **0.5–0.6**
- **Adulthood (esp. middle age)**: often **0.6–0.8**

So, under typical modern Western conditions:
- About **50–80%** of the variation in adult intelligence scores within a population is associated with genetic differences.
- The **rest** (20–50%) reflects environmental influences and measurement error.

### Why does heritability rise with age?

Two main reasons, supported by longitudinal twin/adoption studies:

1. **Genetic amplification**:
   People increasingly select, shape, and stay in environments that fit their genetically influenced traits (e.g., curiosity, persistence, verbal skill). This “gene–environment correlation” magnifies genetic differences over time.

2. **Diminishing impact of shared childhood environment**:
   Factors like parents’ income, home library, etc., that siblings share, have a sizable effect early in life but typically **shrink in importance by adulthood**.
   In many samples, adult IQ variance splits roughly into:
   - genetic: large
   - shared family environment: small to near-zero
   - non-shared (individual-specific) environment: moderate

This doesn’t mean families don’t matter; it means that **among people raised in broadly similar, non-deprived conditions**, the lasting *differences* in adult IQ between siblings are more tied to genes and idiosyncratic experiences than to the family they shared.

---

## 3. Molecular genetics: what specific DNA shows

With large genome-wide association studies (GWAS):

- Tens of thousands of genetic variants are associated with cognitive performance and educational attainment.
- Each variant has **tiny** effects; in aggregate they can form **polygenic scores**.

Current polygenic scores for education or cognitive ability in European-ancestry samples can explain roughly:

- ~10–20% of variance in educational attainment
- ~5–15% of variance in cognitive test performance

This is **less** than the twin-study estimates because:
- GWAS detect only common variants with small effects;
- they miss rare variants, structural variants, and complex interactions.

Still, they **independently confirm** that:
- Many genes, each with small effect, contribute to intelligence.
- The architecture is highly **polygenic**.

---

## 4. The role of environment

Even with high heritability, environment can strongly influence **levels** of intelligence and outcomes.

### Clear environmental effects

- **Severe deprivation** (neglect, malnutrition, institutionalization) can reduce IQ by dozens of points; early intervention can partly reverse this.
- **Iodine deficiency**, lead exposure, extreme poverty, and chronic illness affect cognitive development.
- **Education**: More years of schooling reliably increase test scores (IQ-like tests included). Randomized or quasi-random school reforms and compulsory schooling laws show causal effects.

### Heritability ≠ immutability

A classic illustration:
- **Height** is highly heritable (often >80%) in well-fed populations.
- Yet mean height increased dramatically across the 20th century due to better nutrition and health.

Likewise, IQ distributions have shifted over time (the **Flynn effect**—substantial rises in average test scores across decades), showing that **environmental changes can move the whole distribution**, even when within-population differences are substantially heritable.

---

##


text:
Human intelligence is **substantially heritable**, but **not purely genetic**. The best evidence indicates that:

- **Heritability of general intelligence (g)** in modern high‑income countries is typically around **50–80% in adults**
- **Environmental factors still matter a lot**, particularly in childhood and across generations
- Heritability is **not a fixed number**: it depends on age, social conditions, and how it’s measured

Below is the core of what we know and how we know it.

---

## 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**, **given that population’s environments**.
- It is **not**:
  - The percentage of an individual’s intelligence “caused” by genes
  - A fixed property of a trait across all times and places

Example: If a trait has heritability of 60%, then **60% of the variation between people** in that study population (with its specific mix of environments) can be modeled as due to genetic variation.

---

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

### Twin studies

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

When raised together, if **MZ twins are more similar** in IQ than DZ twins, the excess similarity is attributed to genetic effects.

Findings (meta‑analyses, e.g., Bouchard, Plomin, Polderman 2015):

- **Childhood**: IQ heritability ~0.3–0.4
- **Adolescence**: ~0.5–0.6
- **Adulthood**: ~0.6–0.8

This pattern is robust: **heritability of IQ tends to increase with age**.

### Adoption studies

- Unrelated children adopted into the **same family** have IQ correlations that are **modest and fade with age**.
- Adopted children resemble their **biological parents** more than their adoptive parents in adult IQ.
- But adoption from deprived backgrounds into more advantaged ones often yields **substantial IQ and educational gains**, showing strong environmental impact.

Together, twin and adoption studies consistently support **large genetic contributions plus meaningful environmental effects**.

---

## 3. How heritability changes with age

A well‑replicated pattern:

- **Early childhood**: family environment (shared environment) has a **sizeable effect**; heritability is lower.
- **Adulthood**: people choose and shape environments that match their dispositions (gene–environment correlation), so:
  - **Heritability rises**
  - The influence of the **shared family environment shrinks**, often near zero for IQ in middle adulthood in Western samples
- **Old age**: results are more mixed; some studies suggest a slight decline in heritability as health and random life events accumulate.

This doesn’t mean that environment stops mattering later; it means **within a given adult population**, **differences** are more strongly aligned with genetic differences than with differences in family upbringing.

---

## 4. Molecular genetics: what DNA tells us

Modern genome‑wide association studies (GWAS) and related methods provide a **bottom‑up** check on twin/adoption work.

### GWAS and polygenic scores

- GWAS has identified **thousands of genetic variants** that each have **tiny effects** on intelligence and educational attainment.
- Each variant explains far less than 1% of variance; collectively, **polygenic scores** can now explain:
  - For IQ or cognitive performance: typically **10–25%** of variance in European‑ancestry samples
  - For educational attainment: up to **~10–15%** (sometimes a bit more in large, well‑phenotyped datasets)

### SNP‑based heritability

Methods like GREML / LDSC estimate how much variance is explained by all common SNPs measured:

- Estimates for general cognitive ability: **~20–30%**
- This is **lower than twin estimates (60–80%)**, likely because:
  - GWAS currently capture only **common** variants, not rare ones or structural variants
  - Some assumptions in twin models may inflate estimates, but most behavior‑geneticists think twin estimates are in the right ballpark for broad-sense heritability

The key point: **both family‑based and DNA‑based methods independently show substantial genetic influence**.

---

## 5. Environment still has strong and sometimes massive effects

High heritability ≠ little role for environment. Some important points:

### a) Environmental


text:
Human intelligence is substantially heritable, but not fixed, and not determined by genes alone. The best-supported view from modern behavioral genetics is:

- In contemporary high‑income populations, **about 50–80%** of the *differences between adults* in general cognitive ability (often called g or IQ) are associated with genetic differences.
- The rest—roughly **20–50%**—reflects environmental factors and measurement error.
- This **does not** mean that 50–80% of *your* intelligence is “from genes” and the rest “from environment”; heritability is a population statistic, not a personal breakdown.

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

---

## 1. What “heritability of intelligence” means

- **Heritability (h²)**: the proportion of *variance in a trait within a particular population* that is statistically associated with genetic differences.
- It is:
  - About **groups**, not individuals.
  - About *existing* environments; change the environment radically, and heritability can change.
  - Not a fixed property of IQ itself.

So if heritability of IQ in a given sample is 0.7 (70%), that means: in that population, at that time, about 70% of the observed differences in IQ scores between people are associated with genetic differences, given their current range of environments.

---

## 2. Main sources of evidence

### a. Twin studies

These compare:

- **Monozygotic (MZ) twins**: nearly 100% of genes shared.
- **Dizygotic (DZ) twins**: ~50% of segregating genes shared.

If MZ twins are more similar than DZ twins, that suggests genetic influence.

Key findings (from many large meta-analyses):

- **MZ twins raised together**: IQ correlations typically ~0.80–0.90.
- **DZ twins raised together**: IQ correlations typically ~0.50–0.60.
- **MZ twins raised apart** (reared in different families): correlations typically around ~0.70–0.80.

Statistical models (ACE models: Additive genetics, Common environment, Unique environment) from these data usually estimate:

- **Childhood IQ**: heritability ~30–50%; shared environment plays a substantial role.
- **Adult IQ**: heritability rises to ~60–80%; shared environment drops close to zero; nonshared environment remains.

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

### b. Adoption studies

These compare adopted children with:

- **Biological parents/siblings** (genes, little rearing).
- **Adoptive parents/siblings** (rearing, no genes).

Findings:

- In childhood, adopted children’s IQs correlate modestly with their **adoptive parents**.
- By late adolescence/young adulthood, that correlation with adoptive parents shrinks toward **zero**, while similarity to **biological parents** increases.
- IQ correlations between adopted siblings (no genetic relation, same home) also decline toward zero by adulthood.

Interpretation: family-wide (shared) environment has some impact in childhood but tends to **fade** for general intelligence as people grow up, while genetic influences become more apparent.

### c. Genome-wide association studies (GWAS) and SNP-based estimates

With large genetic datasets:

- **Polygenic scores** (summing the small effects of thousands of variants) can predict **10–20%** of variance in cognitive performance or educational attainment in European-ancestry samples; less so in other ancestries due to data limitations.
- **SNP-based heritability** (variance explained by common SNPs alone) for cognitive ability is often in the **20–40%** range, which is lower than twin-based estimates but clearly nonzero.

This provides **direct molecular-genetic evidence** that intelligence is influenced by very many genetic variants, each with tiny effects.

---

## 3. How heritability changes with age

From large twin/adoption meta-analyses:

- **Early childhood**: h² ~0.20–0.40; shared family environment (C) often ~0.30–0.40.
- **Adolescence**: h² increases to ~0.50–0.60; shared environment decreases.
- **Adulthood**: h² ~0.60–0.80; shared environment often near zero; unique environment (E, including measurement error) ~0.20–0.40.

Reasons proposed:

- **Gene–environment correlation**: as people age, they increasingly select and shape environments that fit their genetic tendencies (e.g., choosing cognitively demanding jobs, educational paths).
- **Brain development**: genetic programming of neural development plays out over time; some influences only manifest later.

---

## 4. Environment still


text:
Human intelligence is **substantially heritable**, but not fixed, and the size of the genetic influence depends on age, environment, and how you measure it.

Below is a structured summary of what the evidence shows.

---

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

- **Heritability (h²)**: The proportion of **differences between people** in a trait (like IQ scores) that can be statistically attributed to genetic differences **in a particular population, in a particular environment**.
- It does **not** mean:
  - “X% of *your* intelligence is genetic.”
  - “Intelligence is unchangeable.”
  - “Genes matter the same way everywhere.”

Heritability is about **variation in a group**, not destiny for individuals.

---

## 2. Twin and adoption studies: classic evidence

These were the first major tools used.

### Twin studies

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

If genes matter a lot, MZ twins should resemble each other more than DZ twins on IQ/cognitive tests.

Findings from large meta-analyses (e.g., Plomin & Deary, 2015; Polderman et al., 2015):

- **Childhood (≈5–12 years)**: heritability often ~0.3–0.4
- **Adolescence**: ~0.5
- **Adulthood**: ~0.6–0.8 in many Western samples

These are **broad-sense heritability** estimates (all genetic effects, including non-additive ones).

### Adoption studies

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

Patterns commonly found:

- In **early childhood**, adopted children’s IQs correlate with **adoptive parents** as well as or more than with biological parents.
- By **adolescence and adulthood**, correlations with **biological parents/siblings** increase, while correlations with adoptive parents often **fall toward zero**.

Interpretation: shared family environment matters a fair bit early on, but genetic differences account for more of the stable, long-term differences.

---

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

A robust pattern:

- Heritability of general intelligence (g) tends to **increase from childhood to adulthood**.
- Often summarized as:
  - ~20–30% in early childhood
  - ~40–50% in late childhood
  - ~60–80% in adulthood

Why might this happen?

- As people age, they increasingly **select, modify, and create environments** that match their genetically influenced traits (gene–environment correlation).
- Early constraints from family, school, etc. relax, letting genetic dispositions express more strongly.

---

## 4. Role of environment

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

### Shared vs non-shared environment

- **Shared environment**: aspects that make siblings similar (home, parents, neighborhood, some schools).
- **Non-shared environment**: experiences that differ between siblings (different friends, teachers, illnesses, accidents, random events).

Twin/adoption studies typically find:

- **Shared environment**:
  - Moderate influence on IQ in **childhood** (~0.2–0.3).
  - Often close to **zero in adulthood** in high-income countries.
- **Non-shared environment**:
  - Substantial across the lifespan, including measurement error.

In other words: by adulthood in many Western samples, **most stable between-person differences in general cognitive ability** are statistically attributed to genes and non-shared environment, not to the aspects of family background siblings share.

### Socioeconomic status (SES) moderation

There’s evidence that **heritability itself can differ by SES**:

- In several **U.S. studies**, heritability of IQ in childhood appears **lower in low-SES environments and higher in high-SES environments**, with shared environment showing the opposite pattern.
  - Interpretation: in deprived contexts, environmental constraints limit the expression of genetic differences; in more advantaged contexts, those constraints are eased, so genetic variation explains more of the observed differences.
- In some **Northern European** samples with stronger social safety nets, this SES moderation is **weaker or absent**.

This underscores that heritability is **context-dependent**.

### Environmental impacts

Environment can and does shift intelligence distributions:

- **Nutrition and health**, toxin exposure, prenatal conditions.
- **Education quality and years of schooling**: robust positive effects on cognitive test performance and measured IQ.
-


text:
Human intelligence is substantially heritable, but not fixed, and it depends strongly on age and environment. Here’s a concise summary of what the evidence shows.

---

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

- **Heritability (h²)**: the proportion of *differences between people* in a population that can be statistically attributed to genetic differences.
- It is:
  - **About variation, not individuals** (it does not say “X% of *your* intelligence is genetic”).
  - **Population- and context-specific** (it can change with environment, age, and inequality).
  - **Not a ceiling on change**: a highly heritable trait can still be strongly altered by environmental interventions (e.g., height and nutrition).

---

## 2. Twin and family studies: broad picture

Most classic estimates of the heritability of general intelligence (g / IQ) come from twin, adoption, and family studies.

### Children
- In early childhood, **heritability estimates** are often around **0.2–0.4** (20–40%).
- Shared environment (home, parenting, early schooling) plays a relatively large role early on.

### Adolescents
- By adolescence, many studies suggest **heritability around 0.4–0.6**.
- Shared environment influence tends to decline, and nonshared environment (individual experiences, chance events) grows.

### Adults
- In adulthood, especially middle age, estimates often reach **0.6–0.8** (60–80%).
- Famous studies:
  - Minnesota Study of Twins Reared Apart
  - Swedish and Danish twin registries
- These consistently find:
  - High similarity of IQ in **identical twins reared apart**.
  - Lower—but still positive—similarity in **fraternal twins and siblings**.
  - Adopted children become less similar to their adoptive parents and more similar to their biological parents over time.

**Key pattern**: heritability of intelligence *increases with age*—sometimes called the “Wilson effect.”

---

## 3. Molecular genetics: what specific DNA can explain

Twin studies measure overall genetic influence but not which genes. Modern methods use actual DNA data.

### SNP-based heritability
- Using genome-wide SNP data and methods like GREML or LD-score regression, studies typically find:
  - **SNP heritability ~20–30%** for intelligence in adults.
  - This is a **lower bound** on true heritability (because common SNPs don’t capture all genetic variation).

### Polygenic scores (PGS)
- Large GWAS (hundreds of thousands to over a million participants) have identified thousands of variants, each with tiny effects.
- Polygenic scores now explain:
  - Roughly **10–20% of variance** in cognitive performance / educational attainment in European-ancestry samples (lower in others due to biases in GWAS data).
- This is substantial but far below twin-based estimates, reflecting:
  - Limited GWAS power (still not capturing all causal variants).
  - Incomplete tagging of rare variants and structural variants.
  - Measurement noise and heterogeneity in cognitive and educational measures.

**Takeaway**: DNA-level data independently confirms that genetic differences strongly contribute to intelligence differences, but also shows intelligence is highly polygenic.

---

## 4. Environment still matters a lot

High heritability does not mean environment is unimportant.

### Evidence for environmental effects
- **Shared environment** (family, schooling, early childhood conditions) clearly affects IQ in childhood, and in some contexts into adulthood.
- **Extreme deprivation and insults** (severe neglect, malnutrition, lead exposure, lack of schooling, major illness) can strongly depress cognitive development.
- **Socioeconomic status (SES)**:
  - In some U.S. samples, heritability of IQ appears *lower* in low-SES families and *higher* in high-SES families (“gene × environment interaction”).
  - In several European and Scandinavian samples (with stronger social safety nets), SES differences in heritability are smaller or absent.
- **Historical change (Flynn effect)**:
  - Average IQ scores rose dramatically (≈2–3 points per decade) across much of the 20th century.
  - Genes did not change that fast; improvements in nutrition, education, health, and complexity of environments are implicated.
  - Some high-income countries now show stagnation or slight declines, again underscoring environmental influence.

**Conceptually**: Genes set up a range of potential; environment influences where within that range a person tends to land, and can shift group averages substantially.

---

## 5. Stability and change across life

- **Rank-order stability**: People’s relative standing on intelligence tests is moderately to highly stable across life, especially after childhood.
- **Mean-level change**: Individuals


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

Here’s what the evidence shows, in layers:

---

## 1. What “heritability” actually means

- **Heritability (h²)** is the proportion of *differences between people in a particular population at a particular time* that can be statistically attributed to genetic differences.
- It is **not**:
  - “How genetic intelligence is in an individual”
  - “How changeable intelligence is”
  - A fixed number that applies to all groups, times, or environments.

So if heritability of IQ is 0.60 in a given population, that means: about 60% of the *variation* in IQ scores between people in that group can be attributed to genetic variation, given that group’s environment.

---

## 2. Twin, adoption, and family studies

### Classic twin studies (monozygotic vs. dizygotic)

- Identical twins (MZ) share ~100% of segregating genes; fraternal twins (DZ) share ~50% on average.
- Meta-analyses of many twin studies find:
  - **Childhood IQ:** heritability ~0.30–0.45
  - **Adolescence:** ~0.50–0.60
  - **Adulthood:** ~0.60–0.80 in many Western samples

This pattern—heritability rising with age—is one of the strongest findings.

Why it rises with age:
- As people grow up, they increasingly select, modify, and create environments that fit their genotypes (called **gene–environment correlation**).
- Shared family environment matters more early in life, then often diminishes.

### Adoption studies

- Adopted children’s IQs:
  - In childhood: modest correlation with their adoptive parents (shared environment effects).
  - By late adolescence and adulthood: IQ correlates more with **biological** parents than adoptive parents.
- Large studies (e.g., Texas Adoption Project, Minnesota Adoption Study) show:
  - Early gains from enriched adoptive homes, but
  - In adulthood, resemblance to biological parents and siblings dominates.

### Shared vs. nonshared environment

From behavioral genetic models of IQ:

- **Shared environment (things making siblings similar):**
  - Moderate in childhood (maybe 20–30% of variance)
  - Often close to zero by adulthood in many samples.
- **Nonshared environment (experiences not shared, plus measurement error):**
  - Typically 20–30% of variance across the lifespan.

---

## 3. Molecular genetic evidence

### Genome-wide association studies (GWAS)

- Very large GWAS (hundreds of thousands to >1 million people) have:
  - Identified thousands of variants associated with cognitive ability and educational attainment.
  - Each variant has *tiny* effects, but collectively they explain a non-trivial amount of variance.

### SNP-based heritability

- Using measured SNPs (common variants), researchers estimate:
  - **SNP-heritability for IQ** often ~0.20–0.30.
  - True heritability (from twin studies) is higher, implying:
    - “Missing” heritability from rare variants, structural variants, imperfect tagging, and methodological limits.

### Polygenic scores (PGS)

- Polygenic scores built from GWAS:
  - Now predict ~10–20% of variance in cognitive test scores in large European-ancestry samples.
  - Higher prediction for educational attainment (often 10–25%).
- PGS predictions:
  - Are statistically meaningful but far from deterministic.
  - Show that measured genetic variation contributes substantially, but leaves most individual variation unexplained.

---

## 4. Development, age, and environment

### Age pattern (very robust finding)

- Heritability of intelligence:
  - Low–moderate in early childhood.
  - Rises into adulthood.
  - Twin estimates in older adults often remain high, although disease/aging processes add environmental variance.

### Socioeconomic status (SES) moderation

Evidence is mixed and varies by country, but:

- In some U.S. samples:
  - Among low-SES families, shared environment plays a larger role and heritability is lower.
  - Among high-SES families, heritability is higher and shared environment smaller.
- In many Northern European samples with more equal schooling and health care:
  - This SES–heritability interaction is smaller or absent.

Interpretation:
- When environments are very unequal or constrained, environmental factors account for more variance.
- When environments are more uniformly supportive, genetic differences can explain more of the remaining variation.

---

## 5. What about interventions and change?

High heritability **does not mean** intelligence can’t be changed.

Examples:

- **Secular “Flynn effect”**:


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

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

- **Heritability (h²)**: the proportion of variation in a trait *within a population at a given time* that is statistically attributable to genetic differences.
- A heritability of 0.5 does **not** mean:
  - “50% of your intelligence comes from genes, 50% from environment,” or
  - “Changing environment can only change intelligence by 50%.”
- It means: in that population, about 50% of the *differences between people* in measured cognitive ability are associated with genetic differences.

Heritability is:
- **Population-specific** (changes between countries, eras, social conditions)
- **Not about individuals** (you can’t assign a personal h²)
- **Not fixed** (it changes over the lifespan and with environmental inequality/equality)

---

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

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 with biological vs adoptive parents/siblings

Patterns that consistently show up:

#### a. Heritability rises with age

Meta-analyses (e.g., Robert Plomin, Wendy Johnson, Ian Deary and others) show something like:

- **Early childhood**: h² ≈ 0.20–0.40
- **Adolescence**: h² ≈ 0.40–0.60
- **Adulthood**: h² ≈ 0.60–0.80

This “rising heritability” is one of the most replicated findings. As people grow up and choose environments that fit their tendencies (gene–environment correlation), genetic differences are more strongly expressed.

#### b. Shared environment effects shrink with age

“Shared environment” = environmental influences that make family members more similar (home, parenting style, neighborhood, etc.)

- In **childhood**, shared environment explains maybe 20–30% of variance in IQ.
- By **late adolescence and adulthood**, shared environment often drops close to zero in many studies; remaining environmental influence is mostly “nonshared” (experiences that differ even within a family: different peers, teachers, illnesses, random life events).

Important nuance: this does **not** mean family environment doesn’t matter. It means that *within the range of environments sampled*, the things families share don’t explain very much of the *differences* in adult intelligence.

#### c. Twin correlations

Rough ballpark from large samples:

- MZ twins raised together: IQ correlation ≈ 0.80–0.90
- MZ twins raised apart: ≈ 0.70–0.80
- DZ twins raised together: ≈ 0.40–0.50
- Adoptive siblings raised together: ≈ 0.20 in childhood, near 0 by adulthood

MZ twins reared apart being nearly as similar as those reared together is a strong indicator of genetic influence, although they are rarely in *wildly* different environments (still usually within the same society).

---

### 3. Molecular genetics: what specific genes say

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

Findings:

- Intelligence is **highly polygenic**: thousands of variants, each with tiny effects.
- Early GWAS with tens of thousands of people explained very little variance (<5%).
- Newer GWAS with hundreds of thousands to over a million participants can produce:
  - **Polygenic scores** that predict ~10–20% of variance in cognitive test performance in European-ancestry samples.
  - Educational attainment scores can explain somewhat more variance in years of education (~10–15%, sometimes more in research samples).

Why is this less than twin-study heritability (~60–80%)?

- GWAS capture only:
  - Common variants that are well-imputed
  - Additive effects (not all interactions)
- This “missing heritability” is common across complex traits (height, mental disorders, etc.).
- When using methods like GREML (genomic-relatedness-based estimates using unrelated individuals), SNP-based heritability for intelligence is often around **20–40%**, indicating that measured common variants explain a substantial, though incomplete, portion of the twin-based estimate.

Polygenic scores work **statistically**, not deterministically. They shift the odds; they do not tell you “your IQ.”

---

### 4. Age, environment, and socioeconomic status (SES)

#### a. SES moderation (the “Sc


text:
Human intelligence is substantially heritable, but not fixed, and environment still matters a lot. The evidence is strongest for “g” (general intelligence) measured with standard cognitive tests.

Here’s a concise overview of what we know:

---

## 1. What “heritable” means

- **Heritability (h²)**: The proportion of differences between people in a trait (in a given population, at a given time) that is statistically attributable to genetic differences.
- It does **not** mean “X% of your intelligence comes from genes” or that the trait is unchangeable.
- Heritability is **population-specific** and **environment-dependent**.

---

## 2. Twin, family, and adoption studies

These provide the classic evidence.

### Twin studies

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

Key findings from meta-analyses and large samples:

- In industrialized countries:
  - **Childhood IQ**: heritability ≈ 0.3–0.5
  - **Adolescence / young adulthood**: ≈ 0.5–0.7
  - **Middle adulthood**: up to ≈ 0.7–0.8

This “increasing heritability with age” is a very robust finding: genetic influences become more predictive as people grow up, partly because people select and shape environments that fit their genetic predispositions (“gene–environment correlation”).

### Adoption studies

- Children adopted away from their biological parents:
  - As young children, IQ correlates modestly with **adoptive parents** and somewhat with biological parents.
  - By late adolescence and adulthood, the correlation with **biological parents’ IQ** is clearly stronger than with adoptive parents; correlations with adoptive parents often drop toward zero.
- Sibling/adoption designs show that:
  - **Shared family environment** (the aspects siblings share: home, SES, parenting style) has a moderate effect in childhood but often shrinks to **near-zero in adulthood** for IQ.
  - **Nonshared environment** (experiences unique to each person) and measurement error account for most of the non-genetic variance.

---

## 3. Molecular genetics (SNP-based estimates)

Genome-wide methods estimate heritability from actual DNA variants (e.g., common SNPs):

- **SNP-heritability of intelligence**: often around **0.2–0.3** (20–30%) with current genotyping panels.
- This is **lower** than twin-based estimates (≈ 0.6–0.8) because:
  - We currently capture only common variants.
  - Not all genetic effects are additive or well tagged.
- This “missing heritability” gap has narrowed over time as GWAS sample sizes grew, but not disappeared.

Large GWAS (n > 1 million) show:
- Intelligence is **highly polygenic**: thousands of variants of very small effect.
- Polygenic scores (PGS) can now explain roughly **10–20%** of variance in educational attainment and **less** in IQ directly, depending on population and measure. That’s meaningful at the group level, but not highly predictive for individuals.

---

## 4. Gene–environment interplay

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

### a) Gene–environment interaction (G×E)

A widely discussed pattern:

- In some lower-SES environments, heritability of IQ appears **lower**, and shared environment relatively more important.
- In higher-SES or more supportive environments, heritability **increases**, because bad environments no longer suppress genetic potential as much.

Findings:
- Strong G×E reported in some US samples.
- Weaker or absent in several Western European / Australian samples.
- Interpretation: where environments are more equal (e.g., better social safety nets, schooling), genetic differences have more room to express; where environments are sharply unequal, environmental constraints dominate.

### b) Gene–environment correlation

People with certain genotypes:
- **Evocative**: elicit different responses (e.g., a verbally precocious child receives more language input).
- **Active**: seek compatible environments (books, friends, schooling, careers).
- **Passive**: inherit genes and environments together (high-IQ parents both pass on genes and provide enriched environments).

These processes help explain why genetic influence seems to grow with age.

---

## 5. Environmental effects that clearly matter

Even with high heritability, interventions and conditions can shift average IQ and the shape of the distribution:

- **Severe deprivation / malnutrition / toxins (e.g., lead)** can markedly reduce IQ.
- **Iodine deficiency**, iron deficiency, and other


text:
Human intelligence is substantially heritable, but not fixed, and strongly shaped by environment. The most robust conclusion from decades of work is:

- In modern, affluent populations, **about 50–80% of the variation in adult IQ is heritable** (broad-sense heritability, h² ≈ 0.5–0.8).
- In childhood it’s lower, **~20–40%**, and **increases with age**.
- This does **not** mean “50–80% of your intelligence is genetic”; it means in a given population, that proportion of *differences between people* in test scores can be statistically attributed to genetic differences.

Below is the core evidence and how to interpret it.

---

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

Heritability (h²) is:

> The proportion of **variance in a trait within a specific population and environment** that is due to genetic differences.

Key implications:

- It is **about variation within a group**, not between groups.
- It is **population- and context-specific**: change the environment, and heritability can change.
- A high h² does **not** imply immutability: height is ~80–90% heritable, yet improved nutrition produces large gains.

When researchers say “IQ heritability is ~0.7 in adults,” they mean:
In that particular setting, **around 70% of the observed differences in IQ scores between individuals** can be statistically linked to genetic variation, given the existing range of environments.

---

## 2. Twin, adoption, and family studies

These are the main sources of “classical” evidence.

### Monozygotic (MZ) vs. dizygotic (DZ) twins

- **MZ twins** share ~100% of segregating genes.
- **DZ twins** share on average ~50%, like ordinary siblings.
- If genes matter, **MZ twins’ IQ scores should correlate more strongly than DZ twins’, especially when raised apart**.

Findings (large meta-analyses):

- MZ twins raised together: IQ correlations ~0.80–0.90.
- MZ twins raised apart: ~0.70–0.80.
- DZ twins raised together: ~0.50–0.60.
- Unrelated children raised together: ~0.20–0.30, declining with age.

These patterns are very difficult to explain without a substantial genetic component.

### Adoption studies

Compare:

- Children adopted into new families vs. their **biological** parents/siblings.
- Same children vs. their **adoptive** parents/siblings.

Findings:

- In childhood, adoptees resemble both biological and adoptive families somewhat.
- By late adolescence and adulthood, **IQ correlates strongly with biological relatives** and **weakly or not at all with adoptive relatives**.
- Shared-environment effects (family upbringing) on IQ largely **fade by adulthood**, especially in middle-class samples.

These data support **both**:
- A substantial genetic effect.
- Environment being more important early in life.

---

## 3. How heritability changes with age

A striking result: **heritability of intelligence tends to increase over development**.

Approximate pattern (from large twin studies):

- Early childhood: h² ≈ 0.20–0.40
- Adolescence: h² ≈ 0.50–0.60
- Adulthood: h² ≈ 0.60–0.80

Explanations include:

- **Genetic amplification**: as people grow, they increasingly choose, elicit, and shape environments that match their genetic dispositions (e.g., seeking out cognitively demanding activities).
- **Diminishing impact of shared family environment**: parental control and home environment matter a lot early; later, peers, education, jobs, and self-selection dominate.

---

## 4. Role of environment

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

- **Differences between groups** (e.g., across countries or historical eras) can be almost entirely environmental even if within-group heritability is high.
- The **Flynn effect**: average IQ scores rose ~2–3 points per decade in many countries during the 20th century—far too rapid for genetic change—implicating education, nutrition, health, and cognitive demands of modern life.
- Severe deprivation (malnutrition, neglect, lack of schooling, toxins like lead) can significantly depress cognitive development regardless of genetic potential.
- **Socioeconomic status (SES)** can moderate heritability:
  - In some U.S. samples, **heritability is lower in low-SES environments** (because constraints suppress genetic potential) and higher in high-SES (where more children can realize their potential).
  - This moderation is not always found in European/welfare


text:
Human intelligence is substantially heritable, but not fixed, and not determined by genes alone. The evidence is strong but nuanced.

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

- **Heritability (h²)** in this context: the proportion of *differences between people* in a particular population that can be statistically attributed to genetic differences.
- It:
  - Does **not** say what fraction of *your* intelligence comes from genes.
  - Depends heavily on the **population and environment** being studied.
  - Can change across the lifespan and across social conditions.

Think of it as: “In this group, under these conditions, how much of the variation in test scores is linked to genetic variation?”

---

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

These are the classical sources of evidence.

**Twin studies**

- Identical (monozygotic, MZ) twins share ~100% of segregating genes; fraternal (dizygotic, DZ) twins share ~50% on average.
- If MZ twins resemble each other more than DZ twins for IQ, that implies genetic influence.
- Large meta-analyses:
  - Typically estimate **heritability of general intelligence (g)** around **0.5–0.8** in adults.
  - A commonly cited range:
    - Childhood: ~0.3–0.5
    - Adolescence: ~0.5–0.6
    - Adulthood: ~0.7–0.8

This “increasing heritability with age” has been replicated many times.

**Adoption studies**

- Compare adopted children to:
  - Their biological parents/siblings (genes, little shared environment).
  - Their adoptive parents/siblings (shared environment, no shared genes).
- Findings:
  - In childhood, IQ correlates modestly with both biological and adoptive family.
  - By late adolescence/young adulthood, IQ correlations with **biological** relatives remain, but with **adoptive** relatives often drop close to zero.
- Interpretation: **early** shared family environment matters, but in the long run, **genetic differences and nonshared environments** (unique experiences) account for most of the IQ variation.

**Family correlations**

- IQ correlations:
  - MZ twins reared together: often ~0.80–0.90
  - MZ twins reared apart: ~0.70–0.80
  - DZ twins reared together: ~0.50–0.60
  - Biological siblings: ~0.40–0.50
  - Adoptive siblings (not genetically related): ~0.00–0.20 in adulthood
- This pattern is consistent with **strong genetic effects plus modest shared-family environmental effects**, especially earlier in life.

---

### 3. Modern molecular genetics (GWAS and polygenic scores)

Classical studies infer genes indirectly. Molecular work looks at DNA directly.

**Genome-wide association studies (GWAS)**

- Large GWAS (hundreds of thousands to >1 million people) have identified **thousands of common variants** each contributing very small effects to intelligence or educational attainment.
- Individual variants have tiny effect sizes; intelligence is a **highly polygenic** trait.

**SNP heritability**

- Using genome-wide SNP data, researchers estimate what fraction of variance can be predicted by all measured common variants.
- Typical **SNP-based heritability** for intelligence: **~0.20–0.30** (lower than twin-based estimates, partly because not all genetic variation is captured).

**Polygenic scores (PGS)**

- A PGS sums effects of many variants into a single score per person.
- For large GWAS:
  - PGS can explain roughly **10–15%** (sometimes a bit more) of variation in educational attainment.
  - For direct cognitive measures (IQ tests, cognitive battery scores), current PGS usually explain **~5–15%** of the variance, depending on sample, ancestry, and phenotype.
- That’s far less than total heritability, but:
  - It’s strong evidence that **common genetic variation is systematically associated** with cognitive ability.
  - Predictive power is **much weaker in non-European populations**, because most GWAS have been Euro-centric.

---

### 4. How heritability changes with age and environment

**Across the lifespan**

- Childhood: shared family environment (home, parents, schooling) plays a larger role; heritability is more modest.
- Adolescence/Adulthood: as people select and shape their own environments (friends, jobs, hobbies), **gene–environment correlation** strengthens, and heritability estimates **rise**.
- Late life: cognitive decline has its own heritability; genetic risks (e.g., APOE for Alzheimer’s) play roles for specific domains


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

---

## 1. What “heritability” means

- Heritability (h²) = proportion of **differences between people** in a population that can be statistically attributed to genetic differences.
- It **does not** mean “X% of your intelligence is genetic” or that IQ is unchangeable.
- Heritability is about **variation within a particular population at a particular time**, not about individuals and not about between-group differences.

---

## 2. Twin and adoption studies

These are the classic lines of evidence.

### Twin studies

Large meta-analyses of identical (MZ) vs fraternal (DZ) twins show:

- In childhood: heritability of IQ ≈ **0.2–0.4**
- In adolescence/young adulthood: ≈ **0.5–0.6**
- In adulthood: often **0.6–0.8**

This means that, among adults in typical modern environments, **about half or more** of the variation in IQ scores is statistically associated with genetic variation.

A key finding: **heritability increases with age**. Children’s intelligence is more strongly shaped by shared environment; as people age and choose environments that fit their genetic predispositions (e.g., hobbies, education level, jobs), genetic influences become more visible.

### Adoption studies

- Adopted children resemble their **biological parents** in IQ more than their adoptive parents, especially by adolescence and adulthood.
- Early in childhood, **shared environment** (home, parenting, schooling) shows clear effects on IQ. By adulthood, those shared-environment effects largely fade in many Western samples; siblings raised together are barely more similar in IQ than siblings raised apart, after controlling for genetics.

---

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

Genome-wide association studies (GWAS) in huge samples (hundreds of thousands to >1 million people) have:

- Identified **thousands** of genetic variants, each with **tiny** effects on cognitive test performance and educational attainment.
- Combined into polygenic scores, these variants can predict:
  - A modest but nontrivial fraction of IQ variance (currently on the order of **5–15%**, depending on population, sample, and measure).
  - A larger fraction of educational attainment (20%+ in some European-ancestry samples).

This is **direct DNA-level evidence** that:
- There are many genetic influences on intelligence, each small.
- Measured genetic variants align with patterns seen in twin studies, though they don’t yet capture all the heritability (the “missing heritability” problem, which is shrinking as data and methods improve).

---

## 4. Role of environment

High heritability does not mean environment doesn’t matter. Evidence for environmental effects includes:

- **Secular “Flynn effect”**: IQ scores rose about 2–3 points per decade across much of the 20th century – too fast to be genetic; driven by changes like schooling, nutrition, test familiarity, complexity of modern life.
- **Early-life deprivation and enrichment**:
  - Severe deprivation (e.g., extreme neglect, malnutrition, lack of schooling) can dramatically reduce cognitive development.
  - High-quality early interventions (intensive preschool, enriched environments) can improve cognitive and educational outcomes, though long-term IQ gains are often modest and less durable than gains in academic and life outcomes.
- **Socioeconomic status (SES)**:
  - In some US data, the **heritability of IQ is lower in low-SES families** and higher in high-SES families (genetic potential can be more fully realized when basic needs and learning opportunities are available).
  - In some European and other contexts with stronger welfare and schooling systems, this SES-by-heritability interaction is weaker or absent.

So: environment can **raise or lower the average** and can especially affect those at risk. Heritability mainly describes **how individuals differ around that average**.

---

## 5. Age patterns and “shared vs nonshared” environment

Behavioral genetics typically partitions variance into:

- **A**: Additive genetic effects
- **C**: Shared environment (family-wide: parenting, home, neighborhood, school system)
- **E**: Nonshared environment (idiosyncratic experiences, peers, illness, measurement error)

Findings for intelligence:

- **Children**:
  - A: ~20–40%
  - C: ~30–40%
  - E: remainder
- **Adults**:
  - A: ~50–80%
  - C: near 0 in many Western samples
  - E: the rest

Interpretation: As


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

**1. What “heritability of intelligence” means**

- “Intelligence” in this context usually means general cognitive ability (“g”), measured by IQ-type tests.
- “Heritability” (h²) is the proportion of **differences between people in a population** that can be statistically attributed to genetic differences.
- It is **not**:
  - The percent of your own intelligence “caused by genes.”
  - A fixed number across ages, groups, or environments.
  - Immune to environmental change: highly heritable traits can still be dramatically shifted by environment (e.g., height and nutrition).

**2. Twin and adoption studies**

These are the classic sources.

- **Meta-analyses of twin studies** (e.g., Plomin et al., Bouchard, Polderman 2015):
  - Identical (MZ) twins share ~100% of their genes.
  - Fraternal (DZ) twins share ~50% on average.
  - Differences in similarity between these groups give estimates of h².

Key findings for general intelligence (g) in Western populations:

- **Childhood**: h² ≈ 0.20–0.40
  Shared environment (family, home, schooling) plays a large role.
- **Adolescence**: h² ≈ 0.40–0.60
  Shared environment still influential, but less so.
- **Adulthood**: h² ≈ 0.60–0.80
  Most variance appears genetic; shared family environment becomes small; “nonshared” environments (different friends, experiences, illnesses, etc.) plus measurement error make up the rest.

Adoption studies show:

- Adopted children correlate in IQ with their **biological** parents more than with their **adoptive** parents by late adolescence/adulthood.
- In early childhood, there is a noticeable IQ similarity between adopted children and adoptive parents (shared environment effect), which fades with age.

**3. Age-related increase in heritability**

A consistent pattern: **heritability of IQ rises with age**. Explanations include:

- **Gene–environment correlation**: as people grow up, they select, modify, and seek environments that fit their genetic dispositions (e.g., intellectually curious people gravitate to cognitively enriching activities), amplifying genetic differences.
- **Accumulation of small genetic effects over time**.
- Early environments (parenting, early schooling, deprivation) have more visible influence in childhood; their relative importance shrinks as individuals gain autonomy.

**4. Molecular genetic evidence (GWAS and polygenic scores)**

Genome-wide association studies (GWAS) scan the genome to find variants statistically associated with educational attainment and cognitive performance.

Findings:

- Each individual DNA variant has a **tiny** effect on IQ.
- Polygenic scores (combining thousands of variants) can:
  - Predict around **10–20%** of variance in educational attainment in European-ancestry samples.
  - Predict a bit less for direct cognitive measures, but still significantly above chance.
- These molecular estimates are **lower than twin-study heritability estimates**, showing that we still haven’t identified all the causal variants or fully captured complex interactions.

The gap between twin-based h² (~0.6–0.8 in adults) and SNP-based h² (~0.2–0.3) is often called “missing heritability,” though better methods and larger samples are narrowing it.

**5. Gene–environment interplay**

The simple “genes vs. environment” framing is misleading. Key interactions:

- **Gene–environment correlation (rGE)**:
  - Passive: parents pass on both genes and environments (intelligent parents may provide more cognitively stimulating homes).
  - Evocative: children’s traits evoke certain responses (a verbally advanced child gets more books and conversations).
  - Active: individuals seek environments that match their abilities/interests.
- **Gene × environment interaction (G×E)**:
  - Some evidence suggests that in **more advantaged environments**, genetic differences are more fully expressed, leading to **higher apparent heritability**.
  - In severely deprived or chaotic environments, environmental constraints can overwhelm genetic differences, reducing observable heritability.

Findings on this “Scarr–Rowe” type interaction vary by country and measurement; some US studies find strong effects, some European studies find weaker or absent ones.

**6. Group and context differences**

Heritability is **population- and context-specific**:

- A trait can be highly heritable in one environment (e.g., well-nourished, universal schooling) and much less so where basic conditions vary widely.
- High heritability **does not** imply:
  - That group differences (e.g., between racial or socioeconomic groups) are genetic in


text:
Human intelligence is **substantially heritable**, but not fixed, and genes are not the whole story. The short summary from modern behavioral genetics and genomics is:

- In wealthy, stable environments, **60–80% of the variation in adult IQ within a population is heritable**.
- In childhood, heritability is typically **30–50%**, rising with age.
- The **environment** (family, education, nutrition, stress, etc.) explains the rest of the variation, and can still have large effects on individuals and groups.
- These are **within-population** estimates; they do *not* directly explain differences between groups.

Below is what the evidence shows, and what it does *not* show.

---

## 1. What “heritability” really means

Heritability (h²) is:

> The proportion of *observed variation in a trait in a specific population and environment* that can be statistically attributed to **genetic differences** between individuals.

Important points:

- It is **about variation, not destiny**. A heritability of 0.7 does *not* mean “70% of your intelligence is from genes”.
- It is **population- and context-specific**. Change the environment a lot, and the heritability estimate can change.
- High heritability does **not** mean the trait can’t be changed (height is highly heritable, yet average height has risen with nutrition).

---

## 2. Evidence from twin and adoption studies

### Twin studies

Comparing:

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

In large samples, for general intelligence (g or IQ):

- Correlation of IQ:
  - MZ twins reared together: often **0.80–0.90**
  - MZ twins reared apart: typically **0.70–0.80**
  - DZ twins reared together: about **0.40–0.60**

These patterns imply substantial genetic influence.

Meta-analyses (e.g., T. J. Bouchard, N. Segal, and later work summarized by Plomin et al.) converge on:

- **Childhood IQ heritability**: ~0.3–0.5
- **Adolescent IQ heritability**: ~0.5–0.6
- **Adult IQ heritability**: ~0.6–0.8 in affluent, developed countries

This **increase with age** is a robust finding.

### Why does heritability increase with age?

Most likely:

- Children have less control over their environments.
- As people age, they **select, modify, and create environments** aligned with their genetic dispositions (called **gene–environment correlation**).
  Example: a child with a genetic tilt toward reading may choose more books, seek intellectually demanding friends and careers, etc.

### Adoption studies

Adoption studies compare:

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

Findings:

- In childhood, adopted kids resemble **both** their biological and adoptive families to some extent.
- By late adolescence/adulthood, their IQ correlates **much more strongly with biological parents** and **weakly or not at all** with adoptive parents.
- This again supports strong genetic influence, plus a **decreasing shared-family environmental effect** with age.

---

## 3. What about specific genes? (Molecular genetics and GWAS)

Older work failed to find “IQ genes” with big effects. Modern genome-wide association studies (GWAS) show why:

- **Intelligence is highly polygenic**: thousands of variants, each with **tiny** effects.
- Large GWAS (e.g., UK Biobank, Sniekers et al. 2017; Savage et al. 2018; de la Fuente et al. 2020 and later) have:
  - Identified **thousands** of SNPs associated with educational attainment and cognitive ability.
  - Built **polygenic scores (PGS)** that can statistically predict part of an individual’s IQ or educational outcomes.

In European-ancestry samples:

- Polygenic scores currently explain roughly **10–20%** of the variance in IQ / cognitive test scores, and **10–25%** for educational attainment, depending on the study and sample.
- That’s **less than twin-based heritability**, due to:
  - Many causal variants not yet found.
  - Rare variants and structural variants not fully captured.
  - Imperfect modeling of gene–environment interplay.
  - Population stratification issues.

The gap between twin-based (~0.6–0.8) and SNP-based (~0.2 or so) heritability is shrinking as data and methods improve,


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

1. **How much of the variation in IQ *within a given population* is due to genetic differences?**
2. **How much can an individual’s abilities change due to environment, education, and life events?**
3. **What (if anything) does this say about group differences?**

### 1. Heritability: numbers and what they mean

**Heritability (h²)** in this context = the proportion of *differences between people* in a particular population, at a particular time, that can be statistically attributed to genetic differences.

Findings from large twin, family, and genomic studies:

- **Childhood:** heritability of IQ ~ **0.3–0.4** (30–40%)
- **Adolescence:** ~ **0.5–0.6**
- **Adulthood:** often **0.6–0.8** in high‑income countries

So by late adulthood, most of the *observable variation* in general cognitive ability within those populations is associated with genetic variation.

Crucial caveats:

- This **does not** mean “intelligence is 60–80% genetic for an individual.”
- Heritability is **population-specific and environment-specific**. In a very unequal or very variable environment, heritability can be lower; in a very uniform environment, it can be higher.
- High heritability **does not imply immutability**; height is highly heritable too, but average height changed greatly with nutrition and health.

### 2. Evidence from different methods

**Twin and adoption studies**

- **Monozygotic (identical) twins** share ~100% of segregating genes; **dizygotic (fraternal) twins** share ~50% on average.
- Meta-analyses show identical twins are **much more similar** in IQ than fraternal twins, even when raised apart, pointing to strong genetic influence.
- **Adoption studies**: adopted children’s IQs correlate more with **biological** than **adoptive** parents by adolescence, indicating genetic effects that strengthen with age.

At the same time:

- Early in life, **shared environment** (family, adopted home, schooling quality, etc.) explains a significant portion of variance.
- In adulthood, shared-family effects often shrink, and **nonshared environment** (individual experiences, peers, illnesses, accidents, education choices) plus genetics dominate.

**Genomic (DNA-based) studies**

- Large genome-wide association studies (GWAS) have found **thousands of variants** each with tiny effects on educational attainment and cognitive ability.
- **SNP-based heritability** (using measured common variants) typically recovers maybe **20–30%** of IQ variance — lower than twin estimates, because:
  - many contributing variants are rare or not well measured,
  - gene–gene and gene–environment interactions are hard to capture.
- **Polygenic scores** constructed from many variants can moderately predict individual differences within a studied population (more strongly for education than for IQ itself), but:
  - they are far from deterministic,
  - they perform substantially worse when moved across ancestral groups or very different environments.

Together, twin, adoption, and genomic studies converge on a picture of **substantial but incomplete genetic influence**.

### 3. Developmental pattern: heritability changes with age

A robust finding: heritability of cognitive ability **increases with age**.

Likely reasons:

- As people age, they have more freedom to **select and shape environments** that fit their genetically influenced preferences and abilities (e.g., seeking cognitively demanding jobs or hobbies).
- Early environments are more controlled by family and childhood circumstances; later environments reflect more “gene–environment correlation.”

So genetics tends to **amplify** with time as people move into niches that match their predispositions.

### 4. Environmental effects are large and real

High heritability does not contradict sizeable environmental influences:

- **Education**: extra years of schooling reliably increase measured IQ and related abilities; natural experiments (e.g., compulsory schooling reforms) support causality.
- **Nutrition and health**: severe malnutrition, iodine deficiency, lead exposure, infectious disease burdens all measurably depress cognitive development.
- **Early childhood adversity** (extreme deprivation, chronic abuse, institutionalization) can have large, lasting negative effects, sometimes partially reversible with intervention.
- **Historical changes**: the **Flynn effect**—substantial IQ gains across much of the 20th century in many countries—occurred far too quickly to be genetic, implicating education, health, and complexity of modern environments.

So while *relative* differences in IQ within a well-off, schooled population may be strongly heritable, **the whole distribution can


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

---

## 1. What “heritability” actually means

- **Heritability (h²)** = the proportion of *variation* in a trait *within a population* that can be statistically attributed to genetic differences.
- It does **not** mean “the percentage of your intelligence that comes from genes.”
- It also:
  - Does *not* apply to individuals (only groups).
  - Depends on that population’s environment—change the environment, and the heritability value can change.

---

## 2. Twin and family studies: broad pattern

Studies of identical (monozygotic, MZ) and fraternal (dizygotic, DZ) twins, adoptees, and families have been the backbone of this research.

Typical findings in modern Western samples:

- **Childhood (early)**:
  - IQ heritability ≈ **0.2–0.4** (20–40%)
- **Adolescence / young adulthood**:
  - IQ heritability ≈ **0.4–0.6** (40–60%)
- **Adulthood**:
  - IQ heritability ≈ **0.6–0.8** (60–80%) in many high-income, relatively equal societies

Key studies and consortia (e.g., the Minnesota Twin Study, Swedish Adoption/Twin studies, and large meta-analyses) repeatedly show that:

- Identical twins reared apart are **much more similar** in IQ than fraternal twins reared together.
- Biological siblings are more similar than adoptive siblings raised together.
- Shared family environment (same household, same parents) explains *some* IQ similarity in childhood, but its influence shrinks markedly by adulthood.

---

## 3. Age effect: why heritability rises over time

The heritability of IQ tends to **increase with age**. Main explanations:

1. **Gene–environment correlation**:
   As people grow, they:
   - Choose and shape environments more in line with their genetic dispositions (e.g., seeking cognitively demanding jobs, hobbies, and peers).
   - This causes genetic differences to become more expressed over time.

2. **Waning impact of shared childhood environment**:
   - Early on, diet, parenting, and schooling matter a lot.
   - In adulthood, individuals diverge in environments and life paths; common family influences fade.

Result:
By middle adulthood, genetic influences account for a larger share of *remaining* differences in IQ among people in those populations.

---

## 4. Role of environment: not trivial

High heritability ≠ “environment doesn’t matter.”

Evidence that environment substantially impacts cognitive ability:

- **Socioeconomic status (SES)**:
  - In the U.S., several studies show **lower heritability of IQ in low-SES families**, and higher heritability in high-SES families.
    - In deprived contexts, environmental constraints limit potential, compressing genetic variation; in better contexts, genetic differences can “show through” more.
  - European and some other countries with stronger social safety nets sometimes show *weaker* SES moderation, implying context matters.

- **Nutrition and health**:
  - Severe malnutrition, iron deficiency, iodine deficiency, exposure to toxins (e.g., lead), and chronic disease can reduce cognitive development.
  - Improving these factors has measurable positive effects on cognitive outcomes.

- **Education and cognitive stimulation**:
  - Extra schooling, richer learning environments, cognitively demanding work, and targeted interventions (especially in early childhood) can raise cognitive performance.
  - Randomized policy changes that extend schooling often show small but robust increases in IQ or closely related cognitive measures.

- **Historical change: the “Flynn effect”**:
  - Across much of the 20th century, many countries saw average IQ scores rise by ~2–3 points per decade.
  - Genes don’t change that fast; this points to powerful environmental effects (education, complexity of modern life, nutrition, smaller families, etc.).
  - In some countries this effect has slowed or reversed, again implicating environment.

Heritability only describes **variance at a given time in a given group**; it does not tell you the *maximum* that environment can do.

---

## 5. Molecular genetics: what specific genes show

With genome-wide association studies (GWAS):

- **Findings**:
  - Intelligence (and strongly related traits like years of education) are **polygenic**: influenced by thousands of genetic variants, each with tiny effects.
  - Polygenic scores derived from these variants can predict:
    - A modest but nontrivial portion of variance in IQ and educational attainment (often ~5–20% in European-ancestry samples


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

### 1. What “heritability” actually means

- In this context, **heritability (h²)** is the proportion of **variation in intelligence within a population** that can be statistically attributed to genetic differences between people.
- It **does not** mean “the percentage of your intelligence that comes from genes.”
- It also **does not** mean immutable: a trait can be highly heritable and still be very responsive to environment (e.g., height and nutrition).

Heritability estimates are about **differences among people in a given population at a given time**, not about individuals or all humans in all environments.

---

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

These are the classic sources of evidence.

**Key patterns:**

- **Monozygotic (identical) twins** raised together: IQ correlations ~0.80–0.90
- **Monozygotic twins raised apart**: ~0.70–0.80
- **Dizygotic (fraternal) twins** raised together: ~0.50–0.60
- **Biological siblings** raised together: ~0.40–0.50
- **Adopted (genetically unrelated) siblings** raised together: near 0 by adulthood

These patterns support:
- A strong **genetic effect** (identical > fraternal > unrelated);
- A **shared family environment effect** that is noticeable in childhood but largely fades in adulthood.

Meta-analyses (e.g., Polderman et al., 2015, >2.7 million twin pairs across traits) find for general cognitive ability:

- **Childhood**: heritability ~0.30–0.40
- **Adolescence**: ~0.50–0.60
- **Adulthood**: often ~0.70–0.80

So in affluent, relatively stable societies, **most of the variation in adult intelligence appears to be genetic** in origin, with the remainder largely due to **non-shared environment** (experiences unique to individuals, measurement error, etc.).

---

### 3. Shared vs non-shared environment

Behavior-genetic models usually partition variance into:

1. **A (Additive genetic)** – what we call heritability.
2. **C (Common/Shared environment)** – aspects of environment that make siblings more similar (same parents, home, schools).
3. **E (Non-shared environment + error)** – aspects that make siblings different (different peers, illnesses, accidents, micro-experiences, plus measurement noise).

For IQ:

- **Shared environment (C)**:
  - More important in childhood (can be ~20–30% of variance).
  - Shrinks toward zero by late adolescence/adulthood in many studies.
- **Non-shared environment (E)**:
  - Typically accounts for **20–30%** of variance across the lifespan.

This means siblings raised together do not end up similar in IQ primarily because of the shared home; instead, genes + unique experiences dominate.

---

### 4. Age trends: “Wilson effect”

The **heritability of intelligence increases with age** (the “Wilson effect”):

- Early childhood: maybe ~20–30% genetic.
- Middle childhood: ~40–50%.
- Adolescence: ~50–60%.
- Adulthood: ~70–80%.

Interpretation: as people age, they gain more freedom to select, shape, and seek environments that align with their genetically influenced traits (gene–environment correlation), amplifying genetic differences.

---

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

Modern genomic studies confirm and refine twin-study findings:

- Intelligence and educational attainment are **highly polygenic**: thousands of DNA variants each with minuscule effects.
- Large GWAS (e.g., UK Biobank, other consortia) identify many associated variants.
- **SNP-based heritability** (using common variants) for cognitive ability is typically:

  - ~20–30% of variance explained at the SNP level (this is a lower-bound on total heritability, since rare variants and other genetic effects are undercounted).

- **Polygenic scores** for cognitive ability or education can now predict:

  - Roughly **10–20%** of the variance in educational attainment;
  - Less (but non-trivial) for directly measured IQ in independent samples.

This is consistent with twin-based heritability once you remember that:
- GWAS capture part of the total genetic variance.
- Twin studies also pick up rare variants, structural variants, and some non-additive effects.

---

### 6. Heritability depends on environment

Heritability is **context-sensitive**:

-


text:
Human intelligence is substantially heritable, but not fixed, and environment still matters a lot. What’s best established is:

- In modern, reasonably well-fed, literate societies, **about 50–80% of the variation in adult IQ scores is due to genetic differences** (broad-sense heritability, h² ≈ 0.5–0.8).
- Heritability is **lower in childhood**, increases into adulthood, and can be **much lower in harsh or unequal environments**.
- High heritability **does not mean** intelligence is unchangeable, nor that genes “determine” individual destiny.

Below is what the main lines of evidence show.

---

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

- **Heritability (h²)**: the proportion of *observed variation* in a trait in a particular population at a particular time that can be statistically attributed to genetic differences among individuals.
- It **does not** mean:
  - “X% of your intelligence comes from genes, Y% from environment.”
  - “Intelligence is mostly genetic, so environment doesn’t matter.”
  - “A high-heritability trait cannot be changed.”

Heritability is a **population statistic**, not an individual breakdown, and it depends heavily on environmental conditions and how varied they are.

---

## 2. Major sources of evidence

### A. Twin studies

Twin studies compare:

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

If MZ twins are more similar in IQ than DZ twins, that implicates genetic influence.

Typical findings (in Western samples):

- **Childhood IQ**:
  - Heritability: ~0.3–0.5
  - Shared environment (family-wide factors): substantial (maybe 0.3–0.4)
- **Adulthood IQ**:
  - Heritability: ~0.6–0.8
  - Shared environment: near-zero; remaining variance is mostly “nonshared” (individual-specific experiences, measurement error, etc.)

These patterns come from many large studies and meta-analyses (e.g., work by Plomin, Deary, Bouchard, etc.).

### B. Adoption studies

Adoption studies separate genes and rearing environment.

Key findings:

- **Biological vs. adoptive parents**:
  - As adopted children grow older, their IQ correlates more strongly with their **biological** parents’ IQ and less with their adoptive parents’.
- **Early vs. late childhood**:
  - Early on, there is a noticeable resemblance to adoptive parents (shared environment effects), which **fades** in adolescence/early adulthood.
- **Adoption effects on mean IQ**:
  - Children adopted from deprived backgrounds into more advantaged families often show **substantial IQ gains** (commonly ~10–15 points, sometimes more), even though heritability is high overall.

So: genes affect individual differences, but **changing environments can noticeably shift average levels**.

### C. DNA-based (molecular) studies

Modern genetics can estimate heritability directly from DNA, and identify specific genetic variants.

1. **SNP-heritability (GREML, etc.)**
   - Looks at how phenotypic similarity tracks with measured genetic similarity for many unrelated people.
   - Typically finds **h² ≈ 0.2–0.5** for cognitive ability, depending on sample, methods, and which variants are captured.
   - This is a **lower bound** on total heritability (it only counts the common measured variants under a specific model).

2. **Genome-wide association studies (GWAS)**
   - Thousands to millions of people genotyped and tested.
   - Hundreds to thousands of loci associated with intelligence/educational attainment.
   - Each variant has **tiny effects**; combined polygenic scores can now explain **~10–20%** of variance in cognitive performance or educational attainment in large European-ancestry samples.
   - Still far from accounting for all twin-based heritability, but the gap has been narrowing with bigger, better studies.

3. **Polygenic scores across environments**
   - Predictive power of polygenic scores often **changes with environment** (e.g., SES, schooling quality), showing **gene–environment interaction**: genetic potentials are more fully expressed in supportive conditions.

---

## 3. How heritable is IQ across the lifespan?

From many longitudinal and meta-analytic studies:

- **Early childhood (3–7 years)**: h² ≈ 0.2–0.4
- **Middle childhood to adolescence (8–18 years)**: h² ≈ 0.4–0.6
- **Young to mid-adulthood


text:
Human intelligence is substantially heritable, but not “genetically fixed,” and environment still matters a lot. The best current summary from mainstream behavioral genetics is:

- In adulthood, about **50–80%** of the *differences between people* in general cognitive ability (often indexed by IQ or g) in a given population are associated with genetic differences.
- In childhood, heritability is typically lower, around **20–40%**, and increases with age.
- None of this means your intelligence is “X% genetic and Y% environmental” in some simple way; it refers to variation *between people* in a particular time, place, and set of environments.

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

---

## 1. What “heritability” actually means

**Heritability (h²)** is:

> The proportion of *observed variation* in a trait in a population that can be statistically attributed to genetic differences among individuals in that population.

Key implications:

- It’s about **variation within a group**, not about what caused the trait to exist at all.
- It **depends on the environment**:
  - In a *more equal* environment (e.g., universal schooling, good nutrition), environmental variation shrinks, so genetic differences explain **more** of the remaining variation → higher heritability.
  - In a *more unequal or harsh* environment, environmental differences loom larger → lower heritability.
- It **does not** tell you:
  - What fraction of one person’s intelligence is “due to genes.”
  - That a trait is “unchangeable.” Height is ~80–90% heritable in many rich countries, but average height rose markedly with improved nutrition and health.

---

## 2. Main lines of evidence

### a. Twin studies

Twin studies compare:

- **Monozygotic (MZ, identical) twins**: ~100% of segregating genes shared.
- **Dizygotic (DZ, fraternal) twins**: ~50% of segregating genes shared, like ordinary siblings.

If MZ twins are more similar than DZ twins in IQ, that points to genetic influence.

Findings from large meta-analyses:

- Young children: IQ heritability ≈ **0.2–0.4**
- Adolescents: ≈ **0.4–0.6**
- Adults: ≈ **0.5–0.8**

Notably:

- IQ similarity **increases with age** among twins reared together and apart. This is somewhat counterintuitive but robust:
  - As people age, they increasingly select and shape environments that fit their predispositions (e.g., seek intellectually demanding jobs or hobbies), amplifying genetic differences (“gene–environment correlation”).

### b. Adoption and “twins reared apart” studies

These allow a separation of family environment from genetics.

- **Adoption studies**:
  - Correlations between *adopted children* and their **biological** parents or siblings: modest but clear and tend to rise with age.
  - Correlations between adopted children and their **adoptive** parents: positive in early childhood but often fade to near zero by late adolescence for general intelligence.
  - Interpretation: the lasting similarity tracks shared genes more than shared home environment, though early environment still matters.

- **MZ twins reared apart**:
  - Average correlations in IQ are usually in the **0.7–0.8** range in adulthood in classic studies.
  - That’s nearly as high as MZ twins raised together, implying:
    - A large genetic contribution.
    - A relatively modest, lasting effect of the specific *shared family environment* (the home).

But note: “reared apart” environments were often still relatively similar in broad terms (same country, language, schooling norms), so this doesn’t rule out environmental effects at the societal level.

### c. Molecular genetics (DNA-based studies)

These look directly at DNA rather than inferring genetic influence from similarity patterns.

- **Genome-wide association studies (GWAS)**:
  - Large studies (hundreds of thousands to millions of people) have identified thousands of genetic variants that each have tiny effects on educational attainment and cognitive performance.
  - Individually, each variant is almost useless for prediction; in aggregate (polygenic scores), they explain:
    - Roughly **10–20%** of the variance in educational attainment in some European-ancestry samples.
    - Less for measured IQ, but still significant.
  - This is lower than twin-based heritability, but that gap is expected:
    - GWAS only capture common variants and are incomplete.
    - “Missing heritability” is shrinking as sample sizes and methods improve.

- DNA-based methods like **GREML** (Genome-based Restricted Maximum Likelihood) estimate heritability from genetic similarity between unrelated individuals and tend to find: