text:
Human intelligence is substantially heritable, but not “genetically fixed.” Genes account for a large fraction of the differences between people in the same population, while environment still matters a lot—especially early in life.

Here’s what the evidence shows.

---

## 1. What “heritability” actually means

- Heritability (h²) = proportion of **variation in a trait within a population** that is due to genetic differences.
- It is:
  - **About differences, not individuals.** You can’t say “John’s IQ is 60% genetic.”
  - **Population- and context-specific.** Heritability can change across time, places, and social conditions.
  - **Not destiny.** High heritability doesn’t mean the trait can’t be changed by environment or policy (e.g., height is highly heritable but changed massively with nutrition and health).

---

## 2. Twin and family studies: broad picture

Most classic evidence comes from comparing:

- **Monozygotic (MZ) twins**: share ~100% of segregating DNA
- **Dizygotic (DZ) twins**: share ~50% on average
- **Adoption studies**: separate genetic from rearing environment

Key patterns:

- MZ twins are **much more similar in IQ** than DZ twins.
- Adopted children resemble their **biological** parents in IQ more than their adoptive parents by adulthood.

Typical estimates (for general intelligence, *g*):

- **Childhood:** h² ≈ 0.2–0.4
- **Adolescence:** h² ≈ 0.4–0.6
- **Adulthood:** h² ≈ 0.6–0.8 (often quoted ~0.7)

So in many high‑income, relatively stable environments, **about half to three‑quarters of IQ variation** in adults is attributable to genetic differences, with the rest due to environmental and measurement factors.

---

## 3. Developmental pattern: heritability increases with age

A robust finding is that the heritability of intelligence **rises from childhood into adulthood**:

- Early life: environments differ a lot (nutrition, illness, school quality, home chaos), so environment explains more variance.
- With age:
  - People **select and shape environments** that match their tendencies (gene–environment correlation: e.g., choosing challenging coursework, reading more).
  - Some early environmental constraints weaken (e.g., nutritional deficits, extreme deprivation).

This produces “genetic amplification”: small genetic differences early on can lead to larger differences in skills and knowledge over time.

---

## 4. Environment is still powerful

Even with high heritability:

- **Severe deprivation** (institutionalization, neglect, extreme poverty) can drastically depress IQ; removal from such conditions can yield large gains.
- **Iodine, iron, lead exposure, prenatal health**, and early nutrition affect cognitive development.
- **Schooling** increases measured IQ and specific cognitive skills; missing years of school reduces IQ scores.
- **The Flynn effect**: 20th‑century rise in IQ scores (about 2–3 points per decade in many countries) is far too fast to be genetic. It reflects environmental changes (education, complexity of daily life, health, etc.).

Implication: a trait can be highly heritable **within** a given environment, while large **between‑cohort** or **between‑country** differences are mostly environmental.

---

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

Large genome‑wide association studies (GWAS) have:

- Identified **thousands** of genetic variants each with **tiny** effects on intelligence and educational attainment.
- Constructed **polygenic scores (PGS)** that aggregate these variants.

Findings:

- In European‑ancestry samples, current PGS explain roughly:
  - ~10–20% of the variance in educational attainment
  - ~5–15% (very roughly) of the variance in cognitive test performance, depending on sample and measure

These “SNP heritability” estimates are **lower** than twin‑study heritability (they don’t capture rare variants, structural changes, etc.), but they **point in the same direction**: intelligence is partly polygenic and highly distributed across the genome.

Caveats:

- PGS are **not precise predictors** of individual outcomes.
- Their predictive accuracy **drops in other ancestries** due to population structure and different linkage patterns.
- Many GWAS associations likely reflect some combination of biology and social pathways (e.g., genes affecting personality, health, or social mobility that in turn affect education and cognition).

---

## 6. Gene–environment interplay

Two crucial concepts:

1. **Gene–environment interaction (G×E)**
   - Genetic influences can be stronger or weaker in different environments.


prompt_id: sci-iq-heritability

provider: openai

text:
Human intelligence is substantially heritable, but not fixed, and strongly shaped by environment. The best-supported estimates and findings look like this:

## 1. What “heritable” means

- **Heritability (h²)**: the proportion of variation in a trait *within a population* that’s attributable to genetic differences *in that population*, at a given time.
- It does **not** mean “X% of your IQ comes from genes and Y% from environment.” It’s a population statistic, not an individual one.
- Heritability can change with context (e.g., quality of schooling, nutrition, inequality).

## 2. Overall heritability of intelligence

Most data focus on **general cognitive ability (g)** or IQ.

### Twin and family studies

- In modern Western populations:
  - **Childhood**: heritability often around **0.3–0.5** (30–50%).
  - **Adolescence**: roughly **0.5–0.6**.
  - **Adulthood**: often **0.6–0.8** (60–80%).

Meta-analyses (e.g., by Ian Deary, Robert Plomin, others) converge on:
- **Adult IQ heritability ≈ 0.5–0.8** in relatively affluent, schooled societies.

### Why does heritability rise with age?

Evidence points to:
- Children’s increasing ability to **select and shape** their environments (books, friends, careers).
- As you age, your experiences become more correlated with your genetic predispositions (“gene–environment correlation”).

## 3. Shared vs nonshared environment

Behavioral genetics divides environment into:
- **Shared environment**: aspects that make siblings similar (same home, parenting style, neighborhood).
- **Nonshared environment**: experiences that make siblings different (different peers, teachers, illnesses, random life events).

Findings:
- **Shared environment** affects childhood IQ more (often 20–30% of variance).
- Its effect tends to **shrink by adulthood**, often approaching near-zero in twin models.
- **Nonshared environment** (plus measurement error) accounts for the remaining variation.

This does *not* mean family and school don't matter; it means:
- Their effects often **don’t make siblings within the same family very different in IQ**, once genes are accounted for.
- Society-level differences (e.g., better schools for everyone, less lead exposure, improved nutrition) **can still have huge effects on average IQ**.

## 4. Molecular genetic evidence

With genome-wide association studies (GWAS):

- Thousands of small-effect variants are associated with intelligence and educational attainment.
- **Polygenic scores** built from these variants:
  - Explain **~10–20% of the variance** in cognitive test scores or educational attainment in large European-ancestry samples.
  - Predict things like school performance and years of education, but far from deterministically.
- This molecular evidence is **consistent with**, but still lower than, twin-based heritability estimates (due to imperfect tagging of causal variants, incomplete models, etc.).

## 5. Environmental effects are real and large

Strong evidence that environment can **meaningfully shift cognitive performance**:

- **Education**: extra years of schooling causally raise IQ and test scores (e.g., natural experiments using school-leaving-age laws).
- **Nutrition & health**: severe malnutrition and some deficiencies (e.g., iodine, iron) depress cognitive functioning.
- **Toxins**: lead exposure reduces IQ; lowering lead exposure increases population-level cognitive outcomes.
- **Early deprivation**: extreme neglect (e.g., in some orphanage studies) yields large cognitive deficits; adoption into enriched homes often yields substantial, lasting gains.
- **Flynn effect**: across the 20th century, many countries saw IQ scores rise ~2–3 points per decade, too fast to be genetic. Likely due to education, complexity of environments, health, and test familiarity.

These effects show intelligence is **highly modifiable**, especially at the population and policy level.

## 6. Socioeconomic status (SES) and heritability

Some studies (notably in the US) suggest:
- In **low-SES** contexts: IQ heritability is **lower** and shared environmental effects **larger**.
- In **high-SES** contexts: heritability is **higher** and shared environment effects smaller.

Interpretation:
- When basic needs and educational resources vary a lot, environment becomes more limiting, so genes can’t fully “show up.”
- Once a decent minimum environment is widely available, genetic differences explain more of what variation remains.

Evidence on this SES–heritability interaction is mixed across countries but robust enough in some datasets to be taken seriously.

## 7. Group differences vs. heritability

A critical distinction:

- **High

finish_reason: length


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

---

## 1. What does “heritability” mean?

- **Heritability (h²)** = the proportion of variation in a trait *within a specific population, at a specific time, in a specific environment* that is due to genetic differences.
- It does **not** mean “X% of your intelligence comes from genes and Y% from environment.” It’s about variation among people, not a split inside one person.
- If environments become more equal, heritability tends to **increase**, because remaining differences are more due to genes.

---

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

Using IQ or g (general cognitive ability):

- **Monozygotic (identical) twins** share ~100% of genes.
- **Dizygotic (fraternal) twins** share ~50% on average.
- If genes matter, identical twins should be more similar in IQ than fraternal twins.

Meta-analyses of twin and family studies consistently find:

- **Childhood:** heritability ≈ 0.3–0.4 (30–40%)
- **Adolescence:** ≈ 0.4–0.6 (40–60%)
- **Adulthood:** ≈ 0.6–0.8 (60–80%)

So, in relatively affluent, industrialized countries, **adult intelligence is often estimated to be about 50–80% heritable**.

Adoption studies back this up:

- Adopted children’s IQs resemble their **biological** parents more than their **adoptive** parents by adolescence/adulthood.
- As they age, correlations with adoptive parents’ IQs fall, and with biological parents’ IQs rise.

This pattern (genetic influence increasing with age) is very robust.

---

## 3. Molecular genetics: what do actual DNA studies say?

### GWAS and polygenic scores

Modern studies use:

- **Genome-wide association studies (GWAS)**: scan the genome for variants linked to educational attainment and cognitive performance.
- **Polygenic scores (PGS)**: combine the small effects of thousands of variants into one score.

Findings:

- Current polygenic scores explain **~10–20%** of the variance in educational attainment and somewhat less for IQ directly in large European-ancestry samples.
- That seems lower than twin estimates because:
  - We haven’t captured all variants yet.
  - Rare variants, gene–gene interactions, epigenetics, etc., aren’t fully modeled.

Still, PGS robustly predict:

- Test scores
- Years of education
- Occupational status
- Even income and some health outcomes (through cognitive and educational pathways)

This **confirms that intelligence-related traits are substantially influenced by many genetic variants**, each with tiny effects.

---

## 4. Environment still matters – a lot

Even with high heritability, **environmental effects are large and consequential**, especially:

- **Early deprivation or malnutrition** can severely impair cognitive development.
- **Lead exposure**, extreme poverty, chronic stress, and lack of schooling lower average IQ.
- **Education, cognitive stimulation, safety, and nutrition** can raise performance substantially.

Two key points:

1. **Heritability is not immutability.**
   Height is ~80–90% heritable in many countries, yet average height changed by many centimeters over the 20th century due to diet and health. Intelligence is similar in that respect.

2. **Heritability varies by environment.**
   In disadvantaged settings, heritability of IQ can be **lower**, because environmental constraints dominate outcomes. In more advantaged, equalized environments, genetic differences explain more of what variation remains.

---

## 5. The age pattern: “growing into your genes”

A consistent finding:

- **Genetic influence appears to increase with age.**
- Shared family environment (home, parenting style, etc.) influences IQ more in childhood, and **much less** by late adolescence/adulthood.
- Nonshared environment (individual experiences, peers, chance events) and measurement error account for much of the rest.

A leading interpretation: as people get older, they **select, modify, and create environments** that fit their genetically influenced preferences and abilities (gene–environment correlation).

---

## 6. Group differences and what heritability cannot tell you

Important limitations:

- High heritability **within** a group says nothing about the **cause of differences between groups**.
  - Example: If two countries have different average heights due to nutrition, height can still be highly heritable within each country.
- For IQ, debates about average differences between racial/ethnic or national groups **cannot be settled from heritability estimates alone**.
  - Those


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

## 1. Heritability: What the numbers look like

Heritability (h²) = the proportion of *differences between people* in a trait that can be statistically attributed to genetic differences in a given population at a given time. It does **not** mean “X% of your intelligence comes from genes.”

**Twin and family studies**

- In modern, developed societies, estimates for general intelligence (g) usually fall around:
  - **Childhood:** h² ≈ 0.3–0.4
  - **Adolescence:** h² ≈ 0.5–0.6
  - **Adulthood:** h² ≈ 0.6–0.8

So, by adulthood, 60–80% of the *variation* in intelligence test scores within a population is linked to genetic variation in that population.

**Genome-wide (molecular) studies**

Using measured DNA (SNP-based heritability):

- SNP heritability for intelligence is typically:
  - **≈ 0.2–0.4** (20–40%)

This is lower than twin-based estimates because:
- Current genotyping captures only common variants.
- Rare variants, structural variants, gene–gene, and gene–environment interactions are missed.
- Twin studies also include some unmodeled environmental effects as “genetic.”

Both lines of evidence, though imperfect, converge on: **intelligence is substantially heritable**.

---

## 2. Environment still matters a lot

High heritability does **not** mean the trait is immune to environment.

- **Shared environment (family, home, schooling)**:
  - Larger role in childhood (≈ 20–30% of variance).
  - Drops toward near-zero in many adult twin samples, meaning that siblings raised together don’t stay similar just because of the family environment they shared. They diverge via genes + individual experiences.

- **Non-shared environment** (experiences unique to each person, randomness, measurement error) remains important across life.

Clear environmental effects:

- **Severe deprivation/neglect** (e.g., Romanian orphanage studies) can reduce IQ by 20–30 points, sometimes more.
- **Iodine and micronutrient deficiency**, lead exposure, and some prenatal insults can noticeably depress cognitive development.
- Dramatic historical changes (nutrition, education, health) are associated with large **population-level IQ gains** (the Flynn effect), far too fast to be genetic.

So, even a highly heritable trait can be *strongly modifiable* by environmental factors, especially early in life.

---

## 3. Heritability changes with age

One of the most robust findings:

- Genetic influence on intelligence **increases with age**.
- Environmental influences, particularly shared family environment, **decrease with age**.

Likely reasons:

- As people grow up, they select, modify, and create environments that fit their genetically influenced preferences and abilities (gene–environment correlation).
- Early on, family and schooling structure experiences; later, people self-sort into niches that let genetic differences “amplify” over time.

---

## 4. What about specific genes?

Intelligence is **highly polygenic**:

- Thousands of variants each have *tiny* effects.
- No single “intelligence gene.”

Large GWAS (genome-wide association studies) have:

- Identified many loci associated with educational attainment and cognitive performance.
- Produced **polygenic scores** that can predict a portion of variance in test scores and educational outcomes.
  - In European-ancestry samples, current polygenic scores explain up to ≈ 10–15% of variance in educational attainment, and somewhat less for direct cognitive measures.
  - Predictive power is substantially lower in non-European ancestries because of biased discovery samples and linkage structure differences.

Important caveats:

- Polygenic scores are **probabilistic**, not deterministic.
- They can be partially confounded by subtle population structure, assortative mating, and social factors correlated with genetics.

---

## 5. Group differences and limits of inference

Evidence for high heritability **within** a population does not automatically explain differences **between** groups (e.g., social classes or racial/ethnic groups).

Key points:

- Heritability is **population- and environment-specific**. Change the environment, and heritability can change.
- Between-group differences (e.g., average IQ differences between countries or socio-economic groups) can be heavily shaped by:
  - Nutrition, health, schooling quality, stress, discrimination, test familiarity, and broader social conditions.
  - The Flynn effect shows that average IQ can rise by >10 points in a couple of generations from environmental changes alone.

The consensus in mainstream behavioral genetics and psychology is:

- Intelligence is substantially heritable within


text:
Human intelligence is substantially heritable, but not fixed, and genes are only part of the story. Here’s what the evidence shows, in a structured way.

---

## 1. What does “heritability” mean?

- **Heritability (h²)**: The proportion of *differences between people* in a trait that can be statistically attributed to genetic differences in a particular population at a particular time.
- It is **not**:
  - The percent of *your* intelligence that “comes from genes.”
  - A measure of immutability (highly heritable traits can still be heavily influenced by environment and policy).

Think of it as: *“Among people living in this context, how much of the variation in test scores is associated with genetic variation?”*

---

## 2. Twin and family studies

These are the classic evidence base.

### Cross‑sectional pattern

- **Childhood IQ:** heritability ~0.3–0.5 (30–50%)
- **Adolescence:** ~0.5–0.7
- **Adulthood:** ~0.6–0.8 (often ~0.7 in well-studied samples)

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

**Key designs and findings:**

- **Monozygotic (MZ) vs dizygotic (DZ) twins**:
  - MZ twins (sharing ~100% of segregating genes) are more similar in IQ than DZ twins (~50% of genes), even when both are raised together.
  - In many adult samples, MZ twin correlations for IQ ≈ 0.75–0.85, DZ ≈ 0.35–0.45.
- **Adoption studies**:
  - Children adopted away at birth resemble their *biological* parents more than their adoptive parents in cognitive ability by adolescence/adulthood.
  - Correlation with adoptive parents’ IQ tends to drop toward zero by late teens; correlation with biological parents rises toward ~0.3–0.4.
- **Reared-apart twins**:
  - Identical twins raised apart show IQ correlations similar to those raised together, indicating genetic influence even in different rearing environments (though usually not *wildly* different in socioeconomic terms).

Meta-analyses (e.g., by Ian Deary and others) consistently suggest an **adult IQ heritability around 50–80%**, depending on sample and method.

---

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

Modern genetics looks at actual DNA rather than just relatedness.

### GWAS (Genome-Wide Association Studies)

- Very large samples (hundreds of thousands to >1 million participants) identify thousands of genetic variants individually associated with cognitive performance and educational attainment.
- Each variant has a tiny effect, but in aggregate they explain a nontrivial share of variance.

### Polygenic scores

- A **polygenic score** (PGS) sums the small effects across many genetic variants.
- For cognitive ability and educational attainment:
  - Early PGS explained <5% of variance.
  - Current, large-scale PGS:
    - Educational attainment: typically explain 10–15% of variance in many European-ancestry samples.
    - Cognitive performance: often ~5–10%, sometimes higher with better measures.
- This is **less** than twin-based heritability estimates; reasons include:
  - Common GWAS mainly capture common variants, not rare ones or structural variants.
  - Incomplete tagging of causal variants.
  - Assumptions in models and measurement error.

Crucially, even with imperfect prediction, the fact that:
- Many variants replicate across studies.
- PGS correlate with IQ, education, and related brain measures.
supports a real genetic contribution.

---

## 4. Gene–environment interplay

High heritability does *not* mean environment is unimportant. Several mechanisms matter:

### a. Gene–environment correlation

- **Passive**: Smarter parents both pass on genes and create enriched environments.
- **Evocative**: Children with higher cognitive propensity evoke more stimulation (teachers, peers, etc.).
- **Active**: Individuals seek out cognitively demanding niches as they age.

This helps explain why heritability appears to **increase with age**: people gradually select and shape their environments in line with their genetic tendencies.

### b. Gene–environment interaction

- The *effect* of genes can depend on environment.
- Examples:
  - In impoverished or chaotic environments, genetic differences may be suppressed (everybody does poorly).
  - In more supportive, high-resource environments, genetic variation can be more fully expressed—raising observed heritability.

Empirically, some studies find **lower heritability in low-SES childhood environments** and higher heritability in high-SES ones, although the pattern is not identical across countries


text:
Human intelligence is substantially heritable, but not fixed, and strongly shaped by environment. The best evidence comes from twin, adoption, family, and molecular genetic studies. Here’s what they show:

---

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

- **Heritability (h²)** is the proportion of variation in a trait *within a particular population, at a particular time, in a particular environment* that can be statistically attributed to genetic differences.
- If intelligence has heritability of 0.5, that means: in that population, about 50% of the *differences between people* in IQ scores are associated with genetic differences.
- It does **not** mean:
  - 50% of *your* intelligence is from genes and 50% from environment.
  - The trait is fixed or unchangeable.
  - The trait is the same across all groups, cultures, or eras.

---

## 2. Twin and adoption studies

### Twin studies
Comparing:
- **Identical (MZ) twins**: share ~100% of genes
- **Fraternal (DZ) twins**: share ~50% of segregating genes

Across many studies:
- MZ twins’ IQ correlations ≈ **0.7–0.8** when raised together, ~0.6 when raised apart.
- DZ twins’ IQ correlations ≈ **0.3–0.5**.

From these patterns, meta‑analyses estimate:
- **Adult IQ heritability ≈ 0.6–0.8** (60–80%) in affluent, industrialized populations.
- In **childhood**, heritability is lower, ≈ **0.2–0.4**, and increases with age.

### Adoption studies
- Children adopted into higher‑SES families show **substantial IQ gains** (often ~10–15 points on average compared with their birth-family background).
- But **within** adoptive families, siblings still differ in IQ partly due to genetics.
- Correlation between adopted children and their **biological** parents in IQ increases as the child grows older; correlation with **adoptive** parents decreases with age.

This pattern suggests:
- Environment has strong effects, especially early.
- Genetic influence becomes more evident as people select and create niches that fit their predispositions (called **gene–environment correlation**).

---

## 3. Age pattern: heritability rises over the lifespan

Consistent finding:
- **Early childhood**: h² for IQ ≈ 0.2–0.4.
- **Adolescence**: ≈ 0.4–0.6.
- **Adulthood**: ≈ 0.6–0.8 in high‑income settings.

Interpretations:
- As people age, they increasingly shape their environments (friends, education, jobs) in ways that align with their genetic tendencies.
- Also, some genetic influences on brain development appear later (e.g., myelination, synaptic pruning).

---

## 4. Role of environment and context

### Environmental influences are large

Key environmental factors that affect population IQ levels:
- **Prenatal environment**: maternal health, nutrition, toxins, stress.
- **Early childhood conditions**: nutrition, disease burden, stimulation, exposure to lead or other toxins.
- **Education**: years of schooling, quality of teaching, literacy and numeracy instruction.
- **Family and neighborhood SES**: resources, safety, cognitive stimulation, stress.
- **Macro conditions**: epidemics, war, economic collapse, policies on public health and education.

The **Flynn effect**: in many countries during the 20th century, average IQ scores rose by ~2–3 points per decade. Genes didn’t change that fast; environmental changes (education, nutrition, test familiarity, complexity of daily life) did. Recently, some countries show a plateau or reversal, again implicating environment.

### Heritability depends on inequality and conditions

Heritability is **not a constant**:
- In **more equal, well‑resourced environments**, environmental differences shrink, so genetic differences explain a larger share of the remaining variation → higher h².
- In **deprived or highly unequal environments**, environmental factors dominate; heritability estimates can be much lower.

Studies in the U.S. suggest:
- In higher‑SES families, IQ heritability is high.
- In lower‑SES families, shared environmental effects (home, schooling, neighborhood) are larger, and heritability lower.
This gradient is less clear or absent in some European samples with stronger social safety nets.

---

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

Genome‑wide association studies (GWAS) have:
- Identified **thousands of genetic variants**, each with very small effects, associated with IQ and educational attainment.
- **Polygenic scores**


text:
Heritability of human intelligence is high but not absolute, and it depends strongly on age and environment. The best overall summary from modern behavioral genetics is:

- In adulthood, about **50–80%** of the *differences between people* in general intelligence (g) within a given population are associated with genetic differences.
- In childhood, that figure is typically **20–40%**, rising steadily through adolescence.
- The remaining variation is due to **environmental factors** and **measurement error**, with “shared family environment” relatively modest in adulthood.

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

---

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

- **Heritability (h²)**: the proportion of *variation in a trait within a specific population at a specific time* that is statistically associated with genetic differences.
- It does **not** mean:
  - “X% of *your* intelligence comes from genes.”
  - That the trait is fixed or unchangeable.
  - That group differences (e.g., between countries or ethnic groups) are genetic. Heritability is about **within-group variation**, not between-group causes.

Heritability can be high even if:
- The trait is very malleable (e.g., height, which responds strongly to nutrition).
- Environmental differences are small or homogenized (which often raises heritability estimates).

---

## 2. Twin and adoption studies

These are the backbone of the evidence.

### Twin studies

- **Monozygotic (MZ, identical) twins** share ~100% of segregating genes;
  **Dizygotic (DZ, fraternal) twins** share ~50% on average.
- If genes matter for intelligence, MZ twins should be more similar in IQ than DZ twins.

Meta-analyses over tens of thousands of twin pairs show:

- **Childhood IQ**: heritability estimates around **0.3–0.4**.
- **Adolescent IQ**: rising to **0.5–0.6**.
- **Adult IQ**: often **0.6–0.8** in affluent, stable societies.

Classic findings:
- MZ twins reared apart have IQ correlations around **0.7+**.
- MZ twins reared together: often around **0.8–0.9**.
- DZ twins reared together: roughly **0.4–0.6**.

The fact that MZ twins raised apart are still so similar is powerful evidence that **genes play a large role**.

### Adoption studies

Compare:
- Children adopted into new families with their **biological** vs **adoptive** parents.
- Siblings adopted into the same vs different homes.

Patterns:
- In early childhood, adopted children resemble their adoptive families somewhat in IQ (shared environment effects).
- By adulthood, their IQ correlates much more strongly with their **biological** parents and siblings, and the correlation with adoptive parents often falls close to zero.

Interpretation:
Family environment early in life can push cognitive development, but **long-term resemblance** in intelligence is driven mainly by genetics plus *non‑shared* environmental factors (individual experiences, etc.).

---

## 3. Age-related increase in heritability

A robust finding: **heritability of intelligence increases with age**.

- Early childhood: environment matters more; heritability modest.
- Adolescence: genetic influence becomes stronger.
- Adulthood: heritability estimates peak.

This is usually explained by **gene–environment correlation**:

- As people grow, they increasingly select, shape, and stay in environments that fit their genetic dispositions (e.g., choosing cognitively challenging hobbies, peers, jobs).
- Genetic differences therefore get *amplified* over time via the environments people build for themselves.

---

## 4. Shared vs non‑shared environment

Behavior genetic models typically partition variance into:

- **A (Additive genetic)** – heritable effects.
- **C (Common/shared environment)** – family-wide influences that make siblings similar.
- **E (Non‑shared environment)** – individual experiences plus measurement error that make siblings different.

For IQ:

- **Shared environment (C)**:
  - Larger in **early childhood** (family SES, parenting, early schooling).
  - Often shrinks toward **near-zero by adulthood** in twin/adoption models, especially in relatively equal, wealthy societies.
- **Non‑shared environment (E)**:
  - Accounts for much of the remaining variance (often 20–30%), including:
    - Unique experiences.
    - Random developmental “noise”.
    - Illness, injuries.
    - Measurement error.

Important nuance: a *small* C estimate in adulthood does **not** mean family environment is unimportant. It can:
- Shift the entire level of cognitive development across a population (e


text:
Human intelligence is moderately to highly heritable, but not in a simple or fixed way, and environment still matters a lot for actual outcomes. Here’s what the evidence shows, in layers:

---

## 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, given a particular time and environment.
- It does **not** mean:
  - “X% of *your* intelligence is genetic.”
  - “Genes are destiny.”
  - “Environment doesn’t matter.”

You can have **high heritability** and still have:
- Large environmental effects (e.g., schooling, nutrition, toxins).
- Big changes in average IQ over time (e.g., the Flynn effect).

---

## 2. Twin, adoption, and family studies

These are the classic sources.

### Typical findings for IQ (in WEIRD, high-income countries)

- **Childhood:**
  - Heritability often estimated around **30–40%**.
- **Adolescence:**
  - Estimates rise to **~50–60%**.
- **Adulthood:**
  - Often **60–80%** in many twin studies.

This pattern—heritability rising with age—is very robust. It’s interpreted as:
- As people grow up, they increasingly choose and shape environments that fit their genetically influenced traits (so-called **gene–environment correlation**).

### Shared vs non-shared environment

- **Shared environment** (factors making siblings similar: family SES, parenting style, neighborhood, schools) accounts for:
  - More variance in **childhood**, but typically **declines by adulthood** (often close to zero in twin models for adult IQ in affluent societies).
- **Non-shared environment** (experiences that differ between siblings, random life events, measurement error) remains important at all ages.

### Adoption studies

- Adopted children’s IQs:
  - Are **initially closer** to their adoptive families in childhood.
  - Move **closer to their biological relatives’ IQs** as they age.
- Adoption into more enriched homes can raise IQ relative to what would be expected otherwise—estimates vary, but **about 4–8 IQ points** is common for typical SES improvements; large, intensive interventions can do more, but effects often fade partly with time.

---

## 3. Molecular genetics findings

These use DNA directly rather than inferring from family resemblance.

### Genome‑wide association studies (GWAS)

- Intelligence (or educational attainment as a proxy) is **highly polygenic**:
  - Thousands of genetic variants; each has **tiny** effects.
- Modern GWAS samples include **millions** of people.
- **SNP-based heritability** (the share of variance explained by common measured variants) for cognitive ability is typically about **20–30%**.
  - This is lower than twin-study estimates but:
    - It only captures *common* variants.
    - It excludes rare variants, structural variants, gene–gene interactions, etc.
- The gap between twin-based and SNP-based heritability is shrinking as methods and sample sizes improve.

### Polygenic scores (PGS)

- A polygenic score aggregates the effects of many variants.
- For **educational attainment**, current PGS explain around **10–15% of the variance** in large European-ancestry samples; for **IQ directly**, usually slightly less.
- This prediction is:
  - **Probabilistic**, not deterministic.
  - Strongly **weakened** when you:
    - Move to other ancestries (because of different allele frequencies and linkage).
    - Control for family-level environment and assortative mating.
- Within-family designs (comparing siblings) show PGS effects that are real but **smaller** than population-level estimates, implying that some of the apparent predictive power reflects **social and environmental stratification** layered on top of genetics.

---

## 4. Gene–environment interplay

Heritability depends on context.

### Socioeconomic context

Evidence suggests:
- In **more equal, high-resource environments**, heritability estimates for IQ tend to be **higher**, because basic environmental needs (nutrition, schooling access) don’t differ as much; genetic differences can express more fully.
- In **more deprived or unequal environments**, **environmental constraints** can cap achievement, and heritability can appear **lower** (a phenomenon sometimes found in US samples, though not always replicated in European data).

### Flynn effect

- IQ scores increased dramatically (roughly **2–3 points per decade**) across much of the 20th century in many countries.
- This large change **cannot** be genetic (too fast), showing that:
  - Environment can shift entire population distributions.
  - Heritability doesn’t prevent big


text:
Human intelligence is substantially heritable, but not fixed, and it arises from many genes interacting with environments over time.

Here’s what the evidence shows, in layers:

---

## 1. What “heritable” means

Heritability (h²) in this context:

- Is the proportion of *observed differences* in intelligence (usually IQ or g) in a population that can be statistically attributed to genetic differences.
- Is *not*:
  - The percent of *your* intelligence that’s “due to genes.”
  - A claim that intelligence cannot change.
  - A universal constant (it depends on the population, time, and environment).

---

## 2. Twin, adoption and family studies

These are the classic sources of evidence. They compare how similar people are in IQ given different degrees of genetic relatedness and different environments.

### Twin studies (monozygotic vs dizygotic)

Meta-analyses of large samples (e.g., work by Plomin, Deary, others) generally find:

- In childhood: heritability of IQ ≈ 0.3–0.5
- In adulthood: heritability of IQ ≈ 0.6–0.8
  (i.e., 60–80% of *variation between adults* in a given population is associated with genetic differences)

Patterns:
- Identical twins raised apart are more similar in IQ than fraternal twins raised together.
- Identical twins reared apart have IQ correlations in the ~0.7 range in many studies, despite different environments.

### Adoption studies

Adoption studies compare:

- Adopted children vs. their biological parents
- Adopted children vs. their adoptive parents

Findings:
- In childhood, both genes and shared environment matter: adopted kids resemble both biological and adoptive parents to some extent.
- By adolescence and adulthood, resemblance to biological parents becomes much stronger; resemblance to adoptive parents declines sharply for IQ and other cognitive traits.

This pattern (increasing resemblance to genetic relatives with age) is one of the most robust findings.

---

## 3. Shared vs non‑shared environment

Most behavioral genetic models break influences into:

- Genetic (A)
- Shared environment (C): aspects that make siblings similar (same home, same parents, often neighborhood/school)
- Non‑shared environment (E): experiences that make siblings different (peer groups, idiosyncratic experiences, measurement error, etc.)

For IQ in many Western populations:

- Shared environment:
  - Substantial in early childhood (can be 20–30% of the variance).
  - Drops toward near-zero in adulthood in many studies.
- Non‑shared environment:
  - Accounts for the rest (often ~20–30%), and includes learning opportunities, accidents, illnesses, chance events, etc.

So: intelligence is highly heritable *and* strongly shaped by non‑shared environmental factors.

---

## 4. Why does heritability increase with age?

Consistently observed pattern: genetic influence on IQ appears to grow from childhood into adulthood.

Main explanations:

- **Gene–environment correlation:** as people age, they increasingly select, modify, and create environments that fit their genetic dispositions (e.g., a child genetically inclined toward reading chooses more reading, more challenging classes, etc.).
- **Longer developmental unfolding:** some genetic influences affect brain maturation, synaptic pruning, myelination, etc., which play out over years.
- **Diminishing impact of the family environment:** once people leave home, parents’ direct influence on daily learning environments is much lower.

This doesn’t mean the environment stops mattering; it means that differences in *shared* family environment explain less and less of the variation.

---

## 5. Molecular genetics and GWAS findings

Older work inferred genetic influence from correlations. Modern studies also look directly at DNA.

### Polygenic nature

- Intelligence is influenced by **many thousands of genetic variants**, each with tiny effects.
- No single “intelligence gene” exists.
- Genome‑wide association studies (GWAS) with hundreds of thousands to over a million participants have identified thousands of SNPs associated with IQ, educational attainment, and related traits.

### Polygenic scores (PGS)

- Researchers can sum the small effects of many variants into a **polygenic score**.
- Such scores currently account for a **modest but nontrivial portion** of IQ variance:
  - Depending on sample and measurement, maybe ~10–20% of variance in cognitive performance or educational attainment at best, sometimes less.
- This is **lower** than twin‑based heritability estimates, reflecting:
  - Incomplete capture of all relevant variants.
  - Limitations of current GWAS (e.g., mainly European ancestry samples).
  - Gene–environment interplay not fully modeled.

The key point: direct DNA evidence is very consistent with twin/adoption findings: intelligence is substantially heritable, highly polygenic, and


text:
Human intelligence is substantially heritable, but not fixed, and genes are not destiny. The evidence is strong but nuanced.

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

- **Heritability (h²)** is the proportion of **differences between people in a population** that can be statistically attributed to genetic differences.
- It does **not** mean “X% of *your* intelligence comes from genes.”
- It depends on:
  - The population (e.g., country, socioeconomic range)
  - The environment (nutrition, schooling, health, etc.)
  - The age at which intelligence is measured

So when we say “intelligence is 50–80% heritable,” we mean: in that particular population and environment, 50–80% of the *variation* in test scores among individuals relates to genetic differences.

---

### 2. Twin and adoption studies

These are the classic sources of evidence.

**Key findings:**

1. **Identical vs fraternal twins**
   - Identical (monozygotic, MZ) twins share ~100% of their genes.
   - Fraternal (dizygotic, DZ) twins share ~50% on average.
   - In large meta-analyses:
     - Correlation of IQ between MZ twins raised together ≈ **0.80–0.85**
     - DZ twins ≈ **0.40–0.45**
   - Higher similarity in MZ twins than DZ twins indicates substantial genetic influence.

2. **Twins reared apart**
   - MZ twins raised in different families show IQ correlations ≈ **0.70–0.75**, only slightly lower than those raised together.
   - This strongly supports genetic influence, because they share genes but not family environment.

3. **Adoption studies**
   - Biological siblings raised apart are more similar in IQ than adoptive siblings raised together.
   - Adopted children’s IQs correlate more strongly with their **biological** parents than with their **adoptive** parents by adulthood.
   - Early in childhood, adoptive-family environment matters more; by adulthood, genetic resemblance dominates.

**Estimated heritability of general intelligence (g) from twin/adoption studies:**
- In children: **~20–40%**
- In adolescence: **~40–60%**
- In adults: **~60–80%**

This pattern—heritability increasing with age—has been replicated many times.

---

### 3. Why heritability increases with age

Several mechanisms are likely:

- **Gene–environment correlation**: As people grow up, they select, create, and gravitate toward environments compatible with their genetic propensities (e.g., a child genetically inclined toward reading seeks books and ends up in cognitively richer environments).
- **Diminishing shared-environment effects**: Early on, family environment (shared environment) has a strong impact; its influence typically **fades by late adolescence**. Non-shared experiences and genetically influenced choices become more important.

By adulthood, family-level factors (e.g., parenting style, number of books at home) explain surprisingly little of the **differences among siblings** in IQ, even though they obviously matter for absolute development and well-being.

---

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

Modern studies look directly at DNA rather than inferring genes from relatedness.

- Large **genome-wide association studies (GWAS)** have identified **thousands** of genetic variants each with tiny effects on cognitive ability and educational attainment.
- Each variant’s effect is minuscule; together, they form **polygenic scores** (PGS).

**What polygenic scores currently show:**

- For individuals of European ancestry, PGS for cognitive ability or educational attainment can explain:
  - Roughly **10–20%** of the variance in educational attainment
  - Roughly **5–15%** of the variance in measured cognitive ability (depending on sample and measure)
- This is **less** than the 60–80% heritability from twin studies, meaning:
  - We still haven’t captured all genetic influence (“missing heritability”).
  - Intelligence is very **polygenic**—thousands of tiny-effect variants, not “the intelligence gene.”

Important caveats:

- Predictive power is **population-specific**: PGS built on European-ancestry samples perform worse in other ancestry groups due to differences in allele frequencies, linkage disequilibrium, and environmental structure.
- PGS are **probabilistic, not deterministic**: People with low scores can have high IQs and vice versa; the distributions overlap heavily.

---

### 5. Environmental influences

High heritability does **not** mean environment is unimportant. It means: given *current* environmental variation, genes account for most *differences*.

Strong environmental factors include:

-


text:
Human intelligence is substantially heritable, but *not* fixed, and genes are far from the whole story.

Here’s what the evidence shows, in structured form.

---

## 1. What “heritability” actually means

- **Heritability (h²)** = proportion of *differences between people* in a trait that can be statistically attributed to genetic differences in a particular population, in a particular environment.
- It does **not** mean:
  - “X% of your intelligence comes from your genes.”
  - “Intelligence can’t be changed.”
- It’s a *population-level*, *environment-specific* statistic.

Example: If IQ heritability = 0.6 in some country, that means roughly 60% of the *variation in IQ scores* within that population is associated with genetic variation, given their current environments.

---

## 2. Evidence from classic behavioral genetics

### Twin studies

Comparisons:

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

If MZ twins are *much* more similar than DZ twins on IQ, genetics are strongly implicated.

Findings from large meta-analyses:

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

So: heritability of intelligence is **moderate in childhood** and **rises to high levels in adulthood**.

Why it rises:
- As people age, they can increasingly choose and shape their environments (schools, jobs, hobbies) in ways that match their genetic tendencies (“gene–environment correlation”).
- Environmental shocks (nutrition, early deprivation, schooling differences) are more impactful early on.

### Adoption studies

- **Adopted children vs. biological vs. adoptive parents**:
  - In childhood, IQ correlates *somewhat* with both biological and adoptive parents.
  - By adulthood, correlations with **biological parents and biological siblings** become much stronger, while correlations with adoptive parents typically **shrink toward zero**.
- Famous example: In the Texas Adoption Project and similar studies, by late adolescence/adulthood, adopted children resembled their biological relatives more than their adoptive families in IQ, despite being raised apart.

Together, twin and adoption studies consistently indicate **substantial genetic influence** on intelligence.

---

## 3. Evidence from molecular genetics (GWAS and polygenic scores)

### GWAS (genome-wide association studies)

- Large studies (hundreds of thousands to >1 million people) have identified **thousands** of genetic variants associated with cognitive performance and educational attainment.
- Each variant has a **tiny effect**; intelligence is **highly polygenic**.

### Polygenic scores (PGS)

- A PGS sums the effects of many variants, each weighted by GWAS estimates, to produce a “genetic propensity” score.
- Modern PGS for cognitive performance/education typically explain:
  - ~10–20% of variance in years of education
  - ~5–15% of variance in cognitive test performance, depending on sample and ancestry
- These numbers are *lower* than twin-study heritability (e.g., ~60–70%) because:
  - GWAS don’t capture all variants or interactions.
  - Effect estimates are noisy.
  - Scores transfer poorly across ancestries (most GWAS are in European-ancestry samples).

The molecular data confirm:
- Intelligence has a measurable genetic basis.
- It is not driven by one or a few “intelligence genes,” but by many variants each with minute effects.
- We’re still far from “explaining” the full genetic contribution at the DNA level (“missing heritability”).

---

## 4. Role of environment

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

### Environmental effects are clear and large:

- **Socioeconomic status (SES)**:
  - In **low-SES contexts**, environmental deprivation (nutrition, toxins, chaotic homes, poor schools) can strongly suppress cognitive development.
  - Some US studies find that IQ heritability is **lower in disadvantaged families** and **higher in advantaged families**, because in harsh conditions environmental constraints dominate.
  - This “Scarr–Rowe effect” (heritability rising with SES) is seen in some countries/datasets, though not all.
- **Macro-environment changes**:
  - The **20th-century “Flynn effect”**: average IQ scores rose dramatically (roughly 2–3 IQ points per decade in many countries) due to improved nutrition, education, health, and cognitive demands of modern life.
  - This large environmental shift occurred despite no comparable genetic change in such a short time.
- **Specific interventions**:
  - Early childhood


text:
Human intelligence is substantially heritable, but not fixed, and it depends heavily on environment. Evidence from twin, adoption, and, more recently, molecular genetic studies converges on a broadly consistent picture.

Below is the core summary, then brief detail on the main evidence and caveats.

---

## 1. How heritable is intelligence?

“Heritability” (h²) here means: in a given population, what proportion of the *differences between people* in cognitive ability can be statistically attributed to genetic differences, given that population’s environments.

From large meta-analyses and reviews:

- **Childhood:** h² ≈ 0.20–0.40
- **Adolescence:** h² ≈ 0.40–0.60
- **Adulthood:** h² ≈ 0.60–0.80
- **Late adulthood:** still high, though some decline and more variability

A commonly cited rough figure for adults in developed countries is **~50–70% heritable** for general intelligence (g).

The rest of the variation (30–50% or more) is due to **environmental factors and measurement error**, especially environments that differ between siblings (peers, illnesses, specific life events, etc.).

---

## 2. Key evidence types

### a) Twin studies

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

Finding:
Across many studies, **IQ correlations** are roughly:

- MZ twins reared together: r ~ 0.80–0.90
- DZ twins reared together: r ~ 0.40–0.60

If both sets share family environment to a similar degree, the much higher similarity in MZ pairs implies substantial genetic influence.

### b) Twins reared apart

MZ twins adopted into different homes and raised apart:

- IQ correlation commonly ~0.70 (somewhat variable by study and age).

This suggests that even when family environments are quite different, genetic similarity still predicts a lot of the similarity in intelligence.

### c) Adoption studies

- Adopted children’s IQs:
  - Initially somewhat correlated with adoptive parents’ IQ and socioeconomic status (reflecting early environmental impact).
  - By adolescence/young adulthood, correlations with adoptive parents tend to drop close to zero.
  - Correlations with biological parents and siblings become stronger with age, even if they never lived together.

Interpretation: Shared family environment effects are moderate early in life but **fade**, and genetic influences become more visible over time.

### d) Molecular genetics (GWAS, SNP-based heritability)

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

- Identify many genetic variants (SNPs) associated with IQ and educational attainment.
- Each individual SNP has a tiny effect; **polygenic scores** that sum thousands of SNP effects can predict:
  - ~10–20% of variance in educational attainment
  - ~5–15% of variance in measured cognitive ability (depending on sample, age, and test).

SNP-based estimates (using only common variants measured in GWAS) give **lower-bound heritability** for intelligence often in the **0.20–0.40** range; twin-based estimates are higher because they implicitly include rare variants, gene–gene interactions, etc. This broad agreement supports a real, polygenic genetic contribution.

---

## 3. Gene–environment interplay

### a) Heritability depends on context

Heritability is *not* a fixed characteristic of intelligence; it’s a property of a population in a particular set of environments.

- In **affluent, relatively equal environments**, basic needs and educational opportunities are broadly met. Differences in IQ then more often reflect genetic and idiosyncratic (nonshared) environmental differences → **higher heritability estimates**.
- In **low-income or highly unequal environments**, variation in nutrition, schooling quality, exposure to stress, toxins, etc., is larger and can overshadow genetic differences → **lower heritability estimates**.

Example: Some studies show that among children in high-SES families, heritability of IQ can be >0.70, while in low-SES families it can be much lower, with environment explaining more variance.

### b) Heritability increases with age

Multiple longitudinal twin and adoption studies show:

- Genetic influences on cognitive ability **increase from childhood to adulthood**.
- Shared family environment influences typically **decrease** over the same period.

Likely mechanisms include:

- As people age, they **select, modify, and create environments** that match their tendencies (gene–environment correlation).
- Early environmental constraints loosen, allowing genetic differences in interests, motivation, and cognitive style to express more strongly.

---

## 4. What herit


text:
Human intelligence is substantially heritable, but not fixed, and strongly shaped by environment. The headline: in rich, stable environments, about half or more of the *differences* in IQ between people are associated with genetic differences; in harsher or more unequal environments, the genetic share can be much lower.

Below is what “heritability” means, what the main kinds of evidence show, and what we can and can’t conclude.

---

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

- **Intelligence** in this research usually means a general cognitive ability factor, *g*, measured by IQ or similar tests.
- **Heritability (h²)** is the proportion of *variation within a particular population at a particular time* that is statistically associated with genetic differences.
- It is **not**:
  - the percent of *your* intelligence caused by genes
  - the extent to which intelligence is unchangeable
  - a fixed value across groups, places, or eras

High heritability can coexist with big environmental effects (e.g., nearly 100% heritable height within a well-fed population, but average height can shift a lot with nutrition).

---

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

### Twin studies

- Compare:
  - **Monozygotic (MZ) twins**: nearly 100% of genes shared
  - **Dizygotic (DZ) twins**: on average 50% of segregating genes shared

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

**Typical estimates in high-income countries:**

- In childhood: h² ≈ 0.3–0.4
- In adolescence: h² ≈ 0.5
- In adulthood: h² ≈ 0.6–0.8

This “increasing heritability with age” is a well‑replicated result: as people select and shape their environments (schools, jobs, hobbies), genetic dispositions have more room to influence the paths they take.

### Adoption and family studies

- **Adopted siblings**, raised together without genetic relatedness, show:
  - Moderate IQ similarity in childhood
  - Much lower similarity by adolescence/adulthood
  → Shared family environment has noticeable early effects that fade over time.
- **Biological relatives** (siblings, parents–children) show IQ correlations that match what is expected from genetic relatedness, especially later in life.

**Takeaway from behavioral genetics:**
In reasonably affluent, stable societies, somewhere around **50–70% of between-person IQ variation in adults** can be statistically attributed to genetic differences; the remainder is due to nonshared environment (experiences that differ even for siblings) and measurement error, with a smaller or vanishing role for shared family environment by adulthood.

---

## 3. Heritability is not the same everywhere: environment matters

Heritability is **context-dependent**. Several lines of work show this:

### Socioeconomic status (SES) moderation

In the U.S. and some other countries:

- Among high-SES families, heritability of IQ is often **higher** (e.g., ~0.7–0.8), and shared environment smaller.
- Among low-SES families, heritability estimates are **lower**, and shared environmental effects larger.

Reason: in deprived or chaotic environments, constraints (poor schools, malnutrition, stress, etc.) can limit how fully genetic potential is expressed, so environment accounts for more of the variance.

Some European and Scandinavian studies, with more equal schooling and healthcare, show weaker SES moderation, again highlighting that **social structure influences heritability**.

---

## 4. Molecular genetics: GWAS and polygenic scores

Twin/adoption studies infer genetic influence indirectly. Modern work examines DNA directly.

### GWAS (genome-wide association studies)

- Very large studies (now millions of people) scan the genome for variants associated with educational attainment and IQ-like measures.
- Each variant’s effect is tiny, but thousands of them add up.
- Summed across the genome, they form a **polygenic score**.

In large, relatively homogeneous samples, **polygenic scores for cognitive ability / education currently explain**:

- Roughly **10–20%** of the variance in educational attainment
- Typically **5–15%** of the variance in cognitive test scores, depending on the sample and measure

This is **lower** than twin-based heritability because:

- We don’t capture all variants (rare ones, structural variants).
- Effect estimates degrade when you apply them to different populations (different ancestries, social structures).
- Measurement noise and gene–environment interplay reduce predictive power.

Still, these scores:

- Predict cognitive test performance, school achievement, and some occupational outcomes.
- Show that **many** genetic variants, each with very small effects, contribute additively to


text:
Human intelligence is substantially heritable, but not fixed, and its development depends strongly on both genes and environment. Here’s what the evidence shows, and what it does *not* show.

---

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

- **Heritability (h²)**: The proportion of *differences between people* in a trait that can be statistically attributed to genetic differences *within that population and environment*.
- It is:
  - About variation **between individuals**, not how “genetic” intelligence is in an absolute sense.
  - Specific to a given **population, time, and environment**.
- Heritability does **not** mean:
  - “X% of your intelligence comes from genes and Y% from environment.”
  - That the trait is fixed or unaffected by environment.
  - That group differences (e.g., between countries or ethnic groups) are necessarily genetic.

---

## 2. Twin and family studies: broad picture

From decades of behavioral genetics:

- **Adult IQ heritability**: commonly estimated around **0.5–0.8** (50–80%) in modern, relatively affluent populations.
  - Meta-analyses of twin studies (e.g., Ian Deary, Robert Plomin, and others) consistently find:
    - Childhood: h² ≈ 0.3–0.5
    - Adolescence: h² ≈ 0.5–0.6
    - Adulthood: h² ≈ 0.6–0.8
- **Shared environment** (family-wide factors that make siblings similar: home, parenting style, neighborhood) tends to:
  - Be substantial in early childhood.
  - Decline in influence by late adolescence/adulthood in many Western samples.
- **Non-shared environment** (experiences unique to each person and measurement error) accounts for the rest.

Key evidence:

- **Monozygotic (identical) vs dizygotic (fraternal) twins**:
  - Identical twins reared together: IQ correlations often ~0.8.
  - Fraternal twins reared together: ~0.4–0.6.
  - The greater similarity of identical twins implies a strong genetic component.
- **Identical twins reared apart**:
  - Show high IQ correlations (often ~0.7), despite different home environments.
- **Adoption studies**:
  - Adopted children’s IQs become *more similar* to their biological parents and *less similar* to adoptive parents as they grow up.
  - This suggests that genetic influence becomes more apparent with age.

Interpretation: Genetic differences explain a large share of IQ variation in typical, developed environments, especially in adulthood, but environmental factors still matter substantially.

---

## 3. Developmental pattern: heritability changes with age

A robust finding:

- **Genetic influence on cognitive ability tends to increase with age**.
- A rough pattern:
  - Early childhood: environment (especially family and early schooling) is relatively powerful.
  - Adolescence/young adulthood: genetic differences play a larger role in how far individuals can and do develop cognitively.
- One explanation: **gene–environment correlation**.
  - As people age, they:
    - Select environments (friends, hobbies, jobs) that fit their predispositions.
    - Amplify their genetic tendencies (e.g., a child slightly better at reading reads more, further increasing ability).

---

## 4. Molecular genetics: what we see at the DNA level

Genome-wide association studies (GWAS):

- Thousands of genetic variants (SNPs) show tiny associations with cognitive ability and educational attainment.
- Combined into **polygenic scores**, these can predict:
  - A modest but non-trivial share of IQ variation: currently around **10–20%** of variance in large European-ancestry samples.
- This is **less** than twin-based heritability (50–80%) because:
  - We haven’t captured all variants.
  - We partially miss rare variants, structural variants, and non-additive effects.

The molecular data, however, confirms:

- Intelligence is **highly polygenic** (influenced by very many genes, each with tiny effect).
- There is a **real, measurable genetic basis** that aligns directionally with twin-study estimates.

---

## 5. Environment still matters a lot

High heritability does not mean environment is weak or irrelevant.

### 5.1. Environmental effects on cognitive ability

Evidence that environment can strongly affect cognitive development:

- **Severe deprivation** (e.g., institutional neglect, extreme poverty, malnutrition, lead exposure, lack of schooling) lowers cognitive functioning.
- **Interventions**:
  - Early childhood education, health and nutrition programs, school quality improvements, and reductions in toxic exposures can raise average performance.
  - Effects are


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

- In affluent, stable environments: **heritability of general intelligence (g) in adulthood is usually estimated around 50–80%.**
- In childhood, and in harsher or more unequal environments: **heritability is lower, sometimes near 20–40%.**
- These numbers are **population-level, context-dependent statistics**, not statements about any individual’s “genetic destiny.”

Below is how we know this, and what it does and does not mean.

---

## 1. What “heritability” actually means

- **Heritability (h²)** = the proportion of **variation in a trait within a specific population and environment** that can be statistically attributed to genetic differences.
- It **does not** mean “X% of your intelligence comes from genes and the rest from environment.”
  Instead: *Among people in this group, at this time, X% of the differences between them are correlated with genetic differences.*

Change the environment or population, and **heritability can change**, even if genes don’t.

---

## 2. Evidence from twin and family studies

These are the classic lines of evidence:

### Monozygotic vs. dizygotic twins

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

Across many studies:

- MZ twins reared together: IQ correlations typically ~0.80–0.90.
- DZ twins reared together: IQ correlations typically ~0.40–0.60.
- Using standard biometric models, this pattern suggests **substantial genetic influence**.

### Twins reared apart

- MZ twins raised in different families still correlate around **0.70–0.80** in IQ in classic studies (e.g., Minnesota Twin Study).
- That’s similar to MZ twins raised together, implying a **large genetic component** plus only moderate shared-family environmental effects.

### Adoption studies

- Adopted children’s IQs correlate:
  - Moderately with their **biological parents** (genetic link).
  - Weakly or modestly with their **adoptive parents** by adulthood.
- Early childhood often shows **stronger shared-environment** effects, which decline with age.

Meta-analyses (e.g., Bouchard & McGue; Polderman et al. 2015, synthesizing >2,700 twin studies across traits) consistently find high heritability for adult general cognitive ability.

---

## 3. How heritability of IQ changes with age

A robust pattern:

- **Childhood (2–10 years)**: heritability estimates often around **20–40%**.
- **Adolescence**: climbs to **40–60%**.
- **Adulthood**: can reach **60–80%** in typical Western, relatively well-off samples.

Why does it increase?

- As people grow, they:
  - Select and shape environments that fit their genetic predispositions (e.g., choosing cognitively stimulating peers, jobs, hobbies).
  - Escape some childhood constraints (such as local school quality or parental control).
- This is sometimes called **gene–environment correlation**: your genes partly influence the environments you seek or create.

---

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

Beyond twin designs, there is direct DNA-based evidence:

### GWAS findings

- Large **genome-wide association studies (GWAS)** have identified **thousands of genetic variants** each with tiny effects on educational attainment and cognitive performance.
- Individually, these variants explain only a trivial fraction of variance; in aggregate as **polygenic scores**, they can explain:
  - ~10–20% of variance in educational attainment in some European-ancestry samples.
  - A smaller but nontrivial share of variance in IQ / cognitive tests (roughly up to ~10–15% with current methods).

This is **below** the twin-based heritability estimates (the gap is “missing heritability”), but shows:

- There are **measurable DNA differences** associated with cognitive ability.
- Effects are **highly polygenic** (many genes, each tiny).

### Consistency with twin studies

DNA-based estimates (e.g., SNP heritability via GCTA, LD-score regression) typically find:

- **SNP-based heritability for cognitive ability** around **20–30%** in European samples.
- This is a lower bound of total heritability, because:
  - Methods capture only common variants well.
  - They miss some rare variants and gene–gene interactions.

Together with twin studies, they strongly support **substantial genetic influence**.

---

## 5. Environment still matters a lot

High her


text:
Human intelligence is substantially heritable, but not fixed, and it is strongly shaped by environment. The key points from the evidence:

---

## 1. What “heritable” means

- **Heritability (h²)** in this context: the proportion of **differences between people** in a given population that can be statistically attributed to genetic differences.
- It **does not** mean:
  - “X% of your intelligence comes from genes.”
  - “Your intelligence is fixed.”
- Heritability is **population- and context-specific**: it depends on the range of environments and genotypes in the studied group.

---

## 2. Twin and family studies

These are the classic sources.

### Overall estimates

Meta-analyses of many twin studies (thousands of pairs) typically find:

- **Children:** heritability often ~0.2–0.4
- **Adolescents:** ~0.4–0.6
- **Adults:** ~0.6–0.8

So in many modern, relatively affluent populations, **about 50–80% of the variance in adult IQ is associated with genetic differences**.

### Shared vs non‑shared environment

- **Shared environment** (family socioeconomic status, parenting style, neighborhood) contributes more in **childhood** (~20–30% of variance), and usually **shrinks** in adulthood (often near 0 in twin models).
- **Non‑shared environment** (unique experiences, different teachers, illness, accidents, peer groups, measurement error) accounts for the rest.

Interpretation: As people grow up and select/shape their own environments (friends, hobbies, jobs), genetic propensities are expressed more fully, and the apparent impact of family-level environment on differences between adult siblings/twins looks smaller.

---

## 3. Adoption studies

Adoption studies separate genes and rearing environment.

Findings:

- Adopted children’s IQs correlate **modestly** with their adoptive parents during childhood, but this correlation tends to **decline with age**, approaching zero in adulthood in many studies.
- Correlations between adopted children and their **biological** parents and siblings **increase with age**.

This pattern supports a **substantial genetic influence** plus an **environmental influence that is stronger earlier in life**.

---

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

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

### What they show

- Intelligence (and related traits like educational attainment) is **highly polygenic**: influenced by **thousands** of variants, each with **tiny** effects.
- **Polygenic scores** (summary measures of many variants):
  - Currently explain on the order of **10–20% of the variance** in cognitive performance and more in educational attainment in large European-ancestry samples.
  - That’s **below** twin-study heritability estimates, reflecting “missing heritability” (uncaptured rare variants, imperfect measurement, gene–gene and gene–environment interactions, etc.).
- As samples grow, the variance explained by genetic scores has been **rising**, which is consistent with a substantial true genetic component.

Important constraints:

- Predictive power and identified variants are **population-specific**; scores built in one ancestry group lose power in others due to different genetic architectures and patterns of linkage disequilibrium.
- Genes identified so far do **not** map cleanly onto simple “intelligence genes”; they’re largely involved in broad brain development and neuronal function.

---

## 5. Gene–environment interplay

Heritability estimates hide several important subtleties.

### Gene–environment correlation

- People with certain genetic tendencies may **seek out or evoke environments** that further develop those tendencies.
  - Example: A child with a genetic tilt toward verbal skill reads more, gets positive feedback, and is channeled into advanced classes, amplifying the initial difference.
- This can make genetic influence **look larger**, because genes and environments are not independent.

### Gene–environment interaction

- The **effect of genes** can depend on the environment.
  - Under severe deprivation (e.g., chronic malnutrition, extreme poverty), **genetic potential may not be expressed**, so heritability can be **lower**.
  - In **more equal and resource-rich environments**, differences attributable to genes often become **more apparent**, raising heritability estimates.
- Some studies find that in low-SES environments, **shared environment** is more important and genetic variance is more suppressed; in high-SES environments, **genetic** influences are stronger. Results vary by country and measure, but the pattern fits the theory.

---

## 6. Stability and change over the lifespan

- Intelligence is **moderately stable**: people who are relatively high (or low) tend to remain so relative to peers, but not perfectly.
- Both genetic and environmental influences


text:
Human intelligence is substantially heritable, but not fixed or determined solely by genes. The evidence points to:

- **Moderate-to-high heritability in adulthood**: around **50–80%** of the variation in IQ within a given population can typically be attributed to genetic differences.
- **Environment still matters a lot**: especially in childhood, and for population-level differences (e.g., between countries or eras), where environmental effects dominate.

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

---

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

- **Heritability** (h²) = the proportion of *observed differences* in a trait within a population that can be statistically attributed to genetic differences.
- It is:
  - **About differences between people**, not about an individual.
  - **Population-specific**: it can change with environment, time, and context.
  - **Not “percent genetic destiny”**: a heritability of 60% does *not* mean your intelligence is 60% from genes and 40% from environment.

Example: In a society where everyone has excellent nutrition and schooling, environmental variation shrinks, and heritability estimates can go *up* even though the environment is extremely important for reaching that level in the first place.

---

## 2. Twin and family studies

These have been the main source of evidence for decades.

### Key findings

- **MZ (identical) twins** share ~100% of their segregating genes.
- **DZ (fraternal) twins** share on average ~50%.
- If genes matter, MZ twins should be more similar than DZ twins.

Meta-analyses (e.g., work by Robert Plomin and colleagues, and large reviews of thousands of twin pairs) find:

- **Childhood IQ:**
  - Heritability ≈ **20–40%** in early childhood.
- **Adolescence and young adulthood:**
  - Heritability rises to ≈ **50–60%**.
- **Adulthood:**
  - Estimates often reach **60–80%**.

This pattern—heritability increasing with age—is one of the most robust findings. The likely reason: as people get older, they select, shape, and create environments that match their genetic tendencies (gene–environment correlation).

### Adoption studies

- Children adopted into new families show:
  - Early in life, measurable influence from adoptive family environment.
  - By late adolescence/early adulthood, their IQ resembles that of their **biological** relatives more than their adoptive relatives.
- Correlations between IQ of:
  - Adoptive siblings often **shrink to near zero** in adulthood.
  - Biological siblings remain substantially positive.

This supports a strong genetic component but also shows that **shared family environment** (home, parenting style) has a modest and often diminishing *within-population* effect on adult IQ.

---

## 3. Molecular genetics (DNA-based evidence)

Earlier estimates came from correlations; newer work uses measured DNA.

### Genome-wide association studies (GWAS)

- Intelligence / educational attainment is **polygenic**: influenced by thousands of genetic variants, each with tiny effects.
- GWAS with millions of people have:
  - Identified many associated variants spread across the genome.
  - Created **polygenic scores (PGS)** that predict:
    - Part of the variation in IQ test scores,
    - Strongly predict years of education.

Current polygenic scores for educational attainment can explain roughly **10–15%** of the variance in years of education, and (depending on sample and measure) a non-trivial fraction of IQ variance. This is noticeably less than twin-based heritability, but consistent with the idea that we haven’t yet captured all the relevant genetic variation (the “missing heritability” problem).

### SNP-based heritability (GREML, LD-score regression)

These methods use measured SNP data to estimate how much phenotypic variance is captured by common variants:

- For intelligence / cognitive ability:
  - SNP-based heritability is often around **20–40%**.
- This is a **lower bound** of total heritability, because:
  - They miss rare variants and structural variants.
  - They’re limited by current sample sizes and statistical power.

The fact that DNA-based estimates are substantial and in the same ballpark as twin-study estimates strongly supports the **reality** of genetic influence.

---

## 4. Environmental effects

High heritability does not mean environment is unimportant.

### Shared vs nonshared environment

Twin studies typically partition variance into:
- **A**: Additive genetic
- **C**: Shared environment (family-level)
- **E**: Nonshared environment + measurement error

Findings for intelligence:
- **Shared environment (C)**:
  - Non-trivial in childhood (e


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)** = the proportion of *differences between people* in a trait that can be statistically attributed to genetic differences *in a given population and environment*.
- It does **not** mean:
  - “X% of your intelligence comes from genes.”
  - “Your IQ is predetermined and unchangeable.”
- It’s a population-level statistic, not an individual-level fate.

---

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

These are the backbone of behavioral genetics.

### 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 suggests genetic influence.

Findings (typical ranges from large meta-analyses):

- **Childhood IQ heritability**: ~0.3–0.4 (30–40%)
- **Adolescent IQ heritability**: ~0.5–0.6 (50–60%)
- **Adult IQ heritability**: ~0.7–0.8 (70–80%)

So in well-studied, mostly Western populations, **most of the variation in adult intelligence appears to be genetic in origin**.

### Adoption studies

- Compare adopted children to:
  - Their **biological parents/siblings** (genes, not environment).
  - Their **adoptive parents/siblings** (environment, not genes).

Findings:

- In **childhood**, adopted children’s IQs correlate modestly with both biological and adoptive parents.
- By **late adolescence and adulthood**, correlations with **biological relatives** remain or increase, while correlations with **adoptive parents** shrink toward zero.
- This supports a strong genetic component that becomes more apparent with age.

---

## 3. Age pattern: heritability increases over the lifespan

One of the most robust findings:

- Heritability of intelligence **increases from childhood to adulthood**.
- Shared family environment (home, parenting style, etc.) **matters more early**, then fades.

Typical pattern:

- **Shared environment**:
  - Childhood: can explain ~20–30% of IQ variance.
  - Adulthood: often close to 0% in twin/adoption studies.
- **Non-shared environment** (experiences unique to each person, plus measurement error) remains important at all ages.

Interpretation: as people grow up, they increasingly **select, shape, and seek out environments** that fit their genetic tendencies (gene–environment correlation), amplifying genetic differences.

---

## 4. Molecular genetics: what specific genes show

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

### SNP-based heritability

- Using common genetic variants (SNPs), researchers estimate:
  - **SNP heritability of intelligence**: ~20–30%.
- This is **lower** than twin-study estimates (~70–80%) because:
  - GWAS captures only common variants and additive effects.
  - Twin studies capture all genetic effects (including rare variants, non-additive interactions, etc.).

The fact that SNP-based estimates are substantial and in the same ballpark supports the reality of genetic influence.

### Polygenic scores

- Intelligence is **highly polygenic**: thousands of variants, each with tiny effects.
- Polygenic scores built from GWAS can:
  - Predict a **modest but non-trivial** portion of IQ and educational attainment (often ~10–20% of variance in large, homogeneous samples).
- These scores:
  - Work best within the same ancestry group used to build them.
  - Lose predictive power across ancestries, highlighting the role of population structure and environment.

---

## 5. Environment still matters a lot

High heritability ≠ environment is unimportant.

Key points:

- **Heritability is context-dependent**:
  - In **more equal, affluent environments**, heritability tends to be **higher** because environmental differences are smaller, so genetic differences explain more of what’s left.
  - In **harsh or highly unequal environments**, environmental factors can dominate, and heritability can be **lower**.
- **Interventions can shift the mean**:
  - Even if heritability is 70%, you can still raise or lower the *average* IQ of a population through nutrition, education, health, and social policy.
  - Classic example: the **Flynn effect**—large IQ gains over the 20th century—occurred far too quickly to


text:
Human intelligence is substantially heritable, but not fixed, and it’s strongly shaped by environment. The best evidence comes from large twin, family, and genomic studies.

Here’s what the data show, in layers:

---

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

- **Heritability (h²)** = the proportion of *differences between people* in a trait that can be statistically attributed to genetic differences *in a given population and environment*.
- It does **not** mean:
  - “X% of your intelligence comes from genes and the rest from environment.”
  - “Intelligence is unchangeable.”
  - “Genes are destiny.”

Heritability is about **variation among people**, not about an individual.

---

## 2. Twin and family studies: broad picture

Across many countries and cohorts, twin and family studies converge on:

- **Childhood IQ**: heritability ~ **0.3–0.5** (30–50%)
- **Adolescent IQ**: heritability ~ **0.5–0.6**
- **Adult IQ**: heritability ~ **0.6–0.8** (60–80%)

Key patterns:

- **Heritability increases with age**: as people grow up and choose their own environments (friends, education, jobs), their genetic tendencies are more fully expressed.
- **Shared family environment** (things siblings share: home, parents, neighborhood) has:
  - Moderate effects in **childhood**.
  - Much smaller effects in **adulthood** on *differences* in IQ, in typical Western samples.
- **Non-shared environment** (unique experiences, accidents, illnesses, random life events, measurement error) always explains a substantial portion of variation.

These estimates come from:
- Monozygotic (identical) vs dizygotic (fraternal) twin comparisons.
- Adoption studies (e.g., adopted children vs biological and adoptive parents).
- Extended family designs (siblings, half-siblings, cousins).

---

## 3. Adoption and environmental effects

Adoption studies show both strong environmental and strong genetic influences:

- **Early in life**, adopted children’s IQs are closer to their **adoptive parents** than to their biological parents, showing a real environmental boost.
- **By late adolescence/adulthood**, their IQs become more similar to their **biological parents** and less to adoptive parents, indicating that genetic influences dominate long-term individual differences.

But:
- Moving children from very deprived to more enriched homes can raise average IQ by **10–20 points** in some cases.
- Large environmental shocks (malnutrition, toxins, extreme poverty, poor schooling) can significantly depress cognitive development.

So:
- Genes explain a lot of *variation within* a given environment.
- Environments can still shift the *overall level* of cognitive performance up or down.

---

## 4. Molecular genetics: what specific genes show

Genome-wide association studies (GWAS) and polygenic scores give a more direct view:

- Intelligence is **highly polygenic**: thousands of genetic variants each have tiny effects.
- Current polygenic scores for educational attainment or cognitive ability can explain roughly:
  - ~10–20% of variance in educational attainment in European-ancestry samples.
  - A bit less for directly measured IQ.
- This is **lower** than twin-based heritability because:
  - We don’t capture all variants.
  - Effect sizes are tiny and noisy.
  - Scores are less predictive across ancestries due to population structure and LD differences.

Still, these scores:
- Predict cognitive test performance, years of education, and related traits.
- Show that **measurable DNA differences** align with the heritability estimates from twin studies, though they don’t yet capture all of it (“missing heritability”).

---

## 5. Gene–environment interplay

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

1. **Gene–environment correlation**:
   - **Passive**: smarter parents provide both genes and more cognitively stimulating homes.
   - **Evocative**: children with higher cognitive tendencies elicit more intellectual engagement from adults.
   - **Active**: people seek out environments that fit their abilities (books, courses, careers).

2. **Gene–environment interaction**:
   - The effect of genes can depend on environment.
   - Some evidence suggests:
     - In **very deprived environments**, genetic potential may be suppressed, lowering heritability.
     - In **more equal, affluent environments**, heritability can appear higher because environmental constraints are reduced and genetic differences stand out more.

Findings on this are mixed across countries and cohorts, but the basic idea is well-supported: **context matters for how genes express**.

---

## 6. Group differences vs individual heritability

A crucial distinction:

- Heritability


text:
Human intelligence is substantially heritable, but not fixed, and environment still matters a lot. The best evidence comes from large twin, family, and genomic studies.

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

- **Heritability (h²)** = the proportion of *differences between people* in a trait that can be statistically attributed to genetic differences **in a given population and environment**.
- It does **not** mean:
  - “X% of your intelligence comes from genes and the rest from environment.”
  - “Intelligence is unchangeable.”
  - “Genes are more important than environment for any given person.”

It’s a population-level statistic, not a personal destiny score.

---

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

These compare:
- **Identical (MZ) twins**: share ~100% of segregating genes.
- **Fraternal (DZ) twins / siblings**: share ~50% on average.
- **Adopted children**: share environment but not genes with adoptive parents.

Key findings (for general intelligence, “g”):

- **Childhood**: heritability ~0.3–0.4 (30–40%)
- **Adolescence**: ~0.5–0.6 (50–60%)
- **Adulthood**: ~0.6–0.8 (60–80%)

This “rising heritability” pattern is one of the most robust findings:
- As people age, they select and shape environments that fit their genetic tendencies (gene–environment correlation).
- Shared family environment (home, parenting style) matters more in childhood, less in adulthood.

Meta-analyses (e.g., Plomin & Deary, 2015; Polderman et al., 2015) consistently find:
- **Substantial genetic influence** on IQ/g.
- **Non-shared environment** (experiences unique to each individual, plus measurement error) explains most of the rest.
- **Shared environment** (family-wide factors) is moderate in childhood, small by adulthood.

---

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

Modern studies use DNA directly:

#### SNP-based heritability (GREML, LD-score regression)
These estimate how much variance is explained by common genetic variants (SNPs) measured on genotyping arrays.

- For intelligence / educational attainment:
  - **SNP heritability** typically ~0.2–0.3 (20–30%) for IQ/g.
  - This is a **lower bound** on total genetic influence, because:
    - It misses rare variants, structural variants, and some gene–gene interactions.
    - It’s limited by measurement noise and sample size.

These values are consistent with twin-based estimates once you account for what SNP methods can and can’t see.

#### GWAS (genome-wide association studies)
- Very large GWAS (hundreds of thousands to millions of people) have:
  - Identified **thousands of variants** associated with educational attainment and cognitive performance.
  - Each variant has a **tiny effect**; intelligence is **highly polygenic**.
- Polygenic scores (PGS) built from these variants:
  - Can explain ~10–20% of variance in educational attainment in European-ancestry samples; somewhat less for IQ directly.
  - Predictive power drops in other ancestries due to population structure and LD differences, not because genes “don’t matter” there.

---

### 4. Gene–environment interplay

Heritability does not mean environment is unimportant. Several key points:

- **High heritability can coexist with large environmental effects.**
  - Example: height is ~80–90% heritable in rich countries, yet nutrition and disease burden can shift average height by many centimeters.
  - Similarly, average IQ can be shifted by schooling, health, toxins, etc., even if within-population differences are substantially genetic.

- **Heritability is context-dependent.**
  - In more **equal, resource-rich environments**, heritability tends to be higher because environmental variation is smaller.
  - In **harsh or unequal environments**, environmental factors can dominate, lowering heritability estimates.

- **Gene–environment correlation**:
  - People with certain genetic predispositions tend to:
    - Seek out certain environments (active rGE).
    - Elicit certain responses from others (evocative rGE).
    - Be born into correlated environments (passive rGE).
  - This makes “genes vs environment” hard to cleanly separate.

- **Gene–environment interaction (G×E)**:
  - The effect of genes can depend on environment (and vice versa).
  - Some evidence suggests genetic influences on IQ are more fully expressed in supportive environments.

---

### 5. Group differences and what herit


text:
Human intelligence is substantially heritable, but not fixed, and strongly shaped by environment. The best evidence comes from large twin, adoption, and genomic studies.

Here’s the core picture:

## 1. What “heritability” means

- Heritability (h²) = the proportion of **differences between people** in a trait that can be statistically attributed to genetic differences **in a given population and environment**.
- It does **not** mean “X% of your intelligence comes from genes.” It’s about variation across people, not about individuals.
- If environments change, heritability can change.

## 2. Twin and adoption studies

These use how similar people are in IQ given how related they are genetically and environmentally.

### Twin studies

- Identical (MZ) twins share ~100% of their genes; fraternal (DZ) twins share ~50% on average.
- If MZ twins are more similar in IQ than DZ twins, that points to genetic influence.

Meta-analyses (e.g., Plomin & von Stumm 2018; Polderman et al. 2015) typically find:

- **Childhood IQ**: heritability ~0.3–0.4 (30–40%)
- **Adolescence**: ~0.5–0.6 (50–60%)
- **Adulthood**: ~0.7–0.8 (70–80%)

So genetic influence on intelligence appears to **increase with age**. This is often interpreted as “genetic amplification”: as people grow up, they select, shape, and seek out environments that fit their genetic tendencies (gene–environment correlation).

### Adoption studies

- Adopted children share environment but not genes with adoptive parents.
- In childhood, adopted children’s IQs correlate moderately with adoptive parents.
- By late adolescence/adulthood, that correlation drops toward zero, while correlation with **biological** parents rises.

This supports the idea that **shared family environment matters more early in life**, and genetic differences explain more of the variation later.

## 3. Shared vs non-shared environment

Behavioral genetics usually partitions variance into:

- **Genetic (A)**: heritable influences.
- **Shared environment (C)**: factors that make siblings similar (same home, SES, parenting style).
- **Non-shared environment (E)**: factors that make siblings different (different peers, teachers, illnesses, random events, measurement error).

Findings:

- **Shared environment**: substantial in childhood IQ (often 20–30% of variance), but tends to **shrink toward zero in adulthood**.
- **Non-shared environment**: consistently important (often 20–30%+), and includes many idiosyncratic experiences plus noise.

So: genes + non-shared environment dominate adult IQ differences; the “family you grew up in” has a strong early effect that fades statistically, even though it may still matter in many ways.

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

Genome-wide association studies (GWAS) look for specific DNA variants associated with cognitive ability or educational attainment.

Findings:

- Intelligence and related traits are **highly polygenic**: thousands of variants, each with tiny effects.
- Polygenic scores (PGS) built from these variants can now explain:
  - Roughly **10–20%** of variance in educational attainment in large European-ancestry samples.
  - Typically **5–15%** of variance in measured cognitive ability, depending on sample and measure.

This is **lower** than twin-based heritability (the “missing heritability” problem), but as sample sizes grow, PGS predictive power has been increasing.

Important caveats:

- Predictive power is **much lower in non-European ancestry groups**, because most GWAS have been Euro-centric.
- PGS are **probabilistic**, not deterministic: people with low scores can have high IQ and vice versa.

Molecular data broadly **confirm** that intelligence is substantially heritable and polygenic, consistent with twin/adoption estimates.

## 5. Environment still matters a lot

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

Examples:

- **Secular IQ gains (Flynn effect)**: average IQ scores rose ~2–3 points per decade in many countries during the 20th century—far too fast to be genetic. Likely drivers: better nutrition, education, health, test familiarity, and more cognitively demanding environments.
- **Socioeconomic status (SES)**:
  - In some U.S. samples, heritability of IQ appears **lower in low-SES families** and higher in high-SES families (suggesting that deprivation can suppress genetic potential).
  - In several European and other countries with stronger social safety nets, this SES–heritability interaction is weaker or absent.
- **Extreme environments