text:
Human intelligence, as measured by standardized cognitive tests (often summarized as IQ or general cognitive ability, “g”), is **substantially heritable**, but it is also **strongly shaped by environment**, especially in childhood. The best summary is: **genetic differences account for a sizable share of variation among individuals within a studied population, but heritability does not mean intelligence is fixed or unaffected by social conditions.**

## What “heritability” means

Heritability is a statistic describing **how much of the variation in a trait within a particular population and environment** is associated with genetic differences.

It does **not** mean:

- “X% of a person’s intelligence is genetic”
- intelligence is immutable
- environment is unimportant
- differences between populations are genetic
- genes determine destiny

A trait can be highly heritable and still be very responsive to environmental change. Height is a classic example: highly heritable, yet average height changed greatly with nutrition and health improvements.

## Typical heritability estimates

Across many studies, estimates for cognitive ability are roughly:

- **Childhood:** about **20–50% heritable**
- **Adolescence:** about **40–60%**
- **Adulthood:** often around **50–80%**

A commonly cited broad adult estimate is around **50–70%**, though it varies by sample, age, socioeconomic context, and measurement.

The heritability of intelligence tends to **increase with age**. One reason is that as people grow older, they increasingly select and shape environments that fit their genetic dispositions—choosing certain hobbies, peer groups, schools, occupations, and intellectual activities.

## Main lines of evidence

### 1. Twin studies

Twin studies compare identical twins, who share nearly all their DNA, with fraternal twins, who share about half their segregating genetic variation.

Findings consistently show:

- Identical twins are more similar in IQ than fraternal twins.
- Identical twins raised apart are still substantially similar.
- Fraternal twins and ordinary siblings are less similar than identical twins.

This pattern supports a significant genetic contribution.

### 2. Adoption studies

Adoption studies compare children to their biological and adoptive relatives.

Typical findings:

- In childhood, adopted children’s IQs are influenced by the adoptive home environment.
- As adopted individuals age, their cognitive scores often become more similar to those of their biological relatives.
- Shared family environment appears important in childhood but often less influential in adulthood.

This does not mean parenting “doesn’t matter.” It means that within the range of families studied, long-term individual differences in measured cognitive ability are less strongly predicted by the shared household environment than by genetic differences and nonshared experiences.

### 3. Molecular genetics and GWAS

Genome-wide association studies identify many DNA variants associated with cognitive performance and educational attainment.

Key findings:

- Intelligence is **highly polygenic**: thousands of genetic variants each have tiny effects.
- Polygenic scores can predict some variation in cognitive outcomes, but they are far from deterministic.
- Current DNA-based predictors explain much less than the total heritability estimated from twin studies, though their predictive power has increased over time.

This supports the idea that genetic influence is real, diffuse, and probabilistic.

## Environmental influences are also large

Many environmental factors affect cognitive development, including:

- prenatal health
- nutrition
- exposure to toxins such as lead
- disease burden
- sleep
- stress and trauma
- quality of schooling
- cognitive stimulation
- family resources
- neighborhood safety
- access to books, language, and enrichment
- social expectations and discrimination

Large environmental effects are evident from phenomena such as the **Flynn effect**, the substantial rise in IQ scores across many countries during the 20th century. Genetic change cannot explain such rapid population-level gains; environmental changes are the likely cause.

## Socioeconomic context matters

Some studies suggest heritability may be lower in disadvantaged environments, especially in childhood, because deprivation can suppress genetic potential. In more supportive environments, genetic differences may explain more of the remaining variation.

However, findings on this vary by country and cohort. In societies with more universal access to health care, nutrition, and education, the socioeconomic moderation of heritability may be weaker.

## Important cautions

Heritability estimates are **population-specific**. A heritability estimate from one country, age group, or historical period may not apply elsewhere.

Also, heritability within a population says little about the causes of average differences between groups. For example, even if individual differences in a given setting are highly heritable, group differences could still be caused largely or entirely by environmental factors.

## Bottom line

The evidence shows that human intelligence is **moderately to highly heritable**, especially in adulthood, with typical estimates around **50–70%** in many studied populations. But intelligence is also meaningfully influenced by environment, development, education, health, and social conditions. Heritability describes sources of variation; it does not imply inevitability, immutability, or genetic explanations for group differences.


prompt_id: sci-iq-heritability

provider: openai

text:
Human intelligence—usually measured with IQ tests or a general cognitive ability factor, *g*—is **substantially heritable**, but not genetically fixed. The best evidence suggests that genetic differences account for a large share of variation in intelligence **within a given population and environment**, especially by adulthood.

## Short answer

- **Childhood IQ heritability:** roughly **20–40%**
- **Adolescent IQ heritability:** roughly **40–60%**
- **Adult IQ heritability:** often **50–80%**, with many estimates around **60%**
- The remaining variation is due to **environmental factors, measurement error, developmental randomness, and gene–environment interaction**.

Importantly, **heritability does not mean immutability**. A trait can be highly heritable and still be strongly affected by schooling, nutrition, disease, poverty, stress, culture, and other environmental factors.

---

## What the evidence shows

### 1. Twin studies show strong genetic influence

Identical twins share essentially 100% of their segregating DNA; fraternal twins share about 50% on average.

A consistent finding is:

- **Identical twins’ IQ scores are much more similar** than fraternal twins’.
- Identical twins reared apart are still quite similar in IQ.
- Fraternal twins and ordinary siblings are less similar.

This pattern strongly suggests a genetic contribution.

Typical correlations:

- Identical twins raised together: often around **0.75–0.85**
- Fraternal twins raised together: often around **0.40–0.60**
- Identical twins raised apart: often around **0.65–0.75**

Those numbers vary by study, age, and sample, but the overall pattern is robust.

---

### 2. Adoption studies support genetic effects, especially with age

Adoption studies compare children to their biological and adoptive relatives.

Common finding:

- In early childhood, adopted children’s IQ may correlate meaningfully with the adoptive family environment.
- As they grow older, their IQ tends to correlate more with their **biological parents’** cognitive ability than with their adoptive parents’.

This does **not** mean the adoptive environment is unimportant. Rather, it suggests that as people mature, genetic differences become more strongly expressed, partly because people select, shape, and respond to environments in ways influenced by their traits.

---

### 3. Heritability increases with age

One of the most replicated findings in behavioral genetics is that the heritability of intelligence tends to **increase from childhood into adulthood**.

A plausible explanation is gene–environment correlation:

- Children have environments largely chosen for them.
- Adults have more freedom to choose friends, education, jobs, hobbies, and intellectual challenges.
- Genetically influenced preferences and abilities can lead people into environments that amplify those differences.

So genetic influence may become more visible over time.

---

### 4. Molecular genetics confirms intelligence is highly polygenic

Genome-wide association studies show that intelligence is influenced by **many thousands of genetic variants**, each with tiny effects.

Key points:

- There is no single “gene for intelligence.”
- Intelligence is **polygenic**: many variants contribute very small amounts.
- DNA-based estimates, called SNP heritability, often explain around **20–30%** of variance in cognitive ability, depending on the sample and methods.
- Polygenic scores can predict some differences in IQ or educational attainment, but their predictive power is still limited and varies across populations.

Twin-study heritability is usually higher than SNP-based heritability because SNP methods capture only part of the genetic contribution currently measurable.

---

## What “heritability” does and does not mean

### Heritability means:

Heritability is the proportion of differences among people in a particular population that can be statistically associated with genetic differences.

For example, if adult IQ is 60% heritable in a population, that means about 60% of the observed variation in IQ within that population is associated with genetic differences.

### Heritability does **not** mean:

- “60% of your intelligence is genetic.”
- “Environment does not matter.”
- “IQ cannot be changed.”
- “Group differences are genetic.”
- “Genes determine destiny.”

Heritability is about **variation within a population**, not about the cause of an individual’s ability or the cause of differences between populations.

---

## Environment still matters a lot

Environmental effects on intelligence are well established. Important factors include:

- Quality and quantity of schooling
- Childhood nutrition
- Prenatal environment
- Exposure to lead or toxins
- Infectious disease burden
- Sleep, stress, and safety
- Family resources
- Language exposure
- Socioeconomic deprivation
- Cultural familiarity with testing
- Educational opportunity

The **Flynn effect**—large IQ gains across generations in many countries during the 20th century—is strong evidence that environments can substantially affect measured intelligence. Genes did not change fast enough to explain those gains.

Schooling also has measurable effects on cognitive test performance. More schooling generally improves scores, especially on skills closely related to reasoning, vocabulary, and abstract problem-solving.

---

## Shared versus non-shared environment

Behavioral genetic studies often divide environmental effects into:

### Shared environment

Factors that make siblings similar:

- Family income
- Neighborhood
- Parenting style
- Home resources
- School district

Shared environment tends to matter more in childhood and often appears smaller in adulthood.

### Non-shared environment

Factors that make siblings different:

- Different peer groups
- Different teachers
- Illnesses or injuries
- Individual experiences
- Random developmental variation
- Different treatment within the same family

Non-shared environment remains important throughout life.

---

## Socioeconomic context may affect heritability

There is evidence that heritability of intelligence can be lower in severely deprived environments, because environmental constraints suppress the expression of genetic potential.

For example:

- In high-opportunity environments, genetic differences may explain more variance.
- In deprived environments, lack of nutrition, poor schooling, stress, or disease may reduce everyone’s outcomes and make environmental factors more dominant.

The strength of this effect varies across countries and studies. It appears more consistent in some U.S. samples than in many European samples, likely because social conditions differ.

---

## Bottom line

The evidence shows that human intelligence is **moderately to highly heritable**, especially in adulthood. A reasonable summary is:

> Genetic differences explain roughly half or more of the variation in adult intelligence within many studied populations, while environmental factors also make a large and meaningful contribution.

The most accurate view is that intelligence develops through a complex interaction of:

- many genes of tiny effect,
- family and social environment,
- education,
- health and nutrition,
- personal experiences,
- and broader cultural and economic conditions.

So intelligence is neither “all genetic” nor “all environmental.” It is strongly influenced by both.

finish_reason: stop


text:
Human intelligence, as measured by IQ tests or broader “general cognitive ability” (often called *g*), is substantially heritable—but heritability is not 100%, varies by age and context, and does not mean intelligence is fixed or unaffected by environment.

## Short answer

Across modern behavioral-genetic research:

- **In childhood:** intelligence is moderately heritable, often around **30–50%**.
- **In adolescence/adulthood:** heritability typically rises to about **50–80%**, with many estimates around **60–70%** in adulthood.
- **Shared family environment**—factors making siblings similar, such as household resources or parenting—matters more in childhood and tends to decline in adulthood.
- **Nonshared environment**, measurement error, illness, schooling differences, peer groups, random developmental factors, and unique life experiences account for much of the remaining variation.

These estimates describe **variation within a studied population at a given time**, not the destiny of individuals and not differences between populations.

---

## What “heritability” means

Heritability is the proportion of variation in a trait within a population that is statistically associated with genetic differences among people.

So if IQ has a heritability of 60% in a given population, that does **not** mean:

- “60% of a person’s intelligence is genetic”
- “IQ cannot be changed”
- “environment does not matter”
- “group differences are genetic”

It means that, in that population and environment, genetic differences explain about 60% of the observed differences among individuals.

A trait can be highly heritable and still be strongly affected by environment. For example, height is highly heritable, but nutrition and disease can dramatically affect average height.

---

## Main lines of evidence

### 1. Twin studies

Twin studies compare identical twins, who share nearly all their DNA, with fraternal twins, who share on average about half of their segregating DNA.

A common finding is:

- Identical twins are more similar in IQ than fraternal twins.
- This is true even when twins are raised apart, although samples of separated twins are smaller and historically less common.

Meta-analyses of twin studies generally find substantial heritability for cognitive ability, with estimates increasing from childhood into adulthood.

### 2. Adoption studies

Adoption studies compare children’s resemblance to their biological and adoptive relatives.

Typical pattern:

- In childhood, adopted children often resemble both adoptive and biological families to some extent.
- As they age, resemblance to biological relatives tends to increase, while resemblance to adoptive relatives often decreases.

This supports the idea that genetic influences on intelligence become more apparent with age, partly because people increasingly select, shape, and respond to environments in ways correlated with their genetic tendencies.

### 3. Genome-wide association studies

Modern genetic studies look directly at DNA variation across very large samples.

Findings:

- Intelligence and educational attainment are **highly polygenic**: influenced by thousands of genetic variants, each with very small effect.
- Current measured common genetic variants explain a meaningful but incomplete share of variation.
- Polygenic scores can predict some cognitive and educational outcomes, but far from perfectly.

For cognitive ability, SNP-based heritability estimates are often lower than twin-based estimates, roughly in the **20–30%** range depending on sample and measure. This gap is sometimes called “missing heritability” and may reflect rare variants, structural variants, nonadditive effects, gene–environment correlation, measurement issues, and limitations of current genomic methods.

---

## Why heritability increases with age

One of the most replicated findings is that the heritability of intelligence tends to rise from childhood to adulthood.

Possible reasons include:

1. **Gene–environment correlation**
   People increasingly choose environments that fit their aptitudes and interests. A child with early verbal strengths may read more, seek intellectually stimulating peers, and pursue challenging coursework.

2. **Amplification over development**
   Small early differences can compound over time through schooling, practice, motivation, and opportunity.

3. **Declining shared family influence**
   As people leave the childhood home, shared family factors become less important relative to individual experiences and self-selected environments.

---

## Environmental effects are still important

Heritability does not imply that environment is unimportant. Evidence shows that intelligence is affected by many environmental factors, including:

- Quality and quantity of education
- Childhood poverty and deprivation
- Nutrition
- Lead and toxin exposure
- Prenatal environment
- Infectious disease burden
- Sleep, stress, and mental health
- Severe neglect or institutionalization
- Access to books, language, and cognitively stimulating environments

The **Flynn effect**—large IQ-score gains across generations in many countries during the 20th century—is especially important evidence that environments can shift population averages substantially over time. Genes did not change fast enough to explain those gains.

---

## Socioeconomic context matters

Some studies suggest that heritability of intelligence may be lower in more deprived environments and higher in more advantaged ones, especially in childhood. The idea is that when environments are highly unequal or limiting, environmental constraints suppress the expression of genetic differences.

However, evidence on this is mixed and appears to vary by country, welfare systems, age, and sample. In societies with broader access to nutrition, schooling, and healthcare, genetic differences may account for more of the remaining individual variation.

---

## What the evidence does *not* show

The evidence does not show that:

- Intelligence is genetically predetermined.
- Education or social policy cannot improve cognitive outcomes.
- Genetic differences explain racial, ethnic, national, or social-class differences in IQ.
- A person’s potential can be inferred reliably from a DNA test.
- High heritability means low malleability.

Heritability estimates apply to variation **within** populations under particular environmental conditions. They cannot automatically be used to explain differences **between** groups, especially when groups differ in history, discrimination, poverty, schooling, nutrition, healthcare, environmental exposure, or test familiarity.

---

## Bottom line

Human intelligence is one of the more heritable psychological traits. The best evidence from twin, adoption, and genomic studies suggests that genetic differences account for a substantial portion of individual differences in cognitive ability, especially in adulthood—often roughly **half or more**.

But intelligence is also strongly shaped by environment, development, education, health, and social conditions. Heritability is a population statistic, not a fixed property of an individual, and high heritability is fully compatible with meaningful environmental influence and societal intervention.


text:
Human intelligence, as measured by IQ or general cognitive ability tests, is **substantially heritable**, but not genetically determined. The best evidence suggests that in many studied populations, especially in high-income countries, **genetic differences account for roughly 50–80% of variation in adult cognitive ability**, with lower estimates in childhood and higher estimates later in life.

## What “heritable” means

Heritability is a statistic about **variation within a population**, not about an individual. If intelligence has a heritability of 60%, that means that, in a given population and environment, about 60% of the observed differences among people are statistically associated with genetic differences.

It does **not** mean:

- 60% of a person’s intelligence is “genetic”
- intelligence is fixed or unchangeable
- environments do not matter
- group differences are genetic
- genes act independently of environment

Heritability can change across societies, ages, and environments.

## Main findings

### 1. Heritability increases with age

A common pattern in behavioral genetics is that the heritability of cognitive ability rises from childhood to adulthood.

Approximate ranges often reported:

- **Early childhood:** ~20–40%
- **Middle childhood/adolescence:** ~40–60%
- **Adulthood:** ~50–80%

One explanation is that as people grow older, they increasingly select, shape, and respond to environments partly in line with their genetic tendencies. For example, a child with an early aptitude for reading may read more, which further strengthens verbal ability. This is called **gene–environment correlation**.

### 2. Twin studies show strong genetic influence

Twin studies compare identical twins, who share nearly all their DNA, with fraternal twins, who share about half of their segregating DNA.

Consistently, identical twins are more similar in IQ than fraternal twins. Typical correlations look roughly like:

- Identical twins raised together: high similarity, often around **0.75–0.85**
- Fraternal twins raised together: moderate similarity, often around **0.40–0.60**
- Identical twins raised apart: still quite similar, often around **0.65–0.75**, though estimates vary and samples are smaller

These patterns imply a substantial genetic contribution. However, twin studies depend on assumptions, such as the idea that identical and fraternal twins experience similarly comparable environments. Researchers debate the details, but the broad finding of meaningful genetic influence is robust.

### 3. Adoption studies also support genetic effects

Adoption studies provide another line of evidence.

In childhood, adopted children’s cognitive scores are influenced by the adoptive home environment. But as they age, their IQ tends to become more similar to that of their biological relatives than their adoptive relatives.

This suggests:

- Family environment matters, especially earlier in development.
- Genetic influences become more apparent with age.
- Shared family environment often explains less adult variation than childhood variation.

### 4. Molecular genetics confirms intelligence is highly polygenic

Modern genome-wide association studies, or GWAS, show that cognitive ability is influenced by **many thousands of genetic variants**, each with a very small effect. There is no “gene for intelligence.”

Current genetic predictors, called **polygenic scores**, can predict some variation in educational attainment and cognitive performance, especially in people of European ancestry, where the largest datasets exist. But they are far from determinative.

Roughly:

- SNP-based heritability estimates for cognitive ability are often around **20–30%**, depending on the sample and method.
- Polygenic scores currently explain a smaller fraction of measured cognitive differences, often in the **single digits to low teens percentage range** in best-studied populations.
- Prediction is weaker in populations underrepresented in genetic studies.

The gap between twin-study heritability and SNP-based estimates is often called part of the “missing heritability” problem. It may reflect rare variants, structural variants, gene–gene interactions, measurement limits, and other complexities.

## Environmental effects are also large

Even when heritability is high, environment can matter enormously. Evidence for environmental influence includes:

- The **Flynn effect**: IQ scores rose substantially across much of the 20th century in many countries, too quickly to be explained by genetic evolution.
- Effects of **education**, literacy, schooling quality, and cognitively stimulating environments.
- Harms from **lead exposure**, malnutrition, prenatal alcohol exposure, infectious disease, chronic stress, and deprivation.
- Improvements from adoption out of severely deprived environments.
- Socioeconomic and cultural differences in opportunities to develop and express cognitive skills.

High heritability within a population can coexist with strong environmental effects between populations or across time.

For example, height is highly heritable in well-nourished populations, yet average height can change substantially with nutrition and health. Intelligence is similar in that genetic differences explain some variation within a given setting, while environmental change can shift averages and outcomes.

## Shared vs nonshared environment

Behavioral genetic studies often divide environmental effects into:

- **Shared environment:** factors that make siblings similar, such as household income, parenting style, neighborhood, and school district.
- **Nonshared environment:** factors that make siblings different, such as different peer groups, illnesses, teachers, experiences, random developmental events, and measurement error.

For IQ:

- Shared environment tends to be more important in childhood.
- Its estimated effect often decreases by adulthood in many studies.
- Nonshared environment remains important throughout life.

That said, “shared” and “nonshared” categories are statistical abstractions and do not map perfectly onto real-world causes.

## Important caveats

1. **Heritability is population-specific.**
   Estimates from wealthy countries may not apply to populations with greater deprivation, inequality, disease burden, or limited schooling.

2. **Genes and environments interact.**
   Genetic potentials may be amplified or suppressed depending on nutrition, education, stress, safety, and opportunity.

3. **IQ is not all of intelligence.**
   IQ tests measure important cognitive abilities, especially reasoning, memory, processing speed, and verbal/spatial skills, but they do not capture every valuable mental trait.

4. **Heritability does not imply immutability.**
   A trait can be highly heritable and still responsive to intervention.

5. **Within-group heritability says little about between-group differences.**
   Evidence that individual differences within a population are partly genetic does not imply that average differences between social, ethnic, national, or racial groups are genetic.

## Bottom line

The evidence from twin, adoption, family, and molecular genetic studies shows that human intelligence is **moderately to highly heritable**, especially in adulthood. A reasonable summary is:

- **Childhood intelligence:** partly heritable, with substantial family and environmental influence.
- **Adult intelligence:** often around **50–80% heritable** in studied high-income populations.
- **Genetic architecture:** highly polygenic, involving many variants of tiny effect.
- **Environment:** crucial, especially through education, health, nutrition, toxins, stress, and opportunity.

So the best answer is: intelligence is significantly influenced by genetic differences, but it is also shaped by environment, development, culture, and life experience. Heritability is real, but it is not destiny.


text:
Human intelligence is **substantially heritable**, but heritability is often misunderstood. The best evidence suggests that **genetic differences explain a sizable share of individual differences in measured cognitive ability within a given population**, especially in adulthood. But this does **not** mean intelligence is fixed, immune to environment, or that group differences are genetic.

## Short answer

Across many studies, the heritability of general cognitive ability/IQ is roughly:

- **Childhood:** about **20–40%**
- **Adolescence:** about **40–60%**
- **Adulthood:** often around **50–80%**, commonly summarized near **60% or more**

The exact number depends on age, population, measurement, and environment.

## What “heritability” means

Heritability is the proportion of **variation among people in a particular population and environment** that is statistically associated with genetic differences.

It does **not** mean:

- “60% of a person’s intelligence is genetic”
- “Intelligence cannot be changed”
- “Environmental interventions do not matter”
- “Differences between social, ethnic, or national groups are genetic”

A trait can be highly heritable and still strongly affected by environment. Height is a classic example: it is highly heritable, but nutrition and disease can substantially affect average height.

## Main sources of evidence

### 1. Twin studies

Identical twins share nearly all their DNA; fraternal twins share about half, on average. If identical twins are more similar in IQ than fraternal twins, that suggests genetic influence.

Findings from many twin studies show:

- Identical twins are more similar in cognitive ability than fraternal twins.
- This remains true across different countries and cohorts.
- Genetic influence tends to increase with age.

Twin studies are one of the strongest traditional lines of evidence for heritability of intelligence.

### 2. Adoption studies

Adoption studies compare children to their biological and adoptive relatives.

Typical findings:

- In childhood, adopted children’s IQs are influenced by the adoptive family environment.
- As adopted children become adults, their IQs tend to resemble their biological relatives more than their adoptive relatives.
- Shared family environment appears to matter more in childhood and less in adulthood for IQ differences.

This supports both environmental and genetic influence.

### 3. Molecular genetics and GWAS

Modern genome-wide association studies, or GWAS, examine measured DNA variants across very large samples.

Findings show:

- Intelligence is **highly polygenic**: thousands of genetic variants each have tiny effects.
- Common DNA variants captured by genotyping explain a meaningful but incomplete portion of IQ variation.
- “SNP heritability” estimates are often lower than twin-study heritability, commonly around **20–30%** for cognitive ability, because they capture only part of genetic variation.
- Polygenic scores can predict some variation in cognitive test performance and educational attainment, but they are far from deterministic.

This molecular evidence independently confirms that genetics contributes to cognitive differences, though current DNA-based predictions are limited.

## Why heritability increases with age

A striking finding is that IQ heritability tends to rise from childhood to adulthood. Several explanations are plausible:

1. **Gene–environment correlation:** As people get older, they increasingly select and shape environments that fit their genetic tendencies. For example, a child who enjoys reading may seek books, advanced classes, and intellectually stimulating peers.

2. **Reduced shared family influence:** The common family environment has a stronger effect when children live at home, but its influence often weakens as people gain independence.

3. **Stability of cognitive traits:** General cognitive ability becomes more stable with age, allowing genetic differences to account for more long-term variation.

## Environmental effects are real

High heritability does not imply that environment is unimportant. Evidence shows intelligence can be affected by:

- Quality and length of schooling
- Childhood nutrition
- Prenatal environment
- Exposure to toxins such as lead
- Infectious disease burden
- Severe deprivation or neglect
- Family stress and socioeconomic conditions
- Access to cognitively stimulating environments

The **Flynn effect**—large IQ score gains across generations in many countries during the 20th century—is strong evidence that environmental change can shift population averages substantially.

## Shared vs non-shared environment

Behavioral genetics often separates environment into:

- **Shared environment:** factors making siblings similar, such as family socioeconomic status, neighborhood, or parenting style.
- **Non-shared environment:** factors making siblings different, such as different peer groups, illnesses, teachers, experiences, or random developmental events.

For intelligence:

- Shared environment tends to have a noticeable effect in childhood.
- Its estimated effect often declines by adulthood.
- Non-shared environmental influences and measurement error account for part of the remaining variation.

## Important caveats

### Heritability is population-specific

A heritability estimate applies to a particular population in a particular range of environments. If environments become more equal, heritability can rise because environmental variation is reduced. If environments become more unequal or deprived, environmental effects can become larger.

### Heritability does not explain group differences

Even if IQ is substantially heritable within a population, that does not tell us why two groups differ on average. Within-group heritability cannot be used to infer between-group causes.

For example, height is highly heritable within wealthy countries, but average height differences between countries have often been largely environmental, due to nutrition and health.

### IQ is not the whole of intelligence

IQ tests measure important cognitive abilities, especially general reasoning, verbal ability, working memory, and processing speed. They predict many outcomes, including academic and occupational performance. But they do not capture all valuable human capacities, such as creativity, wisdom, motivation, emotional skill, practical judgment, or domain-specific expertise.

## Bottom line

The evidence from twin, adoption, and DNA studies converges on the conclusion that **human intelligence is moderately to highly heritable**, especially in adulthood. A reasonable summary is that **genetic differences explain roughly half or more of adult variation in measured general cognitive ability** in many studied populations.

At the same time, intelligence is also shaped by environment, development, education, health, and social conditions. Heritability is not destiny, and it does not imply that environmental improvements are ineffective or that group differences are genetic.


text:
Human intelligence—as typically measured by IQ tests or the general cognitive ability factor *g*—is **substantially heritable**, but it is **not genetically fixed**, and heritability does **not** mean immutability or explain differences between social groups.

## Short answer

Across modern behavioral-genetic studies, the heritability of intelligence is roughly:

- **Childhood:** about **20–40%**
- **Adolescence:** about **40–60%**
- **Adulthood:** often **50–80%**, with many estimates around **60%**

That means that within a given population, under a given range of environments, genetic differences account for a sizable share of the observed variation in cognitive test scores.

But the remaining variation is due to **environmental influences, measurement error, developmental randomness, gene–environment interplay, education, health, nutrition, family conditions, and broader social context**.

## What “heritability” means

Heritability is often misunderstood.

A heritability estimate of, say, 60% does **not** mean:

- “60% of a person’s intelligence is genetic”
- “IQ cannot be changed”
- “environment does not matter”
- “group differences are genetic”

It means:

> In a particular population at a particular time, about 60% of the differences among individuals in measured intelligence are statistically associated with genetic differences.

Heritability can be high even for traits strongly affected by environment. For example, height is highly heritable in well-nourished populations, but average height can still increase dramatically when nutrition and disease conditions improve.

## Evidence from twin and adoption studies

The strongest classical evidence comes from twin, family, and adoption research.

### Twin studies

Identical twins share nearly 100% of their segregating DNA, while fraternal twins share about 50% on average. If identical twins are more similar in IQ than fraternal twins, that suggests genetic influence.

Findings are consistent:

- Identical twins raised together are very similar in cognitive ability.
- Identical twins raised apart are also substantially similar, though not identical.
- Fraternal twins and ordinary siblings are less similar than identical twins.
- Adoptive siblings, who share a home but not genes, tend to become less similar in IQ as they grow older.

These patterns imply a substantial genetic contribution.

### Adoption studies

Adoption studies show that children’s IQs are influenced by the homes they grow up in, especially in childhood. However, as children mature, their IQ scores tend to correlate more strongly with their biological relatives than with their adoptive relatives.

This does not mean adoptive environments do not matter. Rather, it suggests that over development, people may increasingly select, shape, or evoke environments that align with their genetic tendencies.

## Evidence from DNA studies

Modern genome-wide association studies, or GWAS, provide molecular evidence.

Researchers have found that intelligence and educational attainment are influenced by **many thousands of genetic variants**, each with very tiny effects. This is called a **polygenic** architecture.

Important points:

- There is no single “intelligence gene.”
- Each identified variant explains a minuscule amount of variation.
- Polygenic scores can predict some cognitive and educational outcomes, but imperfectly.
- Current DNA-based predictors explain much less variance than twin-study heritability estimates, though they have improved as sample sizes have grown.

This gap between twin-based heritability and DNA-predicted variance is sometimes called part of the “missing heritability” problem.

## Heritability increases with age

One robust finding is that the heritability of intelligence tends to rise from childhood into adulthood.

Possible reasons include:

1. **Gene–environment correlation**
   People with different genetic tendencies may seek out or create different environments. For example, a child who enjoys reading may read more, which further strengthens verbal ability.

2. **Increasing autonomy**
   Adults have more control over their education, work, hobbies, and peer groups than children do.

3. **Environmental differences shared by siblings fade over time**
   The common family environment appears to have stronger effects in childhood than adulthood, at least in many affluent societies.

## Environment still matters a lot

Even when heritability is high, environment can be powerful.

Known environmental influences on cognitive development include:

- Prenatal health
- Childhood nutrition
- Exposure to toxins such as lead
- Infectious disease burden
- Sleep
- Stress and trauma
- Quality and duration of schooling
- Cognitive stimulation
- Family resources
- Neighborhood conditions
- Social stability
- Access to healthcare

Large environmental effects are visible in phenomena such as the **Flynn effect**, the substantial rise in IQ test scores observed across many countries during the 20th century. Such changes occurred too quickly to be genetic and likely reflect improvements in education, health, nutrition, test familiarity, and cognitively demanding environments.

## Heritability depends on the population

Heritability is not a universal constant. It can change depending on environmental conditions.

For example:

- In societies where nutrition, schooling, and healthcare are broadly available, more of the remaining variation may be associated with genetic differences.
- In environments with severe deprivation or inequality, environmental factors may suppress or distort genetic potential, lowering or changing heritability estimates.

So a heritability estimate from one country, age group, or historical period may not apply elsewhere.

## What about group differences?

Heritability within a population does **not** explain differences between populations or social groups.

For example, height is highly heritable within many populations, but average height differences between generations or countries can be largely environmental. The same logic applies to intelligence.

Therefore, evidence that intelligence is heritable among individuals does not by itself imply that observed average differences between socioeconomic, racial, ethnic, national, or historical groups are genetic in origin.

## Bottom line

The evidence shows that human intelligence is:

- **Moderately to highly heritable**, especially in adulthood
- Influenced by **many genes of tiny effect**
- Strongly shaped by **environment and development**
- Not reducible to genetics alone
- Not fixed or immune to intervention
- Poorly summarized by a single universal heritability number

A reasonable summary is:

> Genetic differences explain a substantial portion of individual differences in measured intelligence, especially in adulthood, but environmental conditions play a major role in how cognitive abilities develop and in differences across time, place, and social context.


text:
Human intelligence, as measured by IQ tests or general cognitive ability “g,” is **substantially heritable**, but not genetically fixed. The best summary is:

> **In contemporary industrialized populations, genetic differences account for roughly 40–80% of individual differences in measured intelligence, with heritability increasing from childhood into adulthood.**

A common estimate is about **50% in childhood** and **60–80% in adulthood**, though estimates vary by population, age, environment, and method.

## What “heritable” means

Heritability is about **variation within a population**, not about an individual’s intelligence.

If adult IQ heritability is estimated at 70%, that does **not** mean “70% of your intelligence is genetic.” It means that, in a particular population and environment, about 70% of the observed differences between people are statistically associated with genetic differences.

Heritability also does **not** mean immutability. Height is highly heritable, but nutrition and disease strongly affect height. Intelligence is similar: genes matter, but so do environment, education, health, toxins, stress, family conditions, and culture.

## Main lines of evidence

### 1. Twin studies

Twin studies compare identical twins, who share nearly all their DNA, with fraternal twins, who share about half on average.

Findings are consistent:

- Identical twins are more similar in IQ than fraternal twins.
- Identical twins reared apart are still quite similar.
- Genetic influence tends to increase with age.

Typical results:

| Age/stage | Approximate heritability of IQ |
|---|---:|
| Early childhood | ~20–40% |
| Middle childhood | ~40–50% |
| Adolescence | ~50–60% |
| Adulthood | ~60–80% |

Shared family environment appears more important in childhood and often less important in adulthood.

### 2. Adoption studies

Adoption studies also support genetic influence.

Common findings:

- Adopted children resemble their adoptive families somewhat in childhood.
- As they grow older, their IQs tend to become more similar to their biological relatives.
- Adoptive siblings often become less similar in IQ by adulthood, despite sharing the same household.

This suggests that family environment matters, especially early, but genetic differences become more predictive over time.

### 3. Molecular genetic evidence

Modern DNA studies confirm that intelligence is genetically influenced, but in a highly polygenic way.

That means:

- There is no single “intelligence gene.”
- Thousands of genetic variants contribute.
- Each variant has a tiny effect.
- Genetic prediction is probabilistic, not deterministic.

Genome-wide association studies find that common genetic variants explain a meaningful but incomplete portion of IQ differences. DNA-based “SNP heritability” estimates are often lower than twin-study estimates, roughly in the range of **20–30% or more**, depending on the trait and sample. Polygenic scores can predict some variation in cognitive performance and educational attainment, but they are far from determinative and work best in populations similar to those in which they were developed.

## Why heritability increases with age

One interesting finding is that intelligence becomes more heritable from childhood to adulthood. This may seem surprising, but there are plausible reasons:

- People increasingly select environments that fit their abilities and interests.
- Genetically influenced traits affect which books, friends, schools, hobbies, and careers people pursue.
- Parents and teachers may respond differently to children’s abilities.
- Small early differences can compound over time.

This is called **gene–environment correlation**: genes can influence the environments people experience.

## Environmental effects are real

The evidence does not imply that environment is unimportant. Quite the opposite.

Environmental influences on cognitive development include:

- Quality and length of schooling
- Early childhood nutrition
- Prenatal health
- Exposure to lead and other toxins
- Sleep
- Chronic stress
- Infectious disease burden
- Parenting and language exposure
- Socioeconomic conditions
- Access to books, stimulation, and safe neighborhoods

The **Flynn effect**—large IQ gains across generations during the 20th century—shows that environmental changes can shift population averages substantially over time, even when heritability within a generation is high.

## Important caveats

1. **Heritability is population-specific.**
   Estimates from one society or era may not apply to another.

2. **High heritability does not mean fixed or untreatable.**
   A trait can be highly heritable and still responsive to environmental change.

3. **Genes and environment are intertwined.**
   Genetic effects often depend on environmental opportunities.

4. **Most genetic studies are ancestry-biased.**
   Many large genetic studies have been conducted primarily in people of European ancestry, so results may not transfer well across populations.

5. **Group differences are not explained by heritability estimates.**
   Heritability within a population does not tell us why average differences between groups exist. Between-group differences can be heavily environmental even when within-group heritability is high.

## Bottom line

Human intelligence is **moderately to highly heritable**, especially in adulthood. Twin, adoption, and DNA studies all support a substantial genetic contribution. But intelligence is influenced by many genes of tiny effect, by many environmental factors, and by the interaction between them. The evidence supports neither “IQ is purely genetic” nor “IQ is purely environmental.” It supports a developmental picture in which genetic differences matter a lot, environments matter a lot, and the two shape each other over time.


text:
Human intelligence—as usually measured by IQ tests or a general cognitive ability factor, *g*—is substantially heritable, but not genetically determined. The best evidence suggests that genetic differences explain a meaningful share of individual differences in intelligence within a given population, while environment also matters, especially in childhood and in disadvantaged conditions.

## Short answer

- **Heritability of intelligence is often estimated around ~50% on average**, but it varies by age, population, and environment.
- In **childhood**, estimates are commonly around **20–50%**.
- In **adolescence and adulthood**, estimates often rise to about **50–80%**.
- Shared family environment appears more important in childhood and usually less important by adulthood.
- Modern DNA-based studies confirm a genetic contribution, but current polygenic scores explain only part of the variation and work best in populations similar to those used in the discovery studies.

Importantly, **heritability does not mean immutability**, and it does **not** explain differences between groups or societies by itself.

---

## What “heritability” means

Heritability is the proportion of variation in a trait within a particular population, at a particular time, that is statistically associated with genetic variation.

So if intelligence has a heritability of 60% in a population, that does **not** mean:

- 60% of a person’s intelligence is genetic;
- intelligence cannot be changed;
- environment is unimportant;
- group differences are genetic.

It means that, among people in that population and environment, genetic differences account for about 60% of the observed individual differences.

Heritability can change if environments change. For example, if schooling, nutrition, health care, and safety become more equal, a larger share of the remaining differences may be associated with genetic variation. Conversely, in highly unequal or deprived environments, environmental constraints can suppress genetic potential and reduce heritability estimates.

---

## Evidence from twin studies

Twin studies are one of the strongest traditional sources of evidence.

The basic comparison is:

- **Identical twins** share essentially 100% of their DNA.
- **Fraternal twins** share about 50% of their segregating DNA, like ordinary siblings.

If identical twins are more similar in IQ than fraternal twins, that suggests a genetic contribution.

Across many studies, identical twins tend to be substantially more similar in cognitive ability than fraternal twins. This pattern has been found in different countries, age groups, and testing formats.

Typical findings:

- Identical twin IQ correlations are often around **0.75–0.85**, especially in adulthood.
- Fraternal twin or sibling correlations are often around **0.35–0.55**.
- Twins reared apart are still notably similar, though not identical, which supports genetic influence while also showing environmental effects.

Twin studies generally estimate intelligence heritability as moderate in childhood and high in adulthood.

---

## Evidence from adoption studies

Adoption studies also support genetic influence.

A common finding is that adopted children’s IQs are influenced by both their adoptive environment and their biological family background. In childhood, adopted children often resemble their adoptive families to a meaningful degree. But as they grow older, their cognitive scores tend to correlate more strongly with their biological relatives than with adoptive relatives.

This pattern suggests:

- Family environment matters, especially early in life.
- Genetic differences become more predictive over development.
- People may increasingly select, shape, and respond to environments partly in line with their genetic tendencies.

---

## Evidence from molecular genetics

Modern genome-wide association studies, or GWAS, examine measured DNA directly. They show that intelligence and educational attainment are influenced by many genetic variants, each with very small effects.

Key points:

- Intelligence is **highly polygenic**: thousands of genetic variants are involved.
- No single “intelligence gene” has a large effect in the normal range.
- SNP-based heritability estimates—using measured common genetic variants—are usually lower than twin-study estimates, often around **20–30%** for cognitive ability, depending on method and sample.
- Polygenic scores can predict some variation in IQ or educational attainment, but they are far from deterministic.

Current polygenic scores may explain roughly:

- a modest percentage of variance in cognitive test performance;
- somewhat more for educational attainment in some large European-ancestry samples;
- less in non-European ancestry groups, largely because most discovery datasets have been European-heavy.

This gap is a major limitation and an active area of research.

---

## Why heritability increases with age

One of the most replicated findings is that intelligence heritability tends to rise from childhood to adulthood.

Several mechanisms may contribute:

1. **Gene–environment correlation**
   As people grow up, they increasingly choose and create environments that fit their interests and abilities. A child with a genetic tendency toward verbal ability may read more, seek intellectually stimulating activities, and receive more encouragement.

2. **Reduced shared family influence**
   Siblings raised in the same home often become less similar in cognitive ability as they age, suggesting that common family environment has a stronger effect early on than later.

3. **Cumulative developmental effects**
   Small early differences can compound over time through schooling, peer groups, motivation, and opportunity.

---

## Environmental influences are still important

Even if intelligence is substantially heritable, environment matters a great deal. Environmental factors associated with cognitive development include:

- prenatal health;
- nutrition;
- exposure to toxins such as lead;
- childhood disease burden;
- quality and quantity of schooling;
- family stress and poverty;
- sleep;
- intellectual stimulation;
- neighborhood safety;
- discrimination and social barriers;
- access to books, technology, and skilled teaching.

Large environmental changes can shift average cognitive performance. The **Flynn effect**, the substantial rise in IQ scores over much of the 20th century in many countries, is strong evidence that environment can have major effects on population-level test performance.

---

## Heritability differs by socioeconomic context

Some studies, especially in the United States, have found that heritability of IQ may be lower among children in lower socioeconomic environments and higher in more advantaged environments. The idea is that deprivation can limit the expression of genetic potential.

However, this finding is not equally strong everywhere. In some European samples with stronger social safety nets, the socioeconomic moderation effect appears smaller or absent. So the relationship between heritability and social class likely depends on the broader social environment.

---

## What the evidence does *not* show

The evidence does not show that intelligence is fixed or predetermined.

It also does not justify genetic explanations for average differences between racial, ethnic, national, or social groups. Heritability within a population does not imply that between-group differences are genetic. For example, height is highly heritable, but average height differences between countries can be strongly affected by nutrition, disease, and living conditions.

Similarly, IQ differences across groups or eras can be heavily influenced by schooling, wealth, health, test familiarity, discrimination, language, and other environmental factors.

---

## Bottom line

The evidence from twin, adoption, family, and DNA-based studies shows that human intelligence is **moderately to highly heritable**, especially in adulthood. A reasonable summary is:

- **Children:** roughly **20–50% heritable**
- **Adults:** roughly **50–80% heritable**
- **Overall:** often summarized around **~50%**, with substantial variation by context

But intelligence is shaped by both genes and environment. Genetic influence is real, highly polygenic, and probabilistic—not deterministic. Environmental conditions can raise or lower cognitive development, alter heritability estimates, and strongly affect average performance across time and place.


text:
Human intelligence is **substantially heritable**, but not genetically fixed. The best-supported conclusion is that, **within studied populations**, genetic differences account for a meaningful share of individual differences in cognitive ability, while environment also matters—especially in childhood and in disadvantaged contexts.

## What “heritability” means

Heritability is often misunderstood. A heritability estimate answers:

> In a particular population, at a particular time, what proportion of variation among individuals is statistically associated with genetic differences?

It does **not** mean:
- that intelligence is “determined” by genes,
- that an individual’s intelligence is X% genetic,
- that environment is unimportant,
- or that differences between groups are genetic.

A trait can be highly heritable and still strongly affected by environment. For example, height is highly heritable, but nutrition and disease can dramatically change average height across generations.

## Typical estimates

For general cognitive ability, often called **g** or general intelligence:

- **Childhood:** heritability is often estimated around **30–50%**.
- **Adolescence:** estimates often rise to around **50–60%**.
- **Adulthood:** many twin and family studies estimate around **60–80%**, especially in relatively stable, higher-income environments.

A reasonable summary is:

> Human intelligence is moderately heritable in childhood and substantially heritable in adulthood, with many estimates clustering around **0.5 to 0.8** in adult samples.

However, estimates vary by population, socioeconomic context, measurement, age, and methodology.

## What the evidence shows

### 1. Twin studies

Twin studies compare identical twins, who share nearly all their DNA, with fraternal twins, who share about half of their segregating DNA.

A consistent finding is:

- Identical twins are more similar in IQ than fraternal twins.
- Identical twins reared apart are still substantially similar.
- Fraternal twins and ordinary siblings are less similar.

This pattern strongly suggests genetic influence.

Classic and modern twin studies generally find significant heritability for cognitive ability, with heritability increasing from childhood into adulthood.

### 2. Adoption studies

Adoption studies provide another line of evidence.

Typical findings:

- Adopted children resemble their adoptive families in childhood to some extent, showing environmental influence.
- As adopted children grow older, their cognitive scores tend to resemble their biological relatives more than their adoptive relatives.
- Shared family environment often has a stronger apparent effect in childhood than adulthood.

This supports the idea that both genes and environment matter, and that genetic influences become more visible with age.

### 3. Genome-wide association studies

Modern DNA-based studies show that intelligence is influenced by **many genetic variants**, each with tiny effects. This is called a **polygenic** architecture.

Important findings:

- No single “intelligence gene” exists.
- Thousands of genetic variants are associated with cognitive performance or educational attainment.
- Polygenic scores can predict a modest portion of variation in cognitive outcomes.
- SNP-based heritability estimates are lower than twin-study estimates, often roughly in the **20–30%** range for measured common variants, because they miss rare variants, structural variants, imperfect tagging, and some gene–environment correlations.

This genomic evidence confirms that genetic differences contribute to intelligence, though it also shows that prediction from DNA remains limited.

## Why heritability changes with age

Heritability of intelligence tends to increase from childhood to adulthood. Several mechanisms may contribute:

1. **Gene–environment correlation**
   As people age, they increasingly select, shape, and evoke environments that match their genetic tendencies. For example, a child with a natural interest in reading may read more, improving verbal ability.

2. **Reduced shared family influence**
   Young children share many family environments, but adults create more individualized environments.

3. **Accumulating developmental effects**
   Small early differences can compound over time through education, peer groups, hobbies, and occupational choices.

## Environmental effects are still important

Even if adult intelligence is substantially heritable, environmental influences remain real and important. Relevant factors include:

- prenatal environment,
- nutrition,
- exposure to toxins such as lead,
- childhood illness,
- schooling quality,
- stress and trauma,
- sleep,
- family resources,
- cognitive stimulation,
- social stability,
- discrimination and deprivation.

Severe deprivation can greatly reduce cognitive development. Conversely, improvements in public health, nutrition, schooling, and living conditions likely contributed to large historical IQ gains, often called the **Flynn effect**.

## Heritability depends on social context

Heritability is not a universal constant.

In societies or families where basic needs are widely met and educational opportunities are relatively equal, genetic differences may explain more of the remaining variation. In more deprived or unequal environments, environmental constraints may suppress genetic potential and reduce heritability.

Some studies have found that socioeconomic status moderates IQ heritability, especially in children, though results differ across countries and samples. The general point remains: heritability estimates are context-dependent.

## What heritability does not show

The evidence does **not** justify strong claims about genetic causes of average differences between racial, ethnic, national, or social groups.

Why?

Because heritability within a population says little about causes of differences between populations. For example, height can be highly heritable within two countries, while the average height difference between them may be mostly nutritional or environmental.

Similarly, IQ differences between groups can be affected by schooling, poverty, health, discrimination, language, test familiarity, and many other environmental factors.

## Bottom line

The evidence from twin, adoption, family, and genomic studies shows that:

- Intelligence is influenced by both genes and environment.
- Genetic differences explain a substantial portion of individual variation in intelligence.
- Heritability is lower in childhood and often higher in adulthood.
- Adult heritability is commonly estimated around **60–80%** in many studied populations, though DNA-based estimates of currently measured common variants are lower.
- Intelligence is highly polygenic, shaped by many genetic variants of tiny effect.
- High heritability does not mean immutability, determinism, or genetic explanation of group differences.

A concise summary would be:

> Human intelligence is substantially heritable, especially in adulthood, but it is also meaningfully shaped by environment, development, and social context.


text:
Human intelligence, usually measured with IQ-type tests and related cognitive tasks, is **substantially heritable**, but it is also strongly shaped by environment, development, education, health, and measurement context.

## Short answer

In modern behavioral genetics, the heritability of general cognitive ability is often estimated at roughly:

- **Childhood:** about **20–40%**
- **Adolescence:** about **40–60%**
- **Adulthood:** about **50–80%**, with many estimates clustering around **50–70%**

These numbers mean that, **within a given population and environment**, genetic differences account for that proportion of observed variation in intelligence test scores. They do **not** mean that an individual’s intelligence is “70% genetic,” nor that intelligence is fixed or unaffected by environment.

## What the evidence shows

### 1. Twin and adoption studies

The strongest classical evidence comes from comparisons of:

- Identical twins, who share nearly all their DNA
- Fraternal twins, who share about half their segregating DNA
- Adopted siblings, who share family environment but not much more DNA than unrelated people
- Biological relatives raised apart

The repeated finding is:

- Identical twins are more similar in IQ than fraternal twins.
- Biological relatives resemble each other in cognitive ability even when raised apart.
- Adopted siblings often become less similar in IQ as they grow older, despite sharing a home.
- Genetic influence tends to increase from childhood to adulthood.

This pattern supports a significant genetic contribution.

### 2. Molecular genetic evidence

Modern genome-wide studies also support heritability. Researchers can estimate how much variation in intelligence or educational attainment is associated with measured DNA variants.

Findings include:

- Intelligence is **highly polygenic**: many thousands of genetic variants each have tiny effects.
- Common measured SNPs account for a meaningful but incomplete portion of variation — often called **SNP heritability**.
- Polygenic scores can predict some differences in educational achievement and cognitive performance, but they are still far from determinative.

Typical polygenic scores explain only a modest fraction of individual differences, though their predictive power has improved as sample sizes have grown.

### 3. Heritability increases with age

One of the most robust findings is that heritability of intelligence tends to rise over development. A common interpretation is that as people grow older, they increasingly select, shape, and respond to environments in ways partly correlated with their genetic tendencies. For example, a child with a natural interest in abstract reasoning may seek out books, puzzles, advanced classes, or intellectually stimulating peers.

This is called **gene–environment correlation**.

### 4. Environment still matters a lot

High heritability does not mean environment is unimportant. Environmental factors that can affect cognitive development include:

- Nutrition
- Prenatal health
- Lead exposure and pollution
- Infectious disease burden
- Quality of schooling
- Family stress
- Sleep
- Poverty and instability
- Access to books, language, and intellectual stimulation

The **Flynn effect** — the large rise in IQ scores across many countries during the 20th century — is strong evidence that environmental changes can substantially affect measured cognitive performance at the population level.

### 5. Heritability is population-specific

A heritability estimate applies to a particular population at a particular time under particular environmental conditions.

For example, if everyone in a society had nearly identical schooling, nutrition, and healthcare, then remaining differences might be more attributable to genetic variation. But in a society with large environmental inequality, environmental differences may explain more of the variation.

So heritability is not a universal constant.

## Common misunderstandings

### “If intelligence is heritable, it must be fixed.”

False. Height is highly heritable, but nutrition and disease still affect height. Similarly, intelligence can be heritable while still being influenced by education, health, and life experience.

### “Heritability tells us about an individual.”

False. Heritability is about **variation in a population**, not the causes of a single person’s abilities.

### “Heritability explains group differences.”

Not necessarily. Heritability within a group does not automatically explain average differences between groups. Group differences can be caused by environmental, social, historical, economic, or measurement factors even when individual differences within groups are partly heritable.

### “There is one gene for intelligence.”

False. Intelligence is influenced by many genes of very small effect, plus many environmental factors.

## Bottom line

The evidence from twin, adoption, family, and molecular-genetic studies shows that **genetic differences make a substantial contribution to individual differences in human intelligence**, especially in adulthood. A reasonable summary is that adult intelligence is often estimated to be **about 50–70% heritable** in many studied populations.

But intelligence is not genetically predetermined. Heritability depends on the environment, and environmental changes can have large effects on cognitive development and test performance.


text:
Human intelligence—as typically measured by IQ tests and related cognitive batteries—is **substantially heritable**, but not genetically fixed. The best evidence suggests that genetic differences explain a sizeable share of **individual differences within a given population and environment**, while environment also has large effects, especially across development and across generations.

## Bottom line estimates

Approximate heritability of general cognitive ability:

- **Early childhood:** ~20–40%
- **Adolescence:** ~40–60%
- **Adulthood:** ~50–80%, often summarized around **60–70%**

These numbers mean that, in a particular population at a particular time, genetic differences account for that proportion of observed variation in cognitive test scores. They do **not** mean that an individual’s intelligence is “60% genetic,” nor that intelligence cannot change.

## Main lines of evidence

### 1. Twin studies

Twin studies consistently find that identical twins, who share nearly all their DNA, are more similar in IQ than fraternal twins, who share about half of their segregating DNA.

Typical pattern:

- **Identical twins raised together:** very high similarity
- **Fraternal twins raised together:** moderate similarity
- **Identical twins raised apart:** still substantially similar, though less than those raised together

This supports a strong genetic contribution, while also showing that environment matters.

### 2. Adoption studies

Adoption studies show that children’s IQ scores are influenced by their rearing environment, especially early in life. Adopted children often resemble their adoptive families in childhood.

But as children grow older, their cognitive scores tend to become more similar to those of their biological relatives than to adoptive relatives. This pattern is one reason heritability estimates tend to rise with age.

### 3. Genome-wide association studies

Modern genetic studies find that intelligence is **highly polygenic**: influenced by thousands of genetic variants, each with a tiny effect.

Genome-wide association studies have identified many DNA variants associated with cognitive performance and educational attainment. Current polygenic scores can predict a modest portion of variation in cognitive or educational outcomes, especially in people of European ancestry, but they are far from deterministic.

Typical molecular estimates:

- Common SNPs explain perhaps **20–30%** of variation in cognitive ability.
- Polygenic scores currently predict a smaller share, often roughly **5–15%**, depending on the trait, sample, and population.

The gap between twin-study heritability and DNA-based prediction reflects limits of current methods, rare variants, gene–gene interactions, measurement error, and other complexities.

## Environmental evidence is also strong

High heritability does not imply that environment is unimportant. Many environmental factors affect cognitive development, including:

- Quality and duration of schooling
- Childhood nutrition
- Prenatal environment
- Exposure to lead or other toxins
- Disease burden
- Sleep, stress, and trauma
- Family resources and stimulation
- Social and economic conditions

The **Flynn effect**—large IQ gains across many countries during the 20th century—is especially important evidence that environment can shift average cognitive test performance substantially over time. Genes did not change that quickly; environments did.

## Why heritability increases with age

One robust finding is that intelligence becomes more heritable from childhood into adulthood. Possible reasons include:

- People increasingly choose environments that fit their genetic tendencies.
- Small early differences compound over time.
- Adults have more control over education, work, hobbies, and social environments.
- Shared family environment matters more in childhood and less later.

This is known as **gene–environment correlation**: genes can influence the environments people experience or select.

## Important caveats

Heritability is often misunderstood.

It does **not** mean:

- intelligence is fixed;
- education is pointless;
- a person’s IQ is genetically predetermined;
- environmental interventions cannot work;
- group differences are genetic in origin.

Heritability estimates apply to **variation within a studied population** under its current environmental conditions. If environments change, heritability can change too. A trait can be highly heritable and still strongly affected by environment.

For example, height is highly heritable, but average height has increased dramatically in many populations because of improved nutrition and health.

## Overall conclusion

The evidence from twin, adoption, family, and molecular genetic studies shows that human intelligence is **substantially heritable**, especially in adulthood, with genetic differences explaining perhaps **about half or more** of variation among individuals in many modern populations. But intelligence is also meaningfully shaped by environment, development, education, health, and social conditions. The best view is not “genes versus environment,” but a dynamic interaction between both.


text:
Human intelligence is **substantially heritable**, but not genetically fixed. The best evidence suggests that genetic differences explain a meaningful share of variation in intelligence within a given population, while environment, development, education, health, and social context also matter.

## Short answer

Across modern studies, the heritability of general cognitive ability/IQ is roughly:

- **Childhood:** about **20–40%**
- **Adolescence:** about **40–60%**
- **Adulthood:** often **50–80%**, commonly summarized around **~60%**

These are estimates of **variation among individuals in a particular population and environment**, not a claim that intelligence is “60% genetic” for any one person.

## What “heritability” means

Heritability is the proportion of observed differences between people that can be statistically associated with genetic differences **in a particular population at a particular time**.

So if IQ heritability is estimated at 60%, that means:
> About 60% of the variation in IQ scores among people in that studied population is associated with genetic variation.

It does **not** mean:

- 60% of a person’s intelligence is caused by genes
- intelligence cannot be changed
- environments do not matter
- group differences are genetic
- genes determine destiny

A trait can be highly heritable and still strongly affected by environment. For example, height is highly heritable, but nutrition and disease can greatly affect average height.

## Main lines of evidence

### 1. Twin studies

Twin studies compare identical twins, who share nearly all their DNA, with fraternal twins, who share about half of their segregating DNA.

A consistent finding is that **identical twins are more similar in IQ than fraternal twins**, even when raised apart.

Typical correlations:

- Identical twins raised together: very high similarity
- Fraternal twins raised together: moderate similarity
- Identical twins raised apart: still substantially similar

This pattern supports a genetic contribution.

However, twin studies have assumptions, such as the “equal environments” assumption: that identical and fraternal twins experience equally similar environments relevant to the trait. Researchers have tested this in various ways, and while no design is perfect, the broad conclusion of genetic influence has held up.

### 2. Adoption studies

Adoption studies compare children to their biological and adoptive relatives.

Findings generally show:

- In early childhood, adoptive family environment can matter substantially.
- As children age, their IQ tends to become more similar to that of biological relatives.
- The resemblance to adoptive parents often declines by adulthood.

This suggests that genetic influences become more visible with age, partly because people increasingly select, shape, and respond to environments in ways correlated with their genetic tendencies.

### 3. Genome-wide DNA studies

Modern genetic studies do not rely only on twins. They use measured DNA from very large samples.

These studies show that intelligence is **highly polygenic**: influenced by thousands of genetic variants, each with tiny effects.

Methods such as GWAS and SNP-based heritability estimate that common measured genetic variants account for a notable but incomplete share of intelligence variation, often around **20–30%** depending on the sample and measure. This is lower than twin-study estimates because:

- current DNA arrays capture only some genetic variation,
- rare variants and structural variants may be missed,
- measurement error reduces estimates,
- intelligence is affected by many tiny-effect variants.

Polygenic scores can now predict some variation in educational attainment and cognitive performance, but their predictive power is still limited. They are much better for research than for making strong predictions about individuals.

## Why heritability tends to rise with age

One of the strongest findings is that IQ heritability tends to increase from childhood into adulthood.

Possible reasons include:

1. **Gene–environment correlation**
   People with different dispositions may seek different environments. For example, a child who enjoys reading may read more, improving verbal skills.

2. **Greater personal choice with age**
   Adults have more control over their education, hobbies, occupations, and social environments.

3. **Shared family environment becomes less dominant**
   Siblings raised in the same household often become less similar over time as they develop separate lives.

This does not mean environment stops mattering. Rather, environmental influences become more individualized and less simply tied to the shared household.

## Environmental evidence is also strong

The fact that intelligence is heritable does not mean environment is unimportant. Evidence for environmental influence includes:

- **The Flynn effect:** IQ scores rose substantially across many countries during the 20th century, too quickly to be genetic.
- **Effects of schooling:** More education is associated with higher cognitive test performance, and quasi-experimental studies suggest schooling can causally improve some cognitive skills.
- **Nutrition and health:** Iodine deficiency, lead exposure, prenatal health, infectious disease burden, sleep, and malnutrition can affect cognitive development.
- **Early childhood environment:** Severe deprivation can harm cognitive development; enriched and stable environments can improve outcomes.
- **Socioeconomic conditions:** Poverty, stress, school quality, neighborhood conditions, and access to learning resources can influence cognitive development.

So intelligence is both genetically influenced and environmentally responsive.

## Shared vs non-shared environment

Behavioral-genetic studies often divide environmental effects into:

- **Shared environment:** factors that make siblings similar, such as family income, parenting style, neighborhood, and school.
- **Non-shared environment:** factors that make siblings different, such as different friends, teachers, illnesses, experiences, or random developmental events.

For IQ:

- Shared environment tends to matter more in childhood.
- Its measured effect often declines by adulthood in many studies.
- Non-shared environment and measurement error account for much of the remaining variation.

But “shared environment” estimates can be tricky. A family-wide factor may affect siblings differently, and gene–environment interplay can blur categories.

## Important cautions

### Heritability is population-specific

A heritability estimate from one society does not automatically apply to another. If environments are very unequal, environmental variation may explain more of the differences. If environments are relatively uniform, genetic variation may explain a larger share.

### Heritability within groups does not explain differences between groups

High heritability within a population does not imply that average differences between social, ethnic, national, or racial groups are genetic. Group differences can be caused by environmental factors even when individual differences within groups are highly heritable.

For example, height is highly heritable, but average height differences between populations have changed substantially with nutrition and health.

### Genes influence probabilities, not certainties

Genetic influences on intelligence are distributed across thousands of variants. There is no single “intelligence gene.” Genetic propensities can be amplified, suppressed, or redirected by environments.

### IQ is not all of intelligence

IQ and general cognitive ability predict many outcomes, including academic and occupational performance, but they do not capture all valuable mental traits. Creativity, wisdom, motivation, emotional skills, curiosity, and practical judgment are partly overlapping but not identical.

## Bottom line

The evidence from twin, adoption, family, and molecular-genetic studies converges on the conclusion that **human intelligence is moderately to highly heritable**, especially in adulthood. A reasonable summary is that genetic differences account for **about half or more of individual differences in adult IQ** in many studied populations.

At the same time, intelligence is clearly shaped by environment. Education, health, nutrition, toxins, stress, family conditions, and broader social factors can all affect cognitive development. Heritability does not mean immutability, and it does not justify genetic explanations for group differences.


prompt_id: sci-vaccines-safety

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Yes. Childhood vaccines are considered **safe and effective** at preventing many serious infectious diseases.

- **Effectiveness:** Vaccines greatly reduce the risk of illnesses such as measles, polio, whooping cough, Hib disease, hepatitis B, chickenpox, and others. They also reduce severe outcomes like hospitalization and death. High vaccination rates help protect infants and people with weakened immune systems through community protection.

- **Safety:** Vaccines are tested in clinical trials before approval and continuously monitored afterward. Most side effects are mild and temporary, such as soreness, low fever, or fussiness. Serious allergic reactions or severe side effects are **very rare**.

- **Why schedules matter:** The recommended childhood vaccine schedule is designed to protect children when they are most vulnerable and before they are likely to be exposed to certain infections.

If you have concerns about a specific vaccine, a child’s medical history, allergies, or timing, it’s best to discuss them with a pediatrician or qualified healthcare professional.


text:
Yes. **Recommended childhood vaccines are safe and effective at preventing serious diseases.**

- **Effectiveness:** Vaccines greatly reduce the risk of infections like measles, polio, whooping cough, Hib, pneumococcal disease, chickenpox, and others. If a vaccinated child does get infected, illness is often milder and complications are less likely.
- **Safety:** Vaccines are tested in clinical trials before approval and continuously monitored after use. Serious side effects are **very rare**.
- **Common side effects:** Mild fever, soreness/redness at the injection site, fussiness, or tiredness for a day or two.
- **Rare risks:** Severe allergic reactions can occur but are extremely uncommon, and clinics are prepared to treat them.
- **No link to autism:** Large studies have found **no credible evidence** that vaccines cause autism.

Following the recommended immunization schedule protects your child and helps protect vulnerable people in the community who cannot be vaccinated. If your child has a specific medical condition, allergy, or prior vaccine reaction, discuss it with your pediatrician.


text:
Yes. **Routine childhood vaccines are safe and highly effective at preventing serious diseases** such as measles, polio, whooping cough, Hib meningitis, hepatitis B, and chickenpox.

- **Effectiveness:** Vaccines train a child’s immune system to recognize and fight infections. Many vaccines greatly reduce the chance of getting the disease, and if infection occurs, they often make illness much milder.
- **Safety:** Vaccines are tested in clinical trials before approval and continuously monitored afterward. The most common side effects are mild, such as soreness, low fever, or tiredness.
- **Serious reactions:** Severe allergic reactions or other serious side effects are very rare. The risks from the diseases vaccines prevent are generally much higher than the risks from vaccination.
- **Community protection:** High vaccination rates also help protect babies too young to be vaccinated and people with weakened immune systems.

Following the recommended childhood immunization schedule is one of the most effective ways to protect children’s health. If your child has specific medical conditions, allergies, or prior reactions, discuss the vaccine plan with your pediatrician.


text:
Yes. **Recommended childhood vaccines are safe and highly effective at preventing serious diseases.**

- **Effectiveness:** Vaccines have dramatically reduced or nearly eliminated many diseases in places with high vaccination rates, including measles, polio, Hib meningitis, diphtheria, and whooping cough deaths. Some vaccines mainly prevent infection; others are especially strong at preventing severe illness, hospitalization, and death.
- **Safety:** Vaccines are tested in clinical trials before approval and continuously monitored afterward. The most common side effects are mild and short-lived, such as soreness at the injection site, low fever, or fussiness.
- **Serious reactions:** These are rare. For example, severe allergic reactions can occur but are very uncommon, and vaccination sites are prepared to treat them.
- **Community protection:** High vaccination rates also protect infants too young to be vaccinated and people with weakened immune systems.
- **Schedule:** The childhood vaccine schedule is designed to protect children early, when they are most vulnerable, and to give doses at ages when they work best.

If you have concerns about a specific vaccine or your child’s medical history, it’s best to discuss them with your pediatrician, who can explain risks and benefits for your child’s situation.


text:
Yes. Childhood vaccines are **safe and effective** at preventing serious diseases.

- **Effectiveness:** Vaccines greatly reduce the risk of illnesses such as measles, polio, whooping cough, Hib disease, hepatitis B, and meningitis. They also reduce severe complications, hospitalizations, and deaths.
- **Safety:** Vaccines are tested in clinical trials before approval and continuously monitored afterward. The most common side effects are mild and short-lived, such as soreness at the injection site, low fever, or fussiness.
- **Serious reactions:** Severe allergic reactions can happen but are **very rare**. Clinics are prepared to treat them.
- **Autism:** Large, high-quality studies have found **no link** between vaccines and autism.
- **Community protection:** Vaccinating children also helps protect babies too young to be vaccinated and people with weakened immune systems.

Following the recommended childhood vaccination schedule is considered one of the most effective ways to protect children from preventable diseases. If your child has specific medical conditions or a history of severe allergies, it’s best to discuss the schedule with their pediatrician.


text:
Yes. **Routine childhood vaccines are safe and highly effective at preventing serious diseases** such as measles, polio, whooping cough, Hib meningitis, diphtheria, tetanus, and chickenpox.

- **Effectiveness:** Vaccines train a child’s immune system to recognize and fight infections. They have dramatically reduced—and in some cases nearly eliminated—many dangerous childhood diseases.
- **Safety:** Vaccines are extensively tested before approval and continuously monitored afterward. Most side effects are mild and short-lived, such as soreness at the injection site, low-grade fever, or fussiness.
- **Rare risks:** Serious reactions, such as severe allergic reactions, are very rare. The risk of severe complications from the diseases themselves is generally much higher than the risk from vaccination.
- **Community protection:** Vaccination also helps protect babies too young to be vaccinated and people with weakened immune systems by reducing disease spread.

The best approach is to follow the recommended immunization schedule from trusted public health authorities and discuss any specific concerns—such as allergies, prior reactions, or medical conditions—with your child’s pediatrician.


text:
Yes. Childhood vaccines are **safe and effective** at preventing many serious diseases.

- **Effectiveness:** Vaccines greatly reduce the risk of illnesses such as measles, polio, whooping cough, Hib disease, and severe chickenpox. Some vaccines prevent infection very well; others mainly prevent severe disease, hospitalization, or death.
- **Safety:** Vaccines are tested in clinical trials before approval and continuously monitored afterward. Serious side effects are **rare**. Common side effects—such as a sore arm, mild fever, or fussiness—are usually short-lived.
- **Community protection:** High vaccination rates help protect infants too young to be vaccinated and people with weakened immune systems.
- **Schedules matter:** The recommended childhood vaccine schedule is designed to protect children when they are most vulnerable.

There are a few situations where a vaccine may be delayed or avoided, such as a severe allergy to a vaccine component or certain immune system conditions. If you have concerns about a specific vaccine or your child’s health history, it’s best to discuss them with your pediatrician.


text:
Yes. **Routine childhood vaccines are strongly supported by decades of evidence as safe and effective at preventing serious infectious diseases.**

- **Effectiveness:** Vaccines have dramatically reduced diseases such as measles, polio, Hib meningitis, whooping cough, chickenpox, and diphtheria. Some vaccines prevent infection very well; others mainly reduce severe illness, hospitalization, and death.
- **Safety:** Vaccines are tested in clinical trials before approval and continuously monitored afterward. Most side effects are mild and temporary, such as soreness, low fever, or fussiness.
- **Serious reactions:** Severe allergic reactions or other serious side effects are very rare, and systems exist to detect and investigate them.
- **Community protection:** High vaccination rates also protect infants, immunocompromised children, and others who cannot be vaccinated.
- **Autism:** Large, high-quality studies have found **no link** between vaccines and autism.

For individual concerns—such as allergies, immune disorders, prior reactions, or delayed schedules—it’s best to discuss the child’s situation with a pediatrician or qualified healthcare professional.


text:
Yes. **Childhood vaccines are widely considered safe and effective at preventing serious diseases.** They are extensively tested before approval and continuously monitored for safety after use.

- **Effectiveness:** Vaccines greatly reduce the risk of diseases such as measles, polio, whooping cough, Hib, pneumococcal disease, and others. They also reduce severe illness, hospitalizations, and deaths.
- **Safety:** Most side effects are mild and temporary, such as soreness at the injection site, low-grade fever, or fussiness. Serious side effects are rare.
- **Community protection:** High vaccination rates help protect babies too young to be vaccinated and people with weakened immune systems.
- **Risk comparison:** The risks from vaccine-preventable diseases are generally far higher than the risks from vaccines.

Some children with specific medical conditions or severe allergies may need adjusted vaccine plans, so it’s best to follow the recommended immunization schedule and discuss any concerns with a pediatrician or qualified healthcare professional.


text:
Yes. **Childhood vaccines are safe and effective at preventing many serious diseases.** They are one of the most successful public health tools, greatly reducing illnesses such as measles, polio, whooping cough, Hib meningitis, tetanus, diphtheria, and others.

- **Effectiveness:** Vaccines train a child’s immune system to recognize and fight specific infections. Many provide very strong protection, and if a vaccinated child does get infected, the illness is often milder with lower risk of hospitalization or death.
- **Safety:** Vaccines are tested in clinical trials and continuously monitored after approval. Most side effects are mild and short-lived, such as soreness at the injection site, low fever, or fussiness.
- **Serious reactions:** Severe allergic reactions or other serious side effects are **very rare**. The risks from the diseases vaccines prevent are generally much higher than the risks from vaccination.
- **Community protection:** High vaccination rates also help protect babies too young to be vaccinated and people with weakened immune systems.

If you have concerns about a specific vaccine or your child’s health history, it’s best to discuss them with your pediatrician, who can explain the recommended schedule and any precautions.


text:
Yes. **Routine childhood vaccines are very safe and highly effective at preventing serious infectious diseases.** They are one of the most studied and monitored medical interventions.

- **Effectiveness:** Vaccines train a child’s immune system to recognize and fight specific infections. They have dramatically reduced diseases such as measles, polio, diphtheria, Hib meningitis, whooping cough, and chickenpox. Some vaccines prevent infection very well; others mainly prevent severe disease, hospitalization, and death.
- **Safety:** Before approval, vaccines are tested in clinical trials. After approval, they continue to be monitored through safety systems that can detect rare side effects.
- **Common side effects:** Mild fever, soreness or redness at the injection site, fussiness, and fatigue. These usually resolve within a day or two.
- **Rare risks:** Serious allergic reactions can occur but are extremely rare, and clinics are prepared to treat them.
- **Community protection:** Vaccination also helps protect infants too young to be vaccinated and people with weakened immune systems by reducing spread.

Major medical and public health organizations—including the **CDC, WHO, American Academy of Pediatrics,** and many national health agencies—recommend routine childhood vaccination because the benefits greatly outweigh the risks.

If you have concerns about a specific vaccine or your child’s medical history, it’s best to discuss them with your pediatrician, who can tailor advice to your child’s situation.


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Yes. Childhood vaccines are overwhelmingly **safe and effective** at preventing serious infectious diseases.

- **Effectiveness:** Vaccines have greatly reduced—or in some places nearly eliminated—diseases like measles, polio, diphtheria, Hib meningitis, whooping cough, and severe chickenpox. Some vaccines prevent infection very well; others may not stop all infections but strongly reduce severe illness, hospitalization, and death.
- **Safety:** Vaccines are tested in clinical trials before approval and continuously monitored afterward. Most side effects are mild and short-lived, such as soreness at the injection site, low fever, or fussiness.
- **Serious reactions:** Severe allergic reactions or other serious side effects are very rare. Healthcare providers screen for specific contraindications, such as certain immune conditions or prior severe allergic reactions.
- **Community protection:** High vaccination rates also protect infants too young to be vaccinated and people with weakened immune systems by reducing disease spread.

If you have concerns about a specific vaccine or your child’s medical history, it’s best to discuss them with your pediatrician, who can give guidance based on your child’s situation.


text:
Yes. Childhood vaccines are considered **safe and effective** at preventing many serious diseases.

- **Effectiveness:** Vaccines train the immune system to recognize and fight infections. They have dramatically reduced illnesses such as measles, polio, diphtheria, Hib meningitis, whooping cough, and chickenpox. Some vaccines mainly prevent severe disease and complications, while others also reduce infection and spread.
- **Safety:** Vaccines are tested in clinical trials before approval and continuously monitored afterward. Most side effects are mild and short-lived, such as soreness at the injection site, low-grade fever, or fussiness.
- **Serious reactions:** Severe allergic reactions or other major adverse events are **very rare**. The risks from the diseases vaccines prevent are generally much higher than the risks from vaccination.
- **Community protection:** High vaccination rates help protect infants too young to be vaccinated and people with weakened immune systems.

Parents should follow the recommended immunization schedule and discuss any specific concerns—such as allergies, immune conditions, or prior reactions—with their child’s pediatrician.