text:
Evolution by natural selection is the process by which living populations change over generations because some inherited traits help organisms survive and reproduce better than others.

The basic idea has four main parts:

1. **Variation exists**
   Individuals in a population are not all identical. For example, some beetles may be darker, faster, larger, or more resistant to disease than others.

2. **Some variation is inherited**
   Many differences are caused by genes and can be passed from parents to offspring. If a trait is not heritable, natural selection cannot directly spread it through a population.

3. **More offspring are produced than can survive**
   Organisms often produce more young than the environment can support. This creates competition for food, space, mates, and other resources.

4. **Some traits improve survival or reproduction**
   Individuals with traits that give them an advantage in a particular environment tend to leave more offspring. Over time, the genes associated with those helpful traits become more common.

For example, imagine a population of insects where some are green and some are brown. If the insects live on brown tree bark, brown individuals may be harder for birds to see. They are more likely to survive and reproduce. Over many generations, the population may become mostly brown.

A few important points:

- Natural selection acts on **individuals**, but evolution occurs in **populations**.
- It does not produce “perfect” organisms; it works with existing variation and trade-offs.
- It is not goal-directed. Species do not evolve because they “need” to; traits become common if they improve reproductive success in a given environment.
- If the environment changes, different traits may become advantageous.

In short, evolution by natural selection happens when heritable traits that increase reproductive success become more common in a population over time.


prompt_id: sci-evolution

provider: openai

text:
Evolution by natural selection is the process by which populations of organisms change over generations because some traits help individuals survive and reproduce better than others.

It works through a few key steps:

1. **Variation exists**
   Individuals in a population are not identical. They differ in traits such as size, color, speed, disease resistance, or behavior.

2. **Some variation is inherited**
   Many traits can be passed from parents to offspring through genes.

3. **More offspring are produced than can survive**
   Organisms often produce more young than the environment can support, leading to competition for food, mates, shelter, and other resources.

4. **Some traits improve survival or reproduction**
   Individuals with traits that are better suited to their environment are more likely to survive and have offspring.

5. **Helpful traits become more common**
   Because successful individuals pass their genes to the next generation, advantageous traits tend to increase in frequency over time.

Over many generations, this can cause a population to become better adapted to its environment. If populations become different enough, this process can eventually lead to the formation of new species.

For example, if a population of insects lives on dark tree bark, darker-colored insects may be harder for predators to see. Those insects survive and reproduce more often, so dark coloration becomes more common in future generations.

Natural selection does not mean organisms “try” to evolve. Instead, the environment favors certain inherited traits, and populations change over time as a result.

finish_reason: stop


text:
Evolution by natural selection is the process by which living populations change over generations because some traits help organisms survive and reproduce better than others.

It works through a few key steps:

1. **Variation exists**
   Individuals in a species are not all identical. They differ in traits such as size, color, speed, resistance to disease, or ability to find food.

2. **Some variation is inherited**
   Many traits are influenced by genes and can be passed from parents to offspring.

3. **More offspring are produced than can survive**
   Organisms often produce more young than the environment can support, so there is competition for food, space, mates, and other resources.

4. **Some traits improve survival or reproduction**
   Individuals with traits better suited to their environment are more likely to survive and have offspring. This is called **natural selection**.

5. **Helpful traits become more common**
   Because those successful individuals pass their genes on, the traits that helped them tend to become more common in the population over many generations.

For example, if a population of beetles includes both green and brown beetles, and birds more easily see and eat the green beetles, brown beetles may survive and reproduce more often. Over time, the population may become mostly brown.

Natural selection does not mean organisms “try” to evolve. Instead, evolution happens because inherited differences affect survival and reproduction. Over long periods, this can lead to major changes and even the formation of new species.


text:
Evolution by natural selection is the process by which populations of living organisms change over generations because some inherited traits help individuals survive and reproduce better than others.

It works through a few key steps:

1. **Variation exists**
   Individuals in a population are not all identical. They differ in traits such as size, color, speed, disease resistance, or behavior.

2. **Some variation is inherited**
   Many traits can be passed from parents to offspring through genes.

3. **More offspring are produced than can survive**
   Organisms often produce more young than the environment can support, so there is competition for food, space, mates, and other resources.

4. **Some traits improve survival or reproduction**
   Individuals with traits better suited to their environment are more likely to survive and have offspring. This is called higher “fitness.”

5. **Helpful traits become more common**
   Because those individuals leave more offspring, the genes for their advantageous traits become more frequent in the population over many generations.

Over time, this can lead to significant changes in a population. If populations become different enough, new species can eventually form.

A classic example is antibiotic resistance in bacteria. Some bacteria may naturally have mutations that let them survive an antibiotic. When the antibiotic is used, susceptible bacteria die, while resistant ones survive and reproduce. Over time, the bacterial population becomes mostly resistant.

Natural selection does not mean organisms “try” to adapt. Instead, random genetic variation already exists or arises through mutation, and the environment “selects” which traits are more successful.


text:
Evolution by natural selection is the process by which living populations change over generations because individuals with helpful inherited traits tend to survive and reproduce more successfully.

Here’s how it works:

1. **Variation exists**
   - Individuals in a population are not all identical.
   - For example, some beetles may be darker, some lighter; some animals may run faster than others.

2. **Some variation is inherited**
   - Many traits are passed from parents to offspring through genes.
   - If a trait is genetic, offspring may inherit it.

3. **More offspring are produced than can survive**
   - Organisms often produce more young than the environment can support.
   - This creates competition for food, space, mates, and survival.

4. **Certain traits give an advantage**
   - In a particular environment, some traits help individuals survive or reproduce better.
   - For example, a camouflaged insect may be less likely to be eaten by predators.

5. **Individuals with advantageous traits reproduce more**
   - Those individuals are more likely to pass their traits to the next generation.

6. **The population changes over time**
   - Over many generations, helpful traits become more common.
   - Less helpful traits may become rarer.

A classic example is **peppered moths** in England. During the Industrial Revolution, pollution darkened tree bark. Dark-colored moths became harder for predators to see, so they survived and reproduced more. Over time, dark moths became more common in polluted areas.

Natural selection does **not** mean organisms try to evolve or that evolution has a goal. Instead, traits that happen to be useful in a given environment become more common because their carriers leave more offspring. Over long periods, this process can lead to major changes and even the formation of new species.


text:
Evolution by natural selection is the process by which populations of living organisms change over generations because some traits help individuals survive and reproduce better than others.

It works through a few key steps:

1. **Variation exists**
   Individuals in a population are not identical. They differ in traits such as size, color, speed, disease resistance, or behavior.

2. **Some variation is inherited**
   Many traits are passed from parents to offspring through genes.

3. **More offspring are produced than can survive**
   Organisms often produce more young than the environment can support, leading to competition for food, mates, space, and other resources.

4. **Some traits improve survival or reproduction**
   Individuals with traits better suited to their environment are more likely to survive and have offspring. This is called **natural selection**.

5. **Helpful traits become more common**
   Because successful individuals pass their genes to the next generation, the traits that helped them survive or reproduce tend to become more common in the population over time.

For example, if a population of beetles includes both green and brown individuals, and birds can more easily see and eat the green beetles, brown beetles may survive and reproduce more often. Over many generations, the population may become mostly brown.

Importantly, natural selection acts on individuals, but **evolution happens to populations** over generations. It does not have a goal or plan; it simply favors traits that work well in a particular environment at a particular time.


text:
Evolution by natural selection is the process by which populations of organisms change over generations because some inherited traits help individuals survive and reproduce better than others.

It works through a few key steps:

1. **Variation exists**
   Individuals in a population are not all identical. They may differ in size, color, speed, disease resistance, behavior, and many other traits.

2. **Some variation is inherited**
   Many traits are influenced by genes and can be passed from parents to offspring.

3. **More offspring are produced than can survive**
   In most environments, resources such as food, space, and mates are limited. Not every individual survives or reproduces.

4. **Certain traits give an advantage**
   Individuals with traits better suited to their environment are more likely to survive and have offspring. This is called **higher fitness**.

5. **Helpful traits become more common**
   Because successful individuals pass their genes to the next generation, advantageous traits tend to increase in frequency over time.

Over many generations, this can lead to major changes in a population. If different populations experience different environments or become separated, they may eventually evolve into different species.

A simple example is **antibiotic resistance** in bacteria. Some bacteria may randomly have mutations that let them survive an antibiotic. When the antibiotic kills the non-resistant bacteria, the resistant ones survive and reproduce. Over time, the bacterial population becomes mostly resistant.

Natural selection is not intentional or goal-directed. Organisms do not evolve because they “need” to; rather, individuals with certain inherited traits happen to survive and reproduce more successfully in a particular environment.


text:
Evolution by natural selection is the process by which populations of living organisms change over generations because individuals with certain inherited traits are more likely to survive and reproduce.

It works through a few key steps:

1. **Variation exists**
   Individuals in a population are not identical. They differ in traits such as size, color, speed, disease resistance, or behavior.

2. **Some variation is inherited**
   Many traits are passed from parents to offspring through genes.

3. **More offspring are produced than can survive**
   Organisms often produce more young than the environment can support, so there is competition for food, space, mates, and safety.

4. **Some traits improve survival or reproduction**
   Individuals with traits that help them survive or reproduce in a particular environment tend to leave more offspring.

5. **Helpful traits become more common**
   Over many generations, the traits that increase reproductive success become more frequent in the population.

For example, if a population of insects lives on dark tree bark, darker insects may be harder for birds to see. Those darker insects are more likely to survive and reproduce. Over time, the population may become mostly dark-colored.

A few important points:

- Natural selection acts on **individuals**, but evolution happens in **populations**.
- It does not produce “perfect” organisms; it favors traits that are useful in a specific environment.
- If the environment changes, different traits may become advantageous.
- Evolution has no goal or plan; it is driven by inherited variation and differences in reproductive success.

In short: **natural selection is the process where inherited traits that help organisms survive and reproduce become more common over generations.**


text:
Evolution by natural selection is a process that changes the inherited traits of a population over generations.

It works through a few basic steps:

1. **Variation exists**
   Individuals in a population are not all identical. They may differ in size, color, speed, disease resistance, behavior, and many other traits.

2. **Some variation is heritable**
   Many traits are partly controlled by genes, meaning they can be passed from parents to offspring.

3. **More offspring are produced than can survive**
   Organisms often produce more young than the environment can support. Because resources such as food, space, and mates are limited, not all individuals survive and reproduce equally.

4. **Some traits improve survival or reproduction**
   If a particular inherited trait helps an organism survive, find food, avoid predators, resist disease, or attract mates, individuals with that trait may leave more offspring.

5. **Helpful traits become more common**
   Over many generations, traits that increase reproductive success tend to become more common in the population. Traits that reduce reproductive success may become less common.

A simple example: imagine a population of beetles where some are green and some are brown. If birds can more easily see and eat the green beetles, brown beetles may survive and reproduce more often. If color is inherited, then future generations will contain a higher proportion of brown beetles.

Natural selection does not mean organisms “try” to change, and it does not act with a goal in mind. Individuals do not evolve during their lifetimes; **populations evolve across generations**. Also, a trait that is useful in one environment may be harmful or neutral in another.

New variation can arise through mutations, genetic recombination during reproduction, and other genetic processes. Natural selection then acts on that variation, favoring traits that improve reproductive success in a given environment. Over long periods of time, this process can lead to adaptation and even the formation of new species.


text:
Evolution by natural selection is the process by which populations of organisms change over generations because individuals with certain inherited traits survive and reproduce more successfully than others.

It works through a few key steps:

1. **Variation exists**
   Individuals in a population are not all identical. They may differ in size, color, speed, disease resistance, behavior, or many other traits.

2. **Some variation is inherited**
   Many traits are passed from parents to offspring through genes.

3. **More offspring are produced than can survive**
   Organisms often produce more young than the environment can support, so there is competition for food, space, mates, and survival.

4. **Some traits improve survival or reproduction**
   If a trait helps an organism survive or have more offspring, individuals with that trait are more likely to pass it on.

5. **Helpful traits become more common**
   Over many generations, traits that improve reproductive success tend to increase in frequency in the population.

For example, imagine a population of insects where some are green and some are brown. If birds more easily see and eat the green insects on tree bark, brown insects may survive and reproduce more often. Over generations, the population may become mostly brown.

Natural selection does not mean organisms “try” to evolve. Instead, individuals with advantageous inherited traits tend to leave more descendants. Over long periods, this process can produce adaptations and even new species.


text:
Evolution by natural selection is the process by which populations of living organisms change over generations because some individuals are better suited to survive and reproduce in their environment.

It works through a few key steps:

1. **Variation exists**
   Individuals in a population are not identical. They may differ in traits such as size, color, speed, disease resistance, or behavior.

2. **Some variation is inherited**
   Many traits can be passed from parents to offspring through genes.

3. **More offspring are produced than can survive**
   Organisms often produce more young than the environment can support, so there is competition for food, space, mates, and other resources.

4. **Some traits improve survival or reproduction**
   Individuals with traits that give them an advantage in a particular environment are more likely to survive and have offspring.

5. **Helpful traits become more common**
   Because those successful individuals pass their genes to the next generation, advantageous traits tend to increase in frequency over time.

Over many generations, this can lead to major changes in a population. If populations become different enough, new species can eventually form.

For example, if a population of beetles includes both green and brown individuals, and birds can see green beetles more easily, brown beetles may survive more often. If color is inherited, more brown beetles will be born in future generations. Over time, the population may become mostly brown.

Natural selection does not mean organisms “try” to adapt. Instead, individuals with certain inherited traits happen to survive and reproduce more successfully in a given environment.


text:
Evolution by natural selection is the process by which living populations change over generations because individuals with certain inherited traits survive and reproduce more successfully than others.

It works through a few key steps:

1. **Variation exists**
   Individuals in a population are not all identical. For example, some beetles may be darker, faster, or better able to digest a certain food than others.

2. **Some variation is inherited**
   Many traits are passed from parents to offspring through genes. If a trait has a genetic basis, offspring may resemble their parents in that trait.

3. **More offspring are produced than can survive**
   Organisms often produce more young than the environment can support. This creates competition for food, space, mates, and other resources.

4. **Some traits improve survival or reproduction**
   In a particular environment, certain traits may give individuals an advantage. For example, a bird with a beak shape that is better suited to available seeds may get more food and survive longer.

5. **Those individuals leave more offspring**
   If individuals with advantageous traits reproduce more successfully, their traits become more common in the next generation.

6. **Populations change over time**
   Over many generations, these small changes can accumulate. A population may become better adapted to its environment, and over very long periods, new species can arise.

A simple example: imagine a population of rabbits where some have thicker fur. If the climate becomes colder, rabbits with thicker fur may survive winter better and produce more offspring. Since fur thickness is partly inherited, thick fur becomes more common in later generations.

Natural selection does not mean organisms “try” to evolve, and it does not produce perfect creatures. It simply favors traits that help survival or reproduction in a specific environment at a specific time. If the environment changes, different traits may become advantageous.


text:
Evolution by natural selection is the process by which populations of living organisms change over generations because some inherited traits help individuals survive and reproduce better than others.

It works through a few key steps:

1. **Variation exists**
   Individuals in a population are not all identical. They may differ in size, color, speed, disease resistance, behavior, and many other traits.

2. **Some variation is inherited**
   Many traits can be passed from parents to offspring through genes.

3. **More offspring are produced than can survive**
   Organisms often produce more young than the environment can support, so there is competition for food, space, mates, and other resources.

4. **Some traits improve survival or reproduction**
   Individuals with traits that are better suited to their environment are more likely to survive and have offspring. This is called **higher fitness**.

5. **Helpful traits become more common**
   Because those individuals leave more offspring, the genes linked to beneficial traits become more common in the population over generations.

Over long periods of time, these changes can lead to major adaptations and even the formation of new species.

For example, if a population of insects includes both light and dark individuals, and birds can more easily see the light insects on dark tree bark, the dark insects may survive more often. If color is inherited, dark coloration may become more common in later generations.

Natural selection does not mean organisms “try” to change. Instead, random genetic variation already exists, and the environment “selects” which traits are more successful.


text:
Evolution by natural selection is the process by which living populations change over generations because some inherited traits help organisms survive and reproduce better than others.

It works through a few key steps:

1. **Variation exists**
   Individuals in a population are not all identical. For example, some beetles may be darker, faster, or better at digesting certain foods than others.

2. **Some variation is inherited**
   Many differences are caused by genes and can be passed from parents to offspring.

3. **More offspring are produced than can survive**
   Organisms often produce more young than the environment can support. This creates competition for food, space, mates, and other resources.

4. **Some traits improve survival or reproduction**
   If a trait gives an individual an advantage in its environment, that individual is more likely to survive and have offspring. For example, a camouflaged insect may be less likely to be eaten by predators.

5. **Helpful traits become more common**
   Because successful individuals pass their genes to the next generation, the advantageous traits become more frequent in the population over time.

Over many generations, this can lead to significant changes in a species. If populations become different enough, they may eventually become separate species.

A classic example is **antibiotic resistance** in bacteria. Some bacteria may naturally have mutations that let them survive an antibiotic. When the antibiotic kills the vulnerable bacteria, the resistant ones survive and reproduce. Over time, the bacterial population becomes mostly resistant.

Natural selection does not aim for “perfection” or plan ahead. It simply favors traits that work well in a particular environment at a particular time. If the environment changes, different traits may become advantageous.


text:
Evolution by natural selection is the process by which populations of organisms change over generations because some inherited traits help individuals survive and reproduce more successfully than others.

The basic idea has four main parts:

1. **Variation exists**
   Individuals in a population are not all identical. They may differ in size, color, speed, disease resistance, behavior, or many other traits.

2. **Some variation is inherited**
   Many traits are influenced by genes, which can be passed from parents to offspring. New genetic variation can arise through mutation, recombination during sexual reproduction, and gene flow between populations.

3. **More offspring are produced than can survive**
   Organisms often produce more young than the environment can support. This creates competition for limited resources such as food, space, mates, and shelter.

4. **Some traits improve survival or reproduction**
   Individuals with traits that are better suited to their environment are more likely to survive and have offspring. Because those traits are inherited, they become more common in the population over time.

For example, imagine a population of beetles where some are green and some are brown. If birds can more easily see and eat the green beetles, brown beetles may survive more often. If color is inherited, brown beetles will leave more offspring, and over many generations the population may become mostly brown.

A few important points:

- **Natural selection acts on individuals, but populations evolve.**
  An individual does not evolve during its lifetime; rather, the genetic makeup of the population changes over generations.

- **“Fitness” means reproductive success, not physical strength.**
  A trait is “fit” if it helps an organism survive and reproduce in a particular environment.

- **Selection depends on the environment.**
  A trait that is helpful in one environment may be harmful or neutral in another.

- **Evolution has no goal or plan.**
  Natural selection does not produce “perfect” organisms. It works with existing variation and favors traits that are useful under current conditions.

Over long periods of time, natural selection can lead to major changes, including new adaptations and sometimes the formation of new species.


text:
Evolution by natural selection is the process by which populations of organisms change over generations because some inherited traits help individuals survive and reproduce more successfully than others.

It works through a few key steps:

1. **Variation exists**
   - Individuals in a population are not identical.
   - They may differ in size, color, behavior, disease resistance, speed, and many other traits.

2. **Some variation is inherited**
   - Many traits can be passed from parents to offspring through genes.

3. **More offspring are produced than can survive**
   - Resources such as food, space, and mates are limited.
   - Not every individual will survive long enough to reproduce.

4. **Certain traits give an advantage**
   - In a particular environment, some traits make survival or reproduction more likely.
   - For example, a better-camouflaged insect may be less likely to be eaten.

5. **Those traits become more common**
   - Individuals with helpful inherited traits tend to leave more offspring.
   - Over many generations, those traits increase in frequency in the population.

Over long periods, natural selection can lead to major changes, including the formation of new species.

A simple example: if a population of beetles includes both green and brown beetles, and birds more easily see and eat the green beetles, brown beetles may survive and reproduce more. If color is inherited, future generations will have more brown beetles.

Natural selection does not involve organisms “trying” to evolve. Instead, the environment favors certain inherited traits, and populations gradually change as a result.


text:
Evolution by natural selection is the process by which populations of organisms change over generations because some inherited traits help individuals survive and reproduce better than others.

It works through a few key steps:

1. **Variation exists**
   Individuals in a population are not all identical. They may differ in size, color, speed, disease resistance, behavior, and many other traits.

2. **Some variation is inherited**
   Many traits are influenced by genes and can be passed from parents to offspring.

3. **More offspring are produced than can survive**
   Organisms often produce more young than the environment can support, so there is competition for food, space, mates, and other resources.

4. **Some traits improve survival or reproduction**
   Individuals with traits that are better suited to their environment are more likely to survive, find mates, and produce offspring.

5. **Helpful traits become more common**
   Because those individuals reproduce more successfully, the genes associated with beneficial traits become more frequent in the population over generations.

Over long periods of time, these changes can lead to major adaptations and even the formation of new species.

For example, if a population of insects lives on dark tree bark, darker-colored insects may be harder for predators to see. Those darker insects are more likely to survive and reproduce, so over generations the population may become darker overall.

A key point is that **individual organisms do not evolve during their lifetimes**. Instead, **populations evolve over generations**. Also, mutations and genetic variation arise largely by chance, but natural selection is not random: traits that improve survival or reproduction in a particular environment tend to spread.


prompt_id: sci-iq-heritability

text:
Human intelligence, usually measured with IQ or broader cognitive-test batteries, is **moderately to highly heritable**, but the exact estimate depends on age, population, environment, and method.

## Short answer

- In modern twin and family studies, **heritability of intelligence is often estimated around 40–80%**.
- It tends to be **lower in childhood** and **higher in adulthood**:
  - Childhood: roughly **20–50%**
  - Adolescence/adulthood: roughly **50–80%**
- DNA-based studies using measured genetic variants usually find lower estimates, often around **20–30% SNP heritability**, because they capture only part of genetic influence.
- Heritability does **not** mean intelligence is fixed, unchangeable, or “determined by genes.” It means that, within a particular population and environment, genetic differences explain some proportion of observed variation.

## What “heritability” means

Heritability is a population statistic. If IQ has a heritability of 60%, that means:

> In that population, at that time, about 60% of the variation in IQ scores is statistically associated with genetic differences among individuals.

It does **not** mean:

- 60% of a person’s intelligence is genetic.
- Environment does not matter.
- Intelligence cannot be improved.
- Differences between groups are genetic.
- A high heritability estimate applies everywhere or at all times.

For example, height is highly heritable, but nutrition and disease can still strongly affect height. Similarly, intelligence can be heritable while also being influenced by education, health, stress, poverty, toxins, parenting, and culture.

## Main 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 genetic variation.

Across many studies:

- Identical twins tend to have more similar IQ scores than fraternal twins.
- Identical twins reared apart are still substantially similar.
- This pattern suggests a genetic contribution.

Typical findings:

- Identical twin IQ correlations are often around **0.7–0.9**.
- Fraternal twin correlations are often around **0.4–0.6**.
- Estimates of heritability increase with age.

Twin studies are powerful, though they depend on assumptions, such as how similar the environments of identical and fraternal twins are.

### 2. Adoption studies

Adoption studies compare children with their biological and adoptive relatives.

Findings generally show:

- In childhood, adopted children’s IQ resembles both adoptive-family environment and biological-family background.
- As children grow older, resemblance to biological relatives tends to increase.
- Resemblance to adoptive siblings often decreases by adulthood.

This supports both environmental influence, especially early in life, and genetic influence, especially as people mature and select or shape environments correlated with their abilities and interests.

### 3. Molecular genetics and GWAS

Genome-wide association studies, or GWAS, examine millions of genetic variants across large samples.

Findings show:

- Intelligence is **highly polygenic**: many thousands of genetic variants each have tiny effects.
- No single “intelligence gene” explains much.
- Polygenic scores can predict some variation in cognitive performance and educational outcomes, but far from all of it.
- SNP-based heritability estimates are usually lower than twin-study estimates, often around **20–30%**, because common measured variants do not capture all genetic effects.

Polygenic scores are improving with larger datasets, but they are still limited, especially across ancestries and social contexts.

## Why heritability rises with age

One consistent finding is that the heritability of intelligence appears to increase from childhood into adulthood. Possible reasons include:

- People increasingly choose environments that fit their genetic tendencies.
- Children gain more autonomy over reading, friends, hobbies, courses, and careers.
- Genetic influences may be amplified through accumulated experience.
- Shared family environment becomes relatively less important with age.

This does not mean environment stops mattering. Rather, the kinds of environments people experience become more individually differentiated over time.

## Environmental influences are real and important

Even with substantial heritability, many environmental factors affect cognitive development, including:

- Quality and length of schooling
- Early childhood stimulation
- Nutrition
- Prenatal environment
- Lead and toxin exposure
- Sleep
- Chronic stress
- Poverty and deprivation
- Disease burden
- Parenting and language exposure
- Social stability and safety

Large environmental effects are evident from phenomena such as the **Flynn effect**, where average IQ scores rose substantially across many countries during the 20th century. Genes did not change that quickly, so environmental changes must have played a major role.

## Heritability depends on the environment

Heritability can be higher in environments where people have relatively equal access to nutrition, schooling, healthcare, and safety, because environmental variation is reduced. In more unequal or deprived environments, environmental factors may explain more of the differences.

For example, if everyone has good nutrition, genetic differences may explain more variation in height. If some children are malnourished, environment explains more. The same logic applies to cognitive development.

## What the evidence does not show

The evidence does **not** justify saying:

- Intelligence is genetically fixed.
- Education or interventions do not matter.
- Individual potential can be accurately read from DNA.
- Group differences in IQ are genetic.
- Heritability within a population explains differences between populations.

Heritability estimates are about variation **within** a studied population under particular conditions. They cannot automatically explain differences between countries, social classes, ethnic groups, or historical periods.

## Bottom line

The best evidence indicates that human intelligence is **substantially heritable**, especially by adulthood, with many studies estimating heritability around **50–70%** in developed-country adult samples. However, intelligence is also shaped by environment, development, education, health, and social conditions. Genetic influence is real, but it is distributed across thousands of variants, probabilistic rather than deterministic, and deeply intertwined with environmental opportunity.


text:
Human intelligence—as usually measured by IQ tests or broader “general cognitive ability” tests—is **substantially heritable**, but not genetically fixed. The best evidence suggests that, in many modern populations, genetic differences account for a large share of **individual differences** in measured cognitive ability, especially in adulthood.

## Short answer

- **Childhood:** heritability is often around **30–50%**.
- **Adolescence/adulthood:** heritability often rises to about **50–80%**, with many estimates around **60%** for adult general intelligence.
- **Environment still matters a lot**, especially in childhood and in disadvantaged conditions. Nutrition, schooling, health, stress, toxins, family resources, and social environment can all affect cognitive development.
- Heritability is **not** the same as immutability, and it does **not** say how much of a person’s intelligence is “caused by genes.” It describes variation within a particular population at a particular time.

## What “heritability” means

Heritability is a population statistic. If adult IQ has a heritability of 60%, that means that **about 60% of the variation between people in that population** is statistically associated with genetic differences.

It does **not** mean:

- “60% of your intelligence comes from genes.”
- “IQ cannot be changed.”
- “Environmental interventions do not work.”
- “Group differences are genetic.”
- “Genes determine a person’s intellectual destiny.”

A trait can be highly heritable and still environmentally changeable. Height is a classic example: it is highly heritable, yet average height has increased in many populations because of improved nutrition and health. Intelligence shows a similar pattern: IQ scores rose substantially during the 20th century in many countries, a phenomenon called the **Flynn effect**, which is environmental in origin.

## Main sources 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 genetic variation.

A typical 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 supports a genetic contribution to intelligence.

Approximate correlations often reported:

- **Identical twins reared together:** around **0.75–0.85**
- **Identical twins reared apart:** around **0.65–0.75**
- **Fraternal twins/siblings reared together:** around **0.40–0.60**
- **Adoptive siblings:** often much lower in adulthood

These are not exact constants, but the pattern is robust across many studies.

### 2. Adoption studies

Adoption studies help separate genetic relatedness from shared home environment.

Findings generally show:

- In childhood, adopted children’s IQs are influenced by their adoptive family environment.
- As children grow older, their IQs become more similar to their biological relatives and less similar to unrelated adoptive relatives.
- Shared family environment appears to have a stronger effect in childhood than in adulthood.

This does not mean parenting or schooling “do not matter.” Rather, it suggests that among the environments studied, shared family-level factors explain less of the remaining variation in adult IQ than genetic and non-shared environmental factors.

### 3. Genome-wide association studies, or GWAS

Modern molecular genetics has found that intelligence is **highly polygenic**: influenced by thousands of genetic variants, each with tiny effects.

GWAS results show:

- There is no single “intelligence gene.”
- Many DNA variants are statistically associated with educational attainment and cognitive test performance.
- Polygenic scores can predict some differences in cognitive outcomes, but far from perfectly.

Current polygenic scores explain a meaningful but limited fraction of variation. Depending on the sample and measure, they may explain roughly:

- **5–15%** of variation in cognitive performance,
- often more for educational attainment than for direct IQ,
- less accurately in populations unlike the mostly European-ancestry samples used to build many GWAS models.

The gap between twin-study heritability estimates and DNA-based prediction is sometimes called the “missing heritability” problem. It reflects limitations in measurement, sample size, rare variants, gene–gene and gene–environment interplay, and other complexities.

## Why heritability increases with age

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

Possible reasons include:

1. **Gene–environment correlation:**
   As people age, they increasingly select and shape environments that match their genetic tendencies. A child who enjoys reading may read more, become more skilled, and seek intellectually demanding settings.

2. **Reduced shared environmental influence:**
   The family environment strongly shapes children, but adults diverge into different peer groups, schools, jobs, hobbies, and lifestyles.

3. **Stabilization of cognitive traits:**
   Individual differences become more stable over development.

So heritability rising with age does not mean genes “turn on” and environment disappears. It means genetic differences become increasingly correlated with the environments people experience and choose.

## Environmental effects are real

Even when heritability is high, environment matters. Important environmental influences include:

- prenatal health,
- nutrition,
- exposure to lead or other toxins,
- childhood disease burden,
- sleep,
- stress and trauma,
- schooling quality,
- cognitive stimulation,
- poverty,
- neighborhood conditions,
- discrimination and social exclusion,
- access to books, language, and learning opportunities.

Severe deprivation can have large negative effects. Conversely, improved public health, schooling, and nutrition can raise average cognitive performance across generations.

The Flynn effect is strong evidence that population-level cognitive performance can change substantially over time for environmental reasons.

## Socioeconomic context matters

Some studies suggest that heritability of IQ may be lower in disadvantaged environments and higher in affluent, stable environments. The idea is that when children face deprivation, environmental constraints suppress genetic potential. When basic needs are met, genetic differences may account for more of the remaining variation.

Evidence for this pattern is mixed and appears stronger in some countries, especially the United States, than in others with stronger social safety nets. But the broader point is important: **heritability depends on the range of environments in a population.**

## What the evidence does not show

The evidence does not justify simplistic claims such as:

- “IQ is fixed.”
- “Genes are destiny.”
- “Education does not matter.”
- “Group differences are genetic.”
- “Polygenic scores can accurately rank individuals’ potential.”
- “There is a single biological essence of intelligence.”

Heritability estimates mostly concern **differences among individuals within studied populations**, not differences between nations, social classes, or racial/ethnic groups. Group differences can arise from environmental inequality even when a trait is highly heritable within groups.

## 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:

> Genetic differences explain roughly half or more of individual variation in adult cognitive ability in many studied populations, while environmental factors remain important, especially in childhood and under conditions of deprivation.

Human intelligence is best understood as the product of **many genes of small effect interacting with many environmental influences over development**.


text:
Human intelligence, usually measured with IQ tests and related cognitive batteries, is **substantially heritable**, but not genetically determined in any simple sense. The best summary is:

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

That range depends on age, population, measurement quality, and environment.

## What “heritable” means

Heritability is the proportion of **difences among individuals in a particular population at a particular time** that is statistically associated with genetic differences.

It does **not** mean:

- that intelligence is fixed;
- that environment does not matter;
- that an individual’s IQ is “X% genetic”;
- that differences between groups are genetic;
- that genes operate independently of environments.

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

## Main findings

### 1. Twin and adoption studies show substantial genetic influence

Classic behavioral-genetic evidence comes from comparisons of:

- identical twins, who share nearly all their DNA;
- fraternal twins, who share about half of segregating genetic variation;
- siblings, adoptees, and unrelated individuals raised together.

Across many studies, identical twins tend to be more similar in IQ than fraternal twins, and biological relatives tend to resemble each other more than adoptive relatives, especially by adulthood.

Approximate heritability estimates:

| Age | Typical heritability estimate for IQ/general cognitive ability |
|---|---:|
| Early childhood | ~20–40% |
| Middle childhood | ~40–50% |
| Adolescence | ~50–60% |
| Adulthood | ~60–80% |

The increase with age is one of the more robust findings. One likely reason is **gene–environment correlation**: as people grow older, they increasingly select, shape, and evoke environments that match their dispositions and abilities.

For example, a child with a genetic predisposition toward verbal ability may read more, receive more encouragement, choose more challenging classes, and accumulate further advantages.

### 2. Shared family environment matters more in childhood than adulthood

“Shared environment” means environmental influences that make siblings raised together more similar: socioeconomic status, neighborhood, parenting style, school access, etc.

In many studies:

- shared family environment has a noticeable effect in childhood;
- its measured influence often declines by adulthood;
- non-shared environment — experiences that differ between siblings, measurement error, illness, peers, teachers, random developmental events — remains important.

This does **not** mean parenting or schooling “do not matter.” It means that, in the populations studied, differences between families explain less adult IQ variation than genetic differences and individual-specific experiences.

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

Modern genome-wide association studies, or GWAS, show that cognitive ability and educational attainment are influenced by **many thousands of genetic variants**, each with tiny effects.

No single “intelligence gene” exists.

Polygenic scores constructed from GWAS results can predict a modest but growing fraction of variation in cognitive performance and educational outcomes. They are useful scientifically, but they are far from deterministic and are not accurate enough to make strong predictions about individuals.

A rough current picture:

- twin-study heritability for adult cognitive ability: often ~60–80%;
- SNP-based heritability captured by common measured variants: lower, often ~20–30% or somewhat more depending on method and sample;
- polygenic scores explain a smaller fraction than total heritability, though this has increased with larger datasets.

The gap between twin heritability and measured DNA prediction is sometimes called part of the “missing heritability” problem. It may reflect many small-effect variants, rare variants, structural variants, imperfect measurement, gene–gene interaction, gene–environment interplay, and limitations of current samples.

## Environmental evidence is also strong

The evidence for genetic influence does not negate environmental effects. Several lines of evidence show that environments matter.

### 1. The Flynn effect

Average IQ scores rose substantially in many countries during the 20th century — often by around 2–3 IQ points per decade, though patterns vary and some countries have seen plateaus or reversals.

Such rapid population-level change cannot plausibly be due to genetic evolution. It likely reflects changes in nutrition, health, education, test familiarity, cognitively demanding environments, family size, and other societal factors.

### 2. Deprivation and intervention studies

Severe deprivation, malnutrition, lead exposure, chronic stress, poor schooling, and disease can reduce cognitive development. Conversely, adoption from deprived to enriched environments can produce large gains, especially when it occurs early.

Early-childhood interventions often show immediate IQ gains, though some fade over time; many still improve educational, economic, or health outcomes.

### 3. Socioeconomic context can moderate heritability

Some studies, especially in the United States, have found that heritability of IQ is lower among children in poorer environments and higher among children in more advantaged environments. The interpretation is that genetic potential may be more fully expressed when basic environmental constraints are reduced.

However, this finding is not equally strong in all countries. In societies with more equal access to health care, education, and nutrition, socioeconomic moderation may be weaker.

## Important caveats

### Heritability is population-specific

A heritability estimate applies to a given population and environment. If environments become more equal, heritability can rise because remaining differences are more associated with genetics. If environments become more unequal or harmful, environmental variance can increase and heritability can fall.

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

High heritability of intelligence within a population does **not** imply that average differences between socioeconomic, ethnic, national, or racial groups are genetic.

For example, height is highly heritable within countries, but average height differences between countries have often changed dramatically because of nutrition and health.

### Intelligence tests measure important but limited abilities

IQ and general cognitive ability predict many life outcomes, including academic achievement, job performance, health behaviors, and income, but they do not capture all valuable human capacities: creativity, wisdom, motivation, emotional regulation, practical knowledge, personality, or moral judgment.

### Genetic influence is not determinism

Even highly heritable traits can be changed by environment. Genes influence probabilities, sensitivities, and developmental pathways, not fixed destinies.

## 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 IQ:** roughly 30–50% heritable;
- **adult IQ:** roughly 60–80% heritable in many studied populations;
- **environment:** very important, especially early in life and in disadvantaged or deprived contexts;
- **genetics:** highly polygenic, with thousands of variants of tiny effect;
- **interpretation:** heritability describes variation within populations, not immutability or group causes.

So the best scientific view is neither “IQ is mostly environment” nor “IQ is genetically fixed,” but rather: **intelligence develops through complex interactions between many genes and many environments, with genetic differences explaining a large share of individual variation in many modern populations.**


text:
Human intelligence, as measured by IQ-type cognitive tests, is **substantially heritable**, but it is **not genetically fixed**. The best evidence suggests that genetic differences account for roughly:

- **~20–40% of variation in childhood**
- **~40–60% in adolescence**
- **~50–80% in adulthood**

A reasonable shorthand is: **about half or more of adult differences in measured intelligence within a given population are associated with genetic differences**, with the exact estimate depending on age, environment, measurement, and study design.

Importantly, “heritable” does **not** mean “unchangeable,” “innate,” or “determined.” Heritability describes variation **within a population in a particular environment**, not the cause of an individual’s intelligence and not the size of environmental effects.

---

## What the evidence shows

### 1. Twin studies show strong genetic influence

Identical twins share nearly 100% of their DNA; fraternal twins share about 50% on average. Across many studies:

- **Identical twins’ IQ scores are much more similar** than fraternal twins’.
- This remains true even when twins are raised apart, although such studies are smaller and more difficult to interpret.

Typical findings:

| Relationship | Approximate IQ similarity |
|---|---:|
| Identical twins raised together | High |
| Identical twins raised apart | Still high |
| Fraternal twins/siblings | Moderate |
| Unrelated children raised together | Lower, especially by adulthood |

This pattern is hard to explain without substantial genetic influence.

---

### 2. Adoption studies support both genetic and environmental effects

Adoption studies find that:

- In childhood, adopted children’s IQs often resemble their **adoptive family environment** to some degree.
- As children grow older, their IQs tend to resemble their **biological relatives** more strongly.
- The influence of the shared family environment often appears to decline by adulthood.

This does not mean parenting or schooling “do not matter.” Rather, it suggests that within the range of environments studied, family-level environmental effects on IQ differences often become less visible over time, while genetic influences become more expressed.

---

### 3. Heritability tends to increase with age

One of the most replicated findings is that IQ heritability rises from childhood to adulthood.

Possible reasons include:

- People increasingly choose environments that fit their genetic tendencies.
- Genetic influences may accumulate through development.
- Children gain more autonomy over reading, friendships, hobbies, education, and work.
- Early family environment matters more when children are young.

So intelligence is not simply “born fixed”; rather, genetic effects often become more apparent as people develop.

---

### 4. Modern DNA studies confirm polygenic influence

Genome-wide association studies, or GWAS, show that intelligence-related traits are influenced by **many thousands of genetic variants**, each with tiny effects.

Key points:

- There is no single “gene for intelligence.”
- Intelligence is highly **polygenic**.
- DNA-based estimates currently explain less variance than twin studies, often called the “missing heritability” gap.
- Polygenic scores can predict some variation in cognitive performance and educational outcomes, but they are far from deterministic and work best in populations similar to those in which they were developed.

Modern molecular genetics supports the twin-study conclusion: genetic differences matter, but in a diffuse, probabilistic way.

---

## Environmental evidence is also strong

Even with high heritability, environment can have large effects. Evidence includes:

### The Flynn effect

Average IQ scores rose substantially across the 20th century in many countries. Genes cannot change that fast, so the gains must largely reflect environmental changes such as:

- Better nutrition
- More schooling
- Reduced disease burden
- Smaller families
- More cognitively demanding environments
- Improved test familiarity or abstract reasoning demands

This is a major reminder that high heritability does not imply immutability.

### Schooling matters

More schooling is associated with higher performance on intelligence tests, and quasi-experimental studies suggest schooling itself can raise measured cognitive ability.

### Deprivation and enrichment matter

Severe deprivation, malnutrition, lead exposure, chronic stress, poor schooling, and illness can reduce cognitive development. Conversely, improved health, safety, education, and stimulation can improve outcomes.

---

## What heritability does and does not mean

### Heritability means:

If adult IQ heritability is 60%, then in that population and environment, about 60% of the observed variation in IQ scores is statistically associated with genetic differences.

### It does not mean:

- “60% of a person’s intelligence is genetic.”
- “Environment only matters 40%.”
- “IQ cannot be changed.”
- “Genetic differences explain differences between societies or historical periods.”
- “Polygenic scores can accurately rank individuals’ potential.”

Heritability is about **variation**, not destiny.

---

## A useful analogy

Height is highly heritable, often around 80% or more in well-nourished populations. Yet average height increased dramatically in many countries because of improved nutrition and health. Intelligence is similar in the sense that genetic variation matters, but environmental conditions can shift population averages and individual development.

---

## Bottom line

The evidence from twin, adoption, family, and DNA studies shows that **human intelligence is moderately to highly heritable**, especially in adulthood. A common estimate for adults is roughly **50–80% heritability** in many studied populations.

But intelligence is also shaped by environment, including schooling, nutrition, health, toxins, stress, culture, and opportunity. Genetic influence is real, but it is **polygenic, probabilistic, population-specific, and not deterministic**.


text:
Human intelligence, as measured by standard cognitive tests/IQ tests, is **substantially heritable**, especially in adulthood, but it is also **strongly influenced by environment**. “Heritable” does **not** mean fixed, deterministic, or unaffected by education, nutrition, health, family environment, schooling, or social conditions.

## What “heritability” means

Heritability is a statistic about **variation within a population in a particular environment**.

If the heritability of IQ is estimated at 0.6, that means roughly **60% of the differences among people in that studied population** are statistically associated with genetic differences. It does **not** mean:

- 60% of an individual’s intelligence is “genetic”
- intelligence cannot be changed
- genes explain differences between populations
- environment is unimportant

Heritability can change across age, society, and environmental conditions.

## Typical estimates

Across many studies, the approximate pattern is:

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

A commonly cited summary is that IQ heritability rises from childhood into adulthood, reaching roughly **0.6–0.8** in many adult twin studies.

At the same time, environmental effects are clearly important, especially early in life.

## Main sources of evidence

### 1. Twin studies

Twin studies compare:

- **Identical twins**, who share nearly 100% of their DNA
- **Fraternal twins**, who share about 50% of segregating genetic variants

Identical twins tend to be more similar in IQ than fraternal twins, which supports a genetic contribution.

Studies of identical twins raised apart have also found notable similarity in cognitive ability, suggesting genetics matters even when family environments differ. However, such studies are rare and have limitations, including nonrandom placement and similar broad cultural environments.

### 2. Adoption studies

Adoption studies show that adopted children’s cognitive scores are influenced by both:

- their **biological relatives**, consistent with genetic influence
- their **adoptive environments**, especially in childhood

One repeated finding is that resemblance to adoptive parents tends to be stronger in childhood and weaker in adulthood, while resemblance to biological relatives often increases with age.

### 3. Molecular genetics / GWAS

Genome-wide association studies find that intelligence and educational attainment are influenced by **many thousands of genetic variants**, each with tiny effects.

Modern genomic methods estimate “SNP heritability” — the portion of variation captured by common measured genetic variants — often around **20–30%** for cognitive ability, sometimes higher depending on sample and measure. This is lower than twin-study estimates because GWAS does not capture all genetic influences, such as rare variants, structural variants, nonadditive effects, and imperfect measurement.

Polygenic scores can predict some cognitive and educational outcomes, but they are far from deterministic and explain only a minority of variance.

## Why heritability increases with age

A likely reason is that as people grow older, they increasingly select, shape, and evoke environments that match their genetic tendencies. For example, a child with an early inclination toward reading may seek books, receive encouragement, build vocabulary, and enter more academic settings. This is called **gene–environment correlation**.

So genes and environment are not separate forces acting independently; they interact over development.

## Environmental influences are real and important

Evidence for environmental effects includes:

- improvements in IQ scores over generations, known as the **Flynn effect**
- effects of schooling quality and duration
- impacts of early childhood deprivation or enrichment
- effects of nutrition, toxins such as lead, prenatal health, disease burden, stress, and poverty
- adoption from severely deprived environments into more supportive ones often producing large gains

Shared family environment appears to matter more in childhood than adulthood, while nonshared environmental influences — experiences that make siblings different — remain important.

## Important caveats

### Heritability is population-specific

A high heritability estimate in one society does not imply the same estimate everywhere. If environments are very unequal, environmental factors may explain more variation. If environments are relatively uniform, genetic differences may account for a larger share of remaining variation.

### Heritability does not imply immutability

Height is highly heritable, but average height increased dramatically in many countries because nutrition and health improved. Intelligence is similar: genetic influence can be large while environmental interventions still matter.

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

Heritability estimates mostly address why individuals differ **within a studied population**. They do not by themselves explain average differences between social, ethnic, national, or historical groups. Group differences can arise from environmental, cultural, socioeconomic, educational, health, and historical factors even when within-group heritability is high.

## Bottom line

The evidence shows that human intelligence is **moderately to highly heritable**, with genetic influences becoming stronger from childhood into adulthood. In adults, many studies estimate heritability around **50–80%**, while molecular genetic studies currently capture a smaller but substantial portion.

But intelligence is not genetically predetermined. It is shaped by a complex interplay of many genes, development, education, health, family and social environment, opportunity, and broader cultural conditions.


text:
Human intelligence—as usually measured by IQ tests or broader “general cognitive ability” (*g*)—is **substantially heritable**, but not genetically fixed. The best evidence suggests that **genetic differences account for roughly half of variation in cognitive ability in many studied populations**, with estimates changing across age, environment, and method.

## What “heritable” means

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

It does **not** mean:

- “X% of a person’s intelligence is genetic”
- intelligence cannot be changed
- environment is unimportant
- observed group differences are genetic
- heritability is the same in all societies or historical periods

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

## Typical estimates

Across many twin, adoption, and family studies:

- **Early childhood:** heritability often around **20–40%**
- **Adolescence:** often around **40–60%**
- **Adulthood:** often around **50–80%**

A reasonable summary is that adult general cognitive ability has heritability around **~50–70%** in many modern, high-income populations, though estimates vary.

Shared family environment—features making siblings similar, such as household resources or parenting style—appears more influential in childhood and often declines in measured impact by adulthood. Non-shared environment, measurement error, life experiences, illness, schooling, peers, and random developmental factors also matter.

## Main lines of evidence

### 1. Twin studies

Identical twins share essentially all their segregating DNA; fraternal twins share about half on average.

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 supports genetic influence.

However, twin studies rely on assumptions, such as the “equal environments assumption”—that identical and fraternal twins’ environmental similarity does not differ in ways that fully explain the IQ resemblance difference. Researchers debate details, but the broad genetic signal is robust.

### 2. Adoption studies

Adoption studies show that children’s cognitive scores are influenced by both biological and adoptive environments.

Common findings:

- In childhood, adopted children often resemble their adoptive families to some degree.
- As they age, their resemblance to biological relatives often increases.
- Adoptive-sibling similarity in IQ tends to decline with age.

This supports the idea that genetic influences become more apparent over development, while shared home environment has stronger effects earlier.

### 3. DNA-based studies

Modern genomic studies examine measured DNA directly. Intelligence and educational outcomes are **highly polygenic**: influenced by thousands of genetic variants, each with tiny effects.

Findings include:

- Genome-wide association studies identify many variants associated with cognitive performance or educational attainment.
- Polygenic scores can predict a modest but nontrivial fraction of variation in cognitive and educational outcomes.
- SNP-based heritability estimates—heritability captured by common measured variants—are lower than twin-study estimates, often around **20–30%** for cognitive traits, because they miss rare variants, structural variants, imperfect tagging, gene–environment interplay, and other sources.

DNA evidence confirms that genetic differences contribute to cognitive differences, but current genetic prediction is far from deterministic.

## Why heritability changes with age

Heritability of intelligence often increases from childhood to adulthood. Possible reasons include:

- People increasingly select and shape environments compatible with their dispositions.
- Genetic influences may affect motivation, curiosity, persistence, reading habits, and educational choices.
- Adult environments may allow more self-direction than childhood environments.
- Measurement of stable general ability may become more reliable with age.

This does not mean the environment stops mattering. Rather, environments and genetic tendencies become intertwined over time.

## Environment still matters a lot

Environmental factors known or suspected to affect cognitive development include:

- prenatal health and nutrition
- exposure to toxins such as lead
- childhood disease burden
- schooling quality and duration
- socioeconomic deprivation
- chronic stress
- sleep
- malnutrition
- early language exposure
- family instability or neglect
- broader social and educational opportunities

The **Flynn effect**—large IQ gains across generations in many countries during the 20th century—shows that population-level cognitive test performance can change substantially over time, far faster than genetic evolution could explain. This is strong evidence for environmental effects.

## Heritability depends on context

Heritability is not universal. It can be lower in environments where deprivation, poor schooling, malnutrition, or disease constrain development. In more equal and supportive environments, genetic differences may account for a larger share of remaining variation.

So a high heritability estimate in one population does not imply the same estimate elsewhere.

## Bottom line

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

> Genetic differences explain a substantial portion—often about half or more—of individual differences in measured intelligence within many studied populations, while environmental factors also have major effects and can shift cognitive outcomes significantly.

Intelligence is influenced by many genes of tiny effect, many environmental factors, and their interaction over development. Heritability is real, but it is not destiny.


text:
Human intelligence, usually measured with IQ or broader cognitive-test batteries, is **substantially heritable**, but not genetically fixed. The best summary is:

> In contemporary, well-studied populations, genetic differences explain roughly **40–80%** of individual differences in measured intelligence, with heritability generally **increasing from childhood into adulthood**.

A typical pattern is:

- **Early childhood:** ~20–40% heritable
- **Adolescence:** ~40–60% heritable
- **Adulthood:** ~60–80% heritable in many twin studies
- **Common DNA variants measured by GWAS:** usually explain a smaller share, often ~20–30% of variance, because current genetic data do not capture all inherited influences.

## What “heritable” means

Heritability is a statistic about **variation within a population**. If IQ is 60% heritable in a given population, that means about 60% of the differences among people in that population are statistically associated with genetic differences.

It does **not** mean:

- 60% of a person’s intelligence is “genetic”
- intelligence is unchangeable
- environment is unimportant
- the same estimate applies to every society or historical period
- genetic differences explain average differences between groups

Heritability can be high even for traits strongly affected by environment. For example, height is highly heritable, but nutrition and disease still matter greatly.

## Main lines of evidence

### 1. Twin studies

Identical twins share essentially 100% of their genetic variation, while fraternal twins share about 50% on average. Across many studies, identical twins are much more similar in IQ than fraternal twins.

A simplified pattern often looks like:

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

This is one of the strongest sources of evidence that genetic differences contribute substantially to intelligence differences.

### 2. Adoption studies

Adoption studies show that children often resemble their adoptive families in childhood, but as they grow older, their cognitive scores tend to become more similar to their biological relatives.

Also, adoptive siblings raised in the same home often become less similar in IQ by adulthood than biological siblings do. That suggests that the shared family environment matters, especially early in life, but genetic influences become more visible with age.

### 3. Molecular genetics and GWAS

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

Modern DNA-based methods find that common genetic variants account for a meaningful portion of cognitive differences, but less than twin studies estimate. This gap is partly because:

- many relevant variants are rare or poorly measured
- effects are extremely small and hard to detect
- gene–environment correlations complicate estimation
- twin studies capture more than currently measured common SNP effects

Polygenic scores can predict some variation in intelligence and educational outcomes, but they are far from deterministic and currently work best in populations similar to the mostly European-ancestry samples in which they were developed.

## Why heritability increases with age

One of the most replicated findings is that intelligence becomes more heritable from childhood to adulthood. Possible reasons include:

1. **Gene–environment correlation:** as people age, they increasingly select and shape environments that fit their dispositions. A child inclined toward reading, puzzles, or abstract reasoning may seek more cognitively stimulating settings.

2. **Reduced shared environmental influence:** family-wide environmental effects, such as parenting style or household resources, often have stronger effects in childhood and weaker effects later.

3. **Accumulated feedback loops:** small early differences can compound through schooling, peer groups, interests, and opportunities.

## Environment still matters a lot

The evidence also clearly shows that environment affects intelligence. Important environmental influences include:

- schooling quality and duration
- childhood nutrition
- prenatal environment
- exposure to lead or other toxins
- infectious disease burden
- severe deprivation or neglect
- family stress and poverty
- access to books, language, and cognitively stimulating experiences

The **Flynn effect**, the large rise in IQ scores across many countries during the 20th century, is strong evidence that environmental change can shift cognitive-test performance substantially over time.

Severe deprivation can depress cognitive development, and improvements in education, health, and nutrition can raise average performance.

## Shared vs non-shared environment

Behavioral-genetic studies often divide influences into:

- **Genetic effects**
- **Shared environment:** factors making siblings similar, such as household, neighborhood, parental income
- **Non-shared environment:** factors making siblings different, such as different peers, illnesses, teachers, experiences, and measurement error

For intelligence, shared family environment often appears more important in childhood and less important in adulthood. Non-shared environment remains important throughout life.

## Bottom line

The evidence shows that intelligence is **moderately to highly heritable**, especially in adulthood. Twin, adoption, and DNA-based studies all support a substantial genetic contribution.

But intelligence is also meaningfully shaped by environment, especially early-life conditions, education, health, and social opportunity. Heritability describes differences within a studied population; it does not imply immutability, genetic determinism, or that environments do not matter.


text:
Human intelligence, usually studied with IQ tests or a general cognitive ability factor “g,” is **substantially heritable**, but not genetically fixed. The best short answer is:

> In modern high-income populations, genetic differences typically account for roughly **40–80% of individual differences in measured intelligence**, with heritability tending to be **lower in childhood and higher in adulthood**. Environment still matters greatly, especially early-life conditions, education, nutrition, health, toxins, stress, and socioeconomic opportunity.

## What “heritability” means

Heritability is often misunderstood. A heritability estimate of, say, 60% does **not** mean that 60% of a person’s intelligence is genetic. It means that, within a particular population and environment, about 60% of the **variation among people** in measured intelligence is statistically associated with genetic differences.

Heritability is also not a fixed biological constant. It can change across:

- Age
- Socioeconomic conditions
- Countries and historical periods
- Test types
- Environmental equality or inequality

A trait can be highly heritable and still strongly affected by environment. Height is the classic example: it is highly heritable, but average height changed substantially over generations because of nutrition and disease conditions.

## Typical estimates by age

Across twin, adoption, and molecular-genetic studies, approximate estimates are:

- **Early childhood:** around **20–40%** heritable
- **Middle childhood/adolescence:** around **40–60%**
- **Adulthood:** often **50–80%**, commonly summarized around **60% or more** in affluent populations

The increase with age is one of the more robust findings. As people grow older, they increasingly select and shape environments that fit their genetically influenced interests and abilities—choosing books, friends, hobbies, education, jobs, and intellectual activities.

## Evidence from twin studies

Twin studies are one major source of evidence.

Identical twins share nearly all their DNA, while fraternal twins share about half of their segregating genetic variation. If identical twins are more similar in IQ than fraternal twins, that suggests genetic influence.

Typical findings:

- Identical twins raised together are very similar in IQ.
- Identical twins raised apart are still substantially similar.
- Fraternal twins and ordinary siblings are less similar.
- The similarity between siblings often declines as they move into adulthood and create more independent environments.

These studies consistently show a genetic contribution, though they rely on assumptions about environmental similarity that researchers continue to debate and test.

## Evidence from adoption studies

Adoption studies also support heritability.

Key findings:

- Adopted children’s IQs often resemble their adoptive family environment in childhood.
- As adoptees age, their cognitive scores tend to become more similar to those of their biological relatives.
- Unrelated children raised in the same adoptive home are often moderately similar as children but much less similar as adults.

At the same time, adoption itself shows that environment matters: children adopted from deprived environments into more supportive homes often show large gains.

## Evidence from DNA studies

Modern genomic studies provide independent evidence.

Genome-wide association studies, or GWAS, have found that intelligence and educational attainment are influenced by **many thousands of genetic variants**, each with a tiny effect. There is no single “intelligence gene.”

Important findings:

- Common genetic variants measured on SNP arrays explain perhaps **20–30%** of variation in cognitive ability, depending on sample and method.
- Polygenic scores can predict some differences in cognitive performance and educational outcomes, but they are far from deterministic.
- Prediction is much weaker across populations with different ancestry and social histories, partly because GWAS datasets are still heavily biased toward European-ancestry samples.
- Within-family studies show that polygenic scores still predict differences between siblings, supporting genuine genetic effects, though often smaller than in unrelated-person comparisons.

DNA evidence confirms that intelligence is polygenic and heritable, but it also shows that current genetic prediction is limited.

## Environmental evidence

The evidence for heritability does not erase environmental influence. Many environmental factors affect cognitive development, including:

- Prenatal health
- Lead and other toxins
- Iodine deficiency
- Malnutrition
- Childhood disease burden
- Sleep
- Stress and trauma
- Parenting and language exposure
- School quality
- Neighborhood safety
- Poverty and social instability

Historical changes also matter. The **Flynn effect**—large increases in IQ scores over much of the 20th century—occurred too quickly to be genetic. It likely reflects environmental changes such as education, nutrition, test familiarity, smaller families, cognitively demanding work, and improved public health.

Severe deprivation can strongly depress cognitive development, while enriched and stable environments can improve outcomes.

## Socioeconomic status and heritability

There is evidence, especially from some U.S. studies, that heritability of IQ may be lower in poorer or more deprived environments. The idea is that when children lack basic supports, environmental constraints can suppress genetic potential. In more advantaged environments, where basic needs are more reliably met, genetic differences may account for a larger share of remaining variation.

However, findings differ across countries. In societies with stronger social supports and less extreme deprivation, the SES-by-heritability pattern is sometimes weaker or absent.

## What heritability does not show

High heritability does **not** mean:

- Intelligence is unchangeable.
- Education is pointless.
- Individual destiny is genetically fixed.
- Group differences are genetic.
- A polygenic score can accurately predict a person’s life outcome.

Also, heritability estimates are usually about variation **within** a studied population. They cannot by themselves explain average differences between groups, countries, ethnicities, or historical periods. Group differences can be environmental even when a trait is highly heritable within groups.

## Bottom line

The evidence shows that human intelligence is **meaningfully and substantially heritable**, especially in adulthood. A reasonable summary is that genes account for about **half or more** of individual differences in measured intelligence in many contemporary populations.

But intelligence is also strongly shaped by environment, development, culture, education, health, and opportunity. Genetic influence is real, polygenic, and probabilistic—not deterministic. Heritability describes sources of variation in a population; it does not set a ceiling on what individuals or societies can change.


text:
Human intelligence, as measured by IQ tests and related cognitive-test batteries, is **substantially heritable**, but it is **not genetically determined** and heritability depends strongly on age, population, and environment.

## Short answer

- In well-studied populations, **heritability of general cognitive ability/IQ is often estimated around:**
  - **Childhood:** roughly **20–50%**
  - **Adolescence:** roughly **40–60%**
  - **Adulthood:** roughly **50–80%**, commonly summarized around **60% or so**
- The remaining variation reflects **environmental influences, developmental factors, measurement error, and chance**, including prenatal environment, nutrition, schooling, family conditions, peers, disease burden, stress, and cultural factors.
- Heritability is **not fixed**. It can be lower in environments with more deprivation and higher where basic needs and education are more uniformly available.

## What “heritable” means

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

It does **not** mean:

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

For example, height is highly heritable in many rich countries, but average height has increased substantially over generations due to improved nutrition and health. Intelligence shows a similar distinction: individual differences can be partly heritable while environmental change can still strongly affect average outcomes.

## Main lines of evidence

### 1. Twin studies

Twin studies compare similarity between identical twins, who share nearly all their DNA, and fraternal twins, who share about half of segregating genetic variation.

Consistent findings:

- Identical twins are more similar in IQ than fraternal twins.
- Identical twins raised apart are still substantially similar.
- Heritability estimates tend to rise with age.

A rough pattern often found:

| Age | Approximate heritability of IQ/general cognitive ability |
|---|---:|
| Early childhood | 20–40% |
| Middle childhood | 30–50% |
| Adolescence | 40–60% |
| Adulthood | 50–80% |

The rise with age may occur because people increasingly select, shape, and respond to environments partly in line with genetically influenced traits — for example, interests, motivation, reading habits, peer groups, and educational choices.

### 2. Adoption studies

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

Common findings:

- In childhood, adopted children often resemble their adoptive families to some degree, showing environmental effects.
- As adoptees age, their cognitive scores tend to become more similar to their biological relatives and less similar to adoptive siblings.
- Shared family environment appears important in childhood but often contributes less to adult IQ differences in many affluent populations.

This does not mean family environment is unimportant; rather, within the studied range of environments, its long-term effect on differences among adults is often smaller than genetic and non-shared environmental influences.

### 3. Molecular genetics and GWAS

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

Findings:

- Intelligence is **highly polygenic**: influenced by thousands of genetic variants, each with tiny effects.
- Current polygenic scores predict a meaningful but limited share of variation.
- SNP-based heritability estimates — the portion captured by common measured genetic variants — are often around **20–30%** for cognitive ability, lower than twin-study estimates.

The gap between twin heritability and SNP heritability may reflect rare variants, structural variants, imperfect measurement, gene–environment correlation, non-additive genetic effects, and limits of current data.

### 4. Environmental evidence

Even with substantial heritability, environment clearly matters.

Evidence includes:

- **The Flynn effect:** IQ test scores rose dramatically across much of the 20th century in many countries, too quickly to be genetic.
- **Education effects:** More schooling is associated with higher cognitive-test performance, and natural-experiment studies suggest schooling can causally raise IQ scores.
- **Nutrition and health:** Iodine deficiency, lead exposure, prenatal stress, severe malnutrition, infectious disease, and childhood adversity can affect cognitive development.
- **Socioeconomic context:** Heritability estimates can differ by social conditions. In deprived environments, environmental constraints may suppress genetic potential, reducing measured heritability.

## Best overall interpretation

The evidence supports five broad conclusions:

1. **Genetic differences contribute substantially to individual differences in intelligence.**
2. **Heritability increases from childhood into adulthood.**
3. **Intelligence is influenced by many genes of very small effect, not by a few “intelligence genes.”**
4. **Environmental factors remain important and can shift both individual outcomes and population averages.**
5. **Heritability estimates are population-specific and should not be used to infer that social or group differences are genetic.**

So, a fair summary is:

> In contemporary, mostly well-resourced populations, adult intelligence is moderately to highly heritable — often estimated around 50–80% — but intelligence is also meaningfully shaped by environment, development, education, health, and social conditions. Heritability describes variation within a population, not immutability or genetic destiny.


text:
Human intelligence, usually measured as IQ or a general cognitive ability factor “g,” is **substantially heritable**, but **not genetically fixed**. In modern industrialized populations, the best evidence suggests roughly:

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

These numbers are approximate because heritability depends on the population, age, environment, and measurement method.

## What “heritable” means

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

It does **not** mean:

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

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

## Main lines of evidence

### 1. Twin studies

Identical twins share nearly 100% of their DNA; fraternal twins share about 50% on average. Across many studies:

- Identical twins tend to have more similar IQ scores than fraternal twins.
- Identical twins raised apart are still substantially similar.
- Fraternal twins and ordinary siblings are less similar.

This pattern strongly suggests genetic influence. Twin studies usually estimate adult IQ heritability in the **50–80%** range.

### 2. Adoption studies

Adoption studies help separate genetic and family-environment effects.

Findings generally show:

- In childhood, adopted children’s IQs are influenced by their adoptive home environment.
- As they grow older, their IQs tend to correlate more with their biological relatives than with adoptive relatives.
- Shared family environment appears important in childhood but often becomes smaller by adulthood.

This supports the idea that genetic influences on measured intelligence increase with age.

### 3. Molecular genetics and GWAS

Genome-wide association studies have found that intelligence and educational attainment are influenced by **many thousands of genetic variants**, each with a tiny effect.

Important findings:

- Intelligence is **highly polygenic**: no single “intelligence gene” exists.
- Common genetic variants captured by current DNA chips explain a meaningful but incomplete portion of IQ variation, often around **20–30%** in SNP-heritability estimates.
- Polygenic scores can predict some differences in cognitive performance and educational outcomes, but their predictive power is still modest and much weaker than twin-study heritability estimates.

This confirms that genetic differences matter, while also showing that the genetic architecture is complex.

## Environment still matters a lot

Even if adult IQ is substantially heritable, environmental effects can be large. Relevant factors include:

- Prenatal health
- Nutrition
- Exposure to toxins such as lead
- Childhood illness
- School quality
- Cognitive stimulation
- Family stress
- Poverty and deprivation
- Sleep, mental health, and educational opportunity

The **Flynn effect**—large IQ gains across generations in many countries—also shows that environmental changes can shift average test performance substantially over time.

## Why heritability rises with age

A common finding is that IQ heritability increases from childhood to adulthood. Possible reasons include:

- People increasingly choose environments that match their genetic tendencies.
- Children have less control over their environments; adults have more.
- Small genetic differences may compound through education, hobbies, peer groups, and career choices.
- Gene–environment correlation: genetic tendencies influence the environments people experience.

For example, a child with a tendency to enjoy reading may read more, improving vocabulary and knowledge, which further reinforces cognitive development.

## Important caveats

1. **Heritability is population-specific.**
   In a society where everyone has similar nutrition and schooling, genetic differences may explain more of the remaining variation. In a society with large deprivation, environmental differences may explain more.

2. **High heritability does not imply immutability.**
   Education, health interventions, and reduced deprivation can affect cognitive outcomes.

3. **Within-group heritability does not explain between-group differences.**
   Even if IQ is heritable within a population, that does not mean differences between social, ethnic, national, or historical groups are genetic.

4. **IQ is not the whole of intelligence.**
   IQ tests measure important cognitive abilities, especially reasoning, memory, processing speed, and verbal/spatial skills, but they do not capture every form of human ability, wisdom, creativity, motivation, or practical judgment.

## Bottom line

The evidence from twins, adoption, and modern genomics shows that **human intelligence is moderately to highly heritable**, especially in adulthood. A reasonable summary is that **genes account for about half or more of individual differences in measured intelligence in many modern populations**, while environment also plays a major role, particularly in childhood and under conditions of deprivation. Intelligence is influenced by many genes of tiny effect interacting with many environmental factors; it is neither genetically predetermined nor environmentally arbitrary.