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 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**
   If a trait helps an organism survive or reproduce in a particular environment, individuals with that trait are more likely to leave offspring.

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

For example, if a population of insects includes both light and dark individuals, and birds more easily see and eat the light ones on dark tree bark, dark insects may survive better. If color is inherited, more dark insects will be born in future generations.

Natural selection does not mean organisms “try” to evolve. It is not goal-directed. Instead, the environment favors certain existing inherited variations. Over long periods, this can lead to major changes, including the formation of new species.


prompt_id: sci-evolution

provider: openai

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 basic steps:

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

4. **Some traits give an advantage**
   Individuals with traits that are better suited to their environment are more likely to survive and reproduce. For example, an animal with better camouflage may avoid predators more successfully.

5. **Helpful traits become more common**
   Because those individuals reproduce more, they pass their advantageous traits to more offspring. Over many generations, these traits increase in the population.

Over long periods of time, natural selection can cause major changes. Populations may become better adapted to their environments, and eventually new species can form.

A classic example is antibiotic resistance in bacteria. Some bacteria may randomly have mutations that allow them to survive an antibiotic. When the antibiotic kills the susceptible bacteria, the resistant ones survive and reproduce. Soon, the resistant bacteria become common.

Importantly, natural selection does not “try” to make organisms perfect. It simply favors traits that improve survival and reproduction in a particular environment at a particular time.

finish_reason: stop


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 four main ideas:

1. **Variation**
   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. **Inheritance**
   Some of these differences are genetic and can be passed from parents to offspring.

3. **Competition and survival**
   Organisms often produce more offspring than the environment can support. Because food, mates, space, and other resources are limited, not all individuals survive and reproduce equally.

4. **Differential reproduction**
   Individuals with traits that are better suited to their environment tend to survive and produce more offspring. Their helpful traits become more common in the next generation.

Over many generations, this can cause a population to change significantly. For example, if a population of insects lives on dark tree bark, darker insects may be harder for predators to see. They survive more often and leave more offspring, so dark coloration becomes more common in the population.

A few important points:

- Natural selection acts on individuals, but **populations evolve**.
- It does not have a goal or plan; it simply favors traits that work well in a particular environment.
- A trait that is useful in one environment may be harmful or neutral in another.
- Mutations and genetic recombination create new variation, while natural selection changes how common certain traits become.

In short, evolution by natural selection happens when inherited differences affect survival and reproduction, causing advantageous traits to become more common over generations.


text:
Evolution by natural selection is the process by which populations of living things change over generations because individuals with certain 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 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 other resources.

4. **Some traits improve survival or reproduction**
   If a trait helps an organism survive or have more offspring in a particular environment, that trait becomes more common in the next generation.

5. **Populations change over time**
   Over many generations, helpful traits can spread through a population. This is evolution.

For example, if a population of insects includes both light and dark individuals, and birds can more easily see the light ones on dark tree bark, the dark insects may survive and reproduce more often. Over time, the population may become mostly dark-colored.

Natural selection does not mean organisms “try” to evolve. Instead, individuals with advantageous inherited traits are more likely to pass those traits on. Also, what counts as advantageous depends on the environment, so a trait that is helpful in one situation may not be helpful in another.


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.

The basic steps are:

1. **Variation exists**
   Individuals in a population are not all identical. They differ in traits such as size, color, speed, resistance to disease, 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**
   Resources such as food, space, and mates are limited, so not every individual survives and reproduces.

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 leave more offspring, the genes for beneficial traits become more frequent in the population over many generations.

Over time, this can lead to **adaptations**—features that make organisms better suited to their environments. If populations become different enough, natural selection can eventually contribute 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 birds to see. Those darker insects survive and reproduce more often, so over generations the population may become mostly dark-colored.

Importantly, **individuals do not evolve during their lifetimes**. Populations evolve across generations.


text:
Evolution by natural selection is the process by which populations of living things 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, some lighter; some animals may run faster; some plants may tolerate drought better.

2. **Some variation is inherited**
   - Many traits are passed from parents to offspring through genes.
   - If a trait is genetic, offspring can 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 other resources.

4. **Some traits help 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 on their genes, beneficial traits increase in frequency over generations.
   - Over long periods, this can lead to major changes in a population and even the formation of new species.

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, so over generations the population may become mostly dark-colored.

Natural selection does **not** mean organisms “try” to evolve. Instead, random genetic variation already exists, and the environment “selects” which traits are more useful. Evolution happens to populations over generations, not to individuals during their lifetime.


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 basic steps:

1. **Variation exists**
   Individuals in a population are not 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 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. This creates 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 produce offspring.

5. **Helpful traits become more common**
   Because those individuals leave more offspring, the genes for helpful traits become more frequent in the next generation.

Over many generations, this can cause a population to become better adapted to its environment. If populations become different enough over long periods of time, new species can form.

For example, imagine a population of beetles where some are green and some are brown. If birds can see and eat the green beetles more easily, brown beetles may survive and reproduce more often. Over time, the population may contain mostly brown beetles.

Natural selection does not involve organisms “trying” to change, and it does not work toward a planned goal. It simply favors inherited traits that increase reproductive success in a particular environment.


text:
Evolution by natural selection is the process by which populations of organisms change over generations because individuals with certain heritable 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 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 resources like food, space, mates, and safety.

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. These traits are called **advantageous adaptations**.

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

Over long periods of time, natural selection can lead to major changes in populations. It can produce adaptations, create new species, and explain the diversity of life on Earth.

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

Natural selection does not work toward a goal or create “perfect” organisms. It simply favors traits that are useful in a particular environment at a particular time.


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

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, behavior, or metabolism.

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 survival.

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 **fitness**—not necessarily being strongest, but being better adapted to the current conditions.

5. **Helpful traits become more common**
   Because those individuals reproduce more, the genes associated with helpful traits become more common in the next generation.

Over many generations, this can cause a population to become better adapted to its environment. If populations become different enough over long periods, new species can form.

For example, if a population of insects includes both light and dark individuals, and birds more easily see and eat the light ones on dark tree bark, the darker insects may survive and reproduce more. Over generations, the population may become mostly dark-colored.

Natural selection does not “try” to create perfect organisms. It simply favors traits that help organisms survive and reproduce in a particular environment at a particular time.


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

It works through a few key steps:

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

2. **Some variation is inherited**
   Many traits are passed from parents to offspring through genes. If a trait has a genetic basis, it can become more or less common in future generations.

3. **More offspring are produced than can survive**
   Organisms often produce more offspring 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 are more likely to survive and have offspring. This is called being more “fit” in evolutionary terms.

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.

For example, imagine a population of beetles where some are green and some are brown. If birds can see green beetles more easily on the ground, brown beetles may be eaten less often. Brown beetles then survive and reproduce more, so over many generations the population may become mostly brown.

Natural selection does not mean organisms “try” to evolve, and it does not always produce perfect traits. It simply favors traits that work well enough in a specific environment. If the environment changes, different traits may become advantageous.

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


text:
Evolution by natural selection is the process by which populations of living things change over generations because some inherited traits help organisms 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, metabolism, and many other traits.

2. **Some variation is inherited**
   Many traits can be passed from parents to offspring through genes. New variation can also arise through mutation and genetic recombination.

3. **More offspring are produced than can survive**
   In nature, organisms often produce more offspring than the environment can support. Food, space, mates, and safety are limited.

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. These traits are called **adaptive** in that environment.

5. **Helpful traits become more common**
   Because successful individuals pass their genes to the next generation, traits that improve survival or reproduction tend to become more common in the population over time.

Over many generations, this can lead to major changes in a species, and eventually to the formation of new species.

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

A few important points:

- **Individuals do not evolve; populations evolve.**
- Natural selection does not “try” to make organisms perfect. It simply favors traits that work better in a particular environment.
- A trait that is useful in one environment may be harmful or useless in another.
- Evolution has no goal or direction; it is shaped by environmental pressures and inherited variation.

In short: **natural selection is the filtering of inherited variation by the environment, causing useful traits to become more common over generations.**


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.

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 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, leading to 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.

5. **Helpful traits become more common**
   Because those individuals leave more descendants, the genes associated with their advantageous traits become more frequent in the population over time.

Over many generations, this can cause populations to become better adapted to their environments. If populations become different enough, new species can eventually form.

A classic example is the peppered moth: in polluted areas, darker moths were harder for predators to see on soot-darkened trees, so they survived and reproduced more often. Over time, dark-colored moths became more common.

Natural selection does not work because 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 populations of living organisms change over generations because individuals with certain inherited traits leave more offspring 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 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**
   In a particular environment, certain inherited traits may give individuals an advantage. For example, a faster rabbit may escape predators more often, or a bacterium with antibiotic resistance may survive treatment.

5. **Those individuals reproduce more**
   Individuals with advantageous traits tend to leave more offspring. Their offspring are more likely to inherit those traits.

6. **Traits become more common over generations**
   Over many generations, beneficial traits can spread through the population. This gradual change in inherited traits is evolution.

A classic example is **peppered moths** in industrial England. Dark-colored moths became more common when pollution darkened tree bark, because they were better camouflaged from birds. When pollution decreased and trees became lighter again, lighter moths became more common.

Natural selection does not “try” to make organisms perfect. It has no goal or plan. It simply favors traits that help organisms survive and reproduce in a specific environment. If the environment changes, different traits may become advantageous.


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 four main ideas:

1. **Variation**
   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. **Inheritance**
   Some of these differences are genetic and can be passed from parents to offspring.

3. **Competition and survival**
   Organisms often produce more offspring than can survive. They may compete for food, mates, space, or avoid predators and disease.

4. **Differential reproduction**
   Individuals with traits that help them survive and reproduce in a particular environment tend to leave more offspring. Over many generations, those helpful traits become more common in the population.

For example, if dark-colored beetles are harder for birds to see, they may survive longer and have more offspring. If body color is inherited, dark beetles may become more common over time.

Natural selection does **not** mean organisms “try” to evolve. It also does not produce perfect organisms. It simply favors traits that improve reproductive success in a specific environment. If the environment changes, different traits may become advantageous.

Over long periods, natural selection can lead to major changes in populations and can contribute to the formation of new species.


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 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.
   - This creates competition for food, space, mates, and safety.

4. **Some traits improve survival or reproduction**
   - If a trait helps an organism survive or have more offspring in a particular environment, 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 the population.
   - Traits that reduce reproductive success may become less common.

For example, imagine a population of insects where some are green and some are brown. If birds can 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 “try” to make organisms perfect. It simply favors traits that work well enough in a specific environment at a specific time. If the environment changes, different traits may become advantageous.

Over long periods, natural selection can lead to major changes in populations and can contribute to 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 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, or 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 **differential reproductive success**.

5. **Helpful traits become more common**
   Because successful individuals pass their traits to their offspring, those traits tend to increase in frequency in the population over many generations.

Over time, this can cause populations to become better adapted to their environments. If populations become different enough, new species can eventually arise.

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. After many generations, the population may contain mostly brown beetles.

Natural selection does not involve organisms “trying” to evolve. Instead, it acts on existing variation, and the environment influences which traits are more successful.


text:
Evolution by natural selection is the process by which populations of living things change over generations because individuals with certain heritable 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, behavior, disease resistance, speed, or 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, leading to competition for food, space, mates, and other resources.

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

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

For example, imagine a population of insects where some are green and some are brown. If they live on brown tree bark, birds may more easily spot and eat the green insects. Brown insects are more likely to survive and reproduce, so over generations the population may become mostly brown.

Natural selection does **not** mean organisms “try” to evolve, and it does not act with a goal in mind. It simply results from differences in survival and reproduction among individuals with different inherited traits.

Over long periods, natural selection can lead to adaptations, new species, and the diversity of life seen on Earth today.

request_id: chatcmpl-EIsRxNFuiSNQRZXbvaDlCWWLLSGYO


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 all 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, or 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. 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 traits to their offspring, those traits tend to increase in frequency in the population over many generations.

Over time, this can cause a population to become better adapted to its environment. If populations become different enough, new species can eventually arise.

A simple example: If a population of beetles includes both green and brown beetles, and birds more easily spot and eat the green ones, brown beetles may survive and reproduce more often. After many generations, the population may contain mostly brown beetles.

Natural selection does not have a goal or plan. 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 living populations change over generations because some inherited traits help organisms survive and reproduce better than others.

Here’s how it works:

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 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, 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, traits that improve survival or reproduction tend to increase in frequency over time.

Over many generations, these changes can lead to populations becoming better adapted to their environments. If populations become different enough, they may eventually form new species.

For example, if a population of insects includes both light-colored and dark-colored individuals, and birds more easily see the light ones on dark tree bark, the dark insects may survive and reproduce more. Over time, the population may become mostly dark-colored.

Natural selection does not work toward a goal or “try” to make organisms perfect. It simply favors traits that are useful in a particular environment at a particular time.


prompt_id: sci-iq-heritability

text:
Human intelligence, usually measured as general cognitive ability (“g” or IQ-like test performance), is **substantially heritable**, but heritability varies by age, population, environment, and measurement method.

## Short answer

In modern, relatively advantaged populations:

- **Childhood IQ heritability:** often around **20–40%**
- **Adolescent IQ heritability:** often around **40–60%**
- **Adult IQ heritability:** commonly estimated around **50–80%**

A reasonable summary is: **about half or more of the variation in adult intelligence within a given population is associated with genetic differences**, while the rest reflects environmental influences, developmental factors, measurement error, and gene–environment interplay.

Importantly, “heritable” does **not** mean “fixed,” “unchangeable,” or “unaffected by environment.”

---

## What heritability means

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

> In that population, under those environmental conditions, about 60% of the observed differences among individuals are statistically associated with genetic differences.

It does **not** mean:

- 60% of one person’s intelligence is genetic.
- Environment does not matter.
- Intelligence cannot be improved.
- Differences between social groups are genetic.
- The same estimate applies in every society or historical period.

Heritability can be high even for traits strongly affected by the environment. For example, height is highly heritable in well-nourished societies, but nutrition can still greatly affect height.

---

## Main lines of evidence

### 1. Twin studies

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

Findings are consistent:

- Identical twins are more similar in IQ than fraternal twins.
- Identical twins reared apart are still substantially similar.
- Similarity generally increases with age.

This has been one of the strongest sources of evidence for genetic influence on intelligence.

Typical pattern:

| Age | Approximate heritability |
|---|---:|
| Early childhood | 20–40% |
| Middle childhood/adolescence | 40–60% |
| Adulthood | 50–80% |

The increase with age may seem counterintuitive, but one explanation is that as people grow older, they increasingly select and shape environments that match their genetic tendencies — for example, choosing certain books, friends, hobbies, schools, or careers.

---

### 2. Adoption studies

Adoption studies compare children to their biological and adoptive relatives.

Common findings:

- In childhood, adopted children often resemble both adoptive and biological parents to some degree.
- As adoptees grow older, their IQ tends to become more similar to that of biological relatives and less similar to adoptive relatives.
- Shared family environment has a larger effect in childhood and a smaller effect in adulthood.

This supports both genetic influence and environmental influence, especially early in life.

---

### 3. Molecular genetics and GWAS

Genome-wide association studies examine DNA variants across very large samples.

Findings:

- Intelligence is **highly polygenic**: influenced by thousands of genetic variants, each with tiny effects.
- Common genetic variants captured by SNP arrays explain a meaningful but incomplete portion of IQ variation.
- SNP-based heritability estimates are often lower than twin-study estimates, roughly in the **20–30%** range for cognitive ability, depending on the study and phenotype.
- Polygenic scores can predict some cognitive and educational outcomes, but they are far from deterministic.

For example, current polygenic scores may explain a modest fraction of variation in cognitive performance, and somewhat more for educational attainment, but they cannot accurately predict an individual’s intelligence.

This molecular evidence confirms that genetic differences contribute to intelligence, while also showing that the genetic architecture is very complex.

---

## Environmental influences also matter

Even if intelligence is substantially heritable, environment remains important.

Influential environmental factors can include:

- Prenatal health
- Nutrition
- Exposure to toxins such as lead
- Childhood disease burden
- Quality of schooling
- Family stress and stability
- Sleep
- Cognitive stimulation
- Socioeconomic conditions
- Extreme deprivation or neglect

The **Flynn effect** is especially important: IQ scores rose substantially across many countries during the 20th century, far too quickly to be explained by genetic change. This shows that broad environmental changes can strongly affect test performance.

---

## Shared vs non-shared environment

Behavioral genetics often separates environment into:

### Shared environment

Factors that make siblings similar, such as household income, neighborhood, parenting style, or school access.

### Non-shared environment

Factors that make siblings different, such as different peer groups, illnesses, teacher relationships, accidents, or unique experiences.

For intelligence:

- Shared environment tends to matter more in childhood.
- Its estimated effect often declines by adulthood in affluent populations.
- Non-shared environment and measurement error continue to account for some variation.

However, in disadvantaged or highly unequal environments, shared environmental effects may be larger.

---

## Heritability depends on social conditions

Heritability is not universal. In societies where most people have adequate nutrition, schooling, and healthcare, environmental variation may be reduced, so genetic differences explain more of the remaining variation.

In contrast, where environments vary greatly — severe poverty, malnutrition, poor schooling, disease exposure — environmental factors may explain more of the differences in cognitive outcomes.

So a high heritability estimate in one population does not mean environment is unimportant everywhere.

---

## What the evidence does not show

The evidence does **not** show that:

- IQ is biologically fixed.
- Education or interventions cannot help.
- Genetic influence means inevitability.
- Group differences are genetic in origin.
- A person’s genome can precisely determine their intelligence.

Most evidence concerns **individual differences within populations**, not causes of differences between populations or social groups.

---

## Bottom line

Human intelligence is **moderately to highly heritable**, especially in adulthood. The best evidence from twin, adoption, and DNA studies shows that genetic differences make a substantial contribution to why people differ in cognitive ability.

But intelligence is also shaped by environment, development, education, health, culture, and chance. Heritability describes variation within a given population; it does not imply immutability or genetic determinism.


text:
Human intelligence—as typically measured by IQ tests and related cognitive batteries—is **substantially heritable**, but not genetically fixed. The best evidence suggests that genetic differences account for a sizeable share of differences in cognitive ability **within a given population and environment**, and that this share changes with age and context.

## Short answer

- **Childhood:** heritability is often around **30–50%**.
- **Adolescence/adulthood:** estimates commonly rise to about **50–80%**, with many adult studies clustering around **60–70%**.
- **Shared family environment** matters more in childhood and tends to decline in adulthood.
- **Nonshared environment**, measurement error, illness, schooling differences, peer groups, and life experiences also matter.
- Intelligence is **highly polygenic**: many thousands of genetic variants each have very small effects.
- Heritability does **not** mean immutability, nor does it explain differences between social, ethnic, or national groups.

## What “heritability” means

Heritability is a population statistic: it estimates what proportion of variation in a trait, within a particular population at a particular time, is associated with genetic variation.

So if adult IQ has a heritability of, say, 60%, that does **not** mean 60% of a person’s intelligence is genetic and 40% environmental. It means that, in the studied population, about 60% of the observed differences between individuals are statistically associated with genetic differences.

Heritability can change if the environment changes. For example, if schooling, nutrition, health care, or exposure to toxins vary widely, environmental effects may account for more of the differences.

## Main evidence

### 1. Twin studies

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

A consistent finding is:

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

This pattern strongly supports a genetic contribution to individual differences in intelligence.

However, twin studies rely on assumptions, such as how similarly identical and fraternal twins are treated. Researchers have debated these assumptions, but the overall finding of substantial heritability has held up across many designs.

### 2. Adoption studies

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

Typical findings:

- In childhood, adopted children’s IQs are influenced by the adoptive family environment.
- As children grow older, their IQs tend to become more similar to their biological relatives than to their adoptive relatives.
- Shared family effects are stronger early in life and weaker by adulthood.

This supports both genetic influence and meaningful environmental influence, especially in development.

### 3. Genome-wide association studies

Modern DNA studies examine measured genetic variants across the genome.

They show that intelligence and educational attainment are associated with many genetic variants, each with tiny effects. Polygenic scores constructed from these variants can predict some differences in cognitive performance, but far from all of them.

Current polygenic scores explain a modest fraction of variance—often roughly **5–15%**, depending on the sample, measure, ancestry, and design. This is much lower than twin-study heritability estimates, partly because many causal variants are not fully captured, effects are tiny, and gene-environment correlations complicate interpretation.

These studies confirm that cognitive ability is polygenic and genetically influenced, but they do not make precise individual predictions.

## Why heritability tends to rise with age

One robust finding is that IQ heritability often increases from childhood to adulthood. Several mechanisms may contribute:

1. **Gene-environment correlation:** people increasingly select, shape, and evoke environments that fit their dispositions. A child with stronger verbal ability may read more, receive more encouragement, and seek intellectually stimulating settings.

2. **Reduced shared family influence:** as people leave the family environment, differences between households matter less.

3. **Amplification over development:** small early differences can accumulate through schooling, interests, peer groups, and practice.

This does not mean genes become “more powerful” in a simple biological sense; rather, genetic propensities can become more expressed through life choices and environments.

## Environmental effects are real

Even with high heritability, environment matters. Evidence shows cognitive development can be affected by:

- quality and duration of schooling
- childhood nutrition
- lead and other neurotoxin exposure
- prenatal health
- severe deprivation or neglect
- chronic stress
- disease burden
- sleep, stimulation, and learning opportunities
- socioeconomic conditions

The **Flynn effect**—large IQ score gains across many countries during the 20th century—also shows that population-level cognitive test performance can change substantially over time, too quickly to be explained by genetic evolution. Improved education, health, nutrition, test familiarity, and cognitively demanding environments are likely contributors.

## Important caveats

### Heritability is not destiny

A trait can be highly heritable and still be changeable. Height is highly heritable, but average height has increased in many countries due to improved nutrition and health. Similarly, cognitive outcomes can be affected by environmental conditions.

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

Heritability estimates usually describe variation among individuals within a studied population. They do not by themselves explain differences between groups, countries, social classes, or historical periods. Group differences can be heavily environmental even when within-group heritability is high.

### IQ is not all of intelligence

IQ tests measure important cognitive abilities—reasoning, working memory, processing speed, verbal ability, spatial ability—and predict many life outcomes statistically. But they do not capture every valuable human capability, such as creativity, wisdom, motivation, emotional skills, practical knowledge, or specific talents.

## Overall conclusion

The evidence from twin, adoption, family, and DNA-based studies shows that human intelligence is **meaningfully and substantially heritable**, especially in adulthood. A reasonable summary is that genetic differences explain roughly **half or more** of individual differences in measured intelligence in many modern adult populations, while environmental influences remain important and can have large effects, especially during childhood and under adverse or unequal conditions.

So the best answer is: **intelligence is substantially heritable, highly polygenic, developmentally dynamic, and environmentally influenced.**


text:
Human intelligence, as measured by IQ-type tests and related cognitive batteries, is **substantially heritable**, but not genetically determined. The best-supported conclusion is:

> **In modern, relatively stable, high-income environments, genetic differences account for roughly half or more of the variation in measured intelligence among individuals, with heritability increasing from childhood into adulthood.**

## What “heritability” means

Heritability is the proportion of **variation between 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 is fixed or unchangeable
- environments do not matter
- results automatically generalize to other populations or historical periods
- genetic explanations of within-group differences explain between-group differences

A trait can be highly heritable and still strongly affected by environment. Height is the classic example: it is highly heritable in well-nourished populations, yet average height changes with nutrition, disease burden, and living conditions.

## Approximate heritability estimates

Across many studies, measured cognitive ability shows a pattern like this:

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

These are broad estimates. They vary by country, socioeconomic conditions, test type, age, and research method.

## Main lines of evidence

### 1. Twin studies

Twin studies compare similarity between:

- **Identical twins**, who share ~100% of their DNA
- **Fraternal twins**, who share ~50% on average

A consistent finding is that identical twins are more similar in cognitive test scores than fraternal twins.

Typical adult correlations are roughly:

- Identical twins raised together: ~0.75–0.85
- Fraternal twins raised together: ~0.45–0.60
- Identical twins raised apart: often ~0.65–0.75, though estimates vary

This pattern suggests a large genetic contribution.

However, twin studies rely on assumptions, such as comparable environmental similarity for identical and fraternal twins. Researchers have debated these assumptions, but the broad conclusion of meaningful genetic influence has held up across many designs.

### 2. Adoption studies

Adoption studies ask whether children resemble their biological or adoptive relatives more.

Findings generally show:

- In childhood, adopted children’s IQ scores are influenced by the adoptive home environment.
- As children age, their cognitive scores tend to correlate more strongly with biological relatives and less strongly with adoptive relatives.
- Adoption into more advantaged homes can raise average cognitive outcomes, especially when children come from deprived backgrounds.

This supports both genetic influence and environmental influence.

### 3. Family studies

Siblings, parents, and children resemble each other in cognitive ability. The degree of resemblance generally tracks genetic relatedness, but shared environment also matters, especially earlier in life.

For example:

- Parent–child and sibling correlations are moderate.
- Half-siblings and cousins tend to be less similar.
- Spouses are often cognitively similar partly because of assortative mating—people with similar education or cognitive traits are more likely to pair together.

### 4. Molecular genetic studies

Modern genome-wide association studies, or GWAS, look directly at DNA variants across large samples.

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

Current molecular findings suggest:

- Common genetic variants captured by genotyping arrays explain perhaps **20–30%** of variation in cognitive performance, depending on the sample and method.
- Polygenic scores can predict some variation in educational attainment and cognitive performance, but prediction is still limited.
- Twin-study heritability estimates are generally higher than SNP-based estimates because twin studies can capture broader genetic effects, including rare variants, non-additive effects, and effects not well tagged by common SNPs.

The molecular evidence strongly confirms that genetic differences contribute to cognitive differences, but also shows that there is no single “intelligence gene.”

## Environmental influences

Environment clearly matters. Important influences include:

- Prenatal health
- Nutrition
- Exposure to toxins such as lead
- Infectious disease burden
- Childhood stress and trauma
- Quality and quantity of schooling
- Family resources and language exposure
- Sleep, health, and mental health
- Social stability and neighborhood conditions

Severe deprivation can substantially depress cognitive development. Conversely, improved nutrition, education, public health, and reduced toxins can raise population-level performance.

The **Flynn effect**—large increases in average IQ scores across much of the 20th century—shows that environmental changes can shift average test performance substantially over generations, far too quickly to be due to genetic evolution.

## Shared vs non-shared environment

Behavioral genetics often distinguishes:

- **Shared environment**: influences making siblings more similar, such as family socioeconomic status, neighborhood, and parental education.
- **Non-shared environment**: influences making siblings different, such as different peer groups, illnesses, teachers, experiences, or random developmental factors.

For IQ:

- Shared environment tends to matter more in childhood.
- Its estimated effect often declines by adulthood in affluent societies.
- Non-shared environment and measurement error account for a meaningful portion of remaining variation.

This does not mean family environment is irrelevant. It means that, within the range of environments studied, siblings raised together often become less similar than one might expect from family environment alone.

## Socioeconomic status may affect heritability

Some studies, especially in the United States, have found that heritability of IQ is lower in poorer families and higher in wealthier families. One interpretation is that poverty constrains the development of genetic potential.

However, this finding is not equally strong everywhere. In countries with stronger social supports, healthcare, and educational access, socioeconomic moderation appears weaker or less consistent.

So the heritability of intelligence is not a fixed biological constant; it depends on environmental context.

## What the evidence does and does not show

The evidence shows:

- Genetic differences contribute substantially to individual differences in measured intelligence.
- Environmental differences also matter.
- Heritability rises with age in many studies.
- Intelligence is influenced by many genetic variants of tiny effect.
- Environmental changes can raise or lower average cognitive performance.
- Heritability estimates are population-specific.

The evidence does **not** show:

- Intelligence is immutable.
- Education and environment are unimportant.
- A person’s potential can be read from their genes.
- Genetic findings about individuals within a population explain average differences between social, ethnic, national, or racial groups.
- IQ tests capture every form of valuable human cognitive ability.

## Bottom line

A fair summary is:

> Human intelligence is moderately to highly heritable, especially in adulthood, with many studies estimating adult heritability around **50–80%** in affluent societies. But environmental factors remain important, heritability varies by context, and high heritability does not imply immutability or genetic determinism. The strongest evidence comes from converging twin, adoption, family, and molecular genetic studies.


text:
Human intelligence—as usually measured by IQ tests and the general cognitive factor “g”—is **substantially heritable**, but not genetically fixed. The best evidence suggests that, in contemporary high-income populations, **genetic differences account for roughly 50% of variation in intelligence overall**, with estimates often **rising from childhood into adulthood**.

## Short answer

- **Childhood:** heritability often around **20–40%**
- **Adolescence:** around **40–60%**
- **Adulthood:** often around **60–80%**
- **Across the lifespan:** a reasonable broad estimate is **about 50% or more**, depending on age, population, and environment.

But heritability does **not** mean intelligence is unchangeable, nor does it mean “IQ is 50% genes and 50% environment” for an individual. Heritability describes **variation within a population at a time**, not the cause of any one person’s intelligence.

---

## What the evidence shows

### 1. Twin studies show strong genetic influence

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

Typical findings:

| Relationship | Approximate IQ similarity |
|---|---:|
| Identical twins raised together | High, often ~0.75–0.85 |
| Fraternal twins raised together | Moderate, often ~0.40–0.60 |
| Siblings | Moderate |
| Unrelated children raised together | Low to moderate, often declining with age |

The fact that identical twins resemble each other more than fraternal twins is one of the strongest pieces of evidence for heritability.

Twin studies also suggest a pattern known as the **Wilson effect**: heritability of intelligence tends to increase with age. This may happen because people increasingly select and shape environments compatible with their genetic tendencies—choosing books, friends, hobbies, schools, jobs, etc.

---

### 2. Adoption studies support genetic effects

Adoption studies separate biological relatedness from shared upbringing.

Common findings:

- Adopted children resemble their **adoptive parents** somewhat in childhood.
- As they grow older, their IQ tends to become **more similar to their biological relatives**.
- The influence of the shared family environment appears stronger in childhood and weaker in adulthood.

This does not mean family environment is irrelevant. It means that, in many studied populations, the long-term differences between adults in IQ are less strongly explained by the family environment shared by siblings than by genetic and individual-specific environmental factors.

---

### 3. DNA-based studies confirm intelligence is highly polygenic

Modern genome-wide association studies, or GWAS, examine measured DNA directly. They show that cognitive ability is influenced by **thousands of genetic variants**, each with tiny effects.

Important findings:

- There is no “intelligence gene.”
- Intelligence is **highly polygenic**.
- Common genetic variants measured by SNP arrays explain a meaningful but incomplete portion of IQ differences—often around **20–30%** in some genomic estimates, depending on the sample and method.
- Polygenic scores can predict some variation in educational attainment and cognitive performance, but they are far from deterministic and currently explain only a minority of the variance.

The fact that DNA-based results point in the same direction as twin and adoption studies strengthens the conclusion that genetic differences matter.

---

## Environment also matters a lot

A heritability estimate of, say, 60% still leaves substantial room for environmental influence. Relevant environmental factors include:

- Prenatal health
- Nutrition
- Exposure to toxins such as lead
- Childhood disease burden
- Quality and quantity of education
- Family stress and poverty
- Sleep, safety, and mental health
- Cultural and test-taking familiarity

The **Flynn effect**—large IQ gains across generations in many countries during the 20th century—shows that environmental change can substantially affect population-level cognitive test performance. Genes did not change fast enough to explain those gains.

---

## Heritability depends on the population and environment

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

For example:

- In environments where nutrition, schooling, and healthcare are broadly available, genetic differences may explain more of the remaining variation.
- In environments with severe deprivation, environmental constraints may suppress genetic potential, lowering heritability.
- Some studies find that heritability is lower in poorer or more unequal contexts, though results vary by country and sample.

So saying “IQ is 60% heritable” is shorthand for something like: “In this studied population, under these environmental conditions, around 60% of the observed differences are statistically associated with genetic differences.”

---

## What heritability does *not* mean

Heritability does **not** mean:

- Intelligence is fixed.
- Education does not matter.
- A person’s IQ is determined at birth.
- Group differences are necessarily genetic.
- Environmental interventions are useless.
- We can accurately predict an individual’s intelligence from DNA.

A highly heritable trait can still be environmentally changeable. Height is highly heritable, but average height increased greatly in many countries because of improved nutrition and health. Intelligence is similar in that genetic variation matters, but environment can still have large effects.

---

## Bottom line

The evidence from twin, adoption, family, and DNA-based studies converges on the conclusion that **human intelligence is substantially heritable**, especially by adulthood. A reasonable summary is:

> In modern, relatively well-resourced populations, genetic differences explain roughly half or more of individual differences in intelligence, with heritability increasing from childhood into adulthood. Environmental factors remain important, especially early in life and in conditions of deprivation, and heritability does not imply immutability or genetic determinism.


text:
Human intelligence, as measured by cognitive tests/IQ and related “general cognitive ability” (*g*), is **substantially heritable**, but heritability is **not 100%**, is **population- and context-specific**, and does **not mean intelligence is fixed or unaffected by environment**.

## What “heritability” means

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

So if IQ heritability is estimated at 50%, that does **not** mean “50% of a person’s intelligence is genetic.” It means roughly 50% of the observed differences in IQ within that studied population are associated with genetic variation.

It also does **not** imply that group differences between populations are genetic. Heritability within a population says little by itself about the causes of differences between populations.

## Typical estimates

Across many studies, intelligence heritability is often estimated roughly as:

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

A common summary is that IQ is **moderately heritable in childhood and highly heritable in adulthood**.

The remaining variation is due to environmental influences, measurement error, developmental randomness, and gene–environment interplay.

## 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 DNA.

A consistent finding is:

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

This pattern supports a genetic contribution.

However, twin studies rely on assumptions, such as whether identical and fraternal twins experience equally similar environments. Researchers debate details, but the broad finding of genetic influence is robust.

### 2. Adoption studies

Adoption studies show that children’s IQ scores tend to resemble:

- Their **adoptive families** more in early childhood.
- Their **biological relatives** more as they grow older.

This suggests that shared family environment matters, especially early in life, but genetic influence becomes more visible with age.

### 3. Genome-wide association studies

Modern genetic studies have identified many DNA variants associated with educational attainment and cognitive performance. Each variant has a tiny effect, but together they can form **polygenic scores**.

Findings:

- Intelligence is **highly polygenic**: influenced by thousands of genetic variants, each with very small effects.
- Polygenic scores can predict some variation in cognitive performance and educational outcomes.
- Current polygenic scores explain only a fraction of IQ variation, much less than twin-study heritability estimates.

This gap is sometimes called “missing heritability,” though it has narrowed as sample sizes have grown.

## Environment clearly matters

Even if intelligence is heritable, environment has large effects. Evidence includes:

- **Flynn effect:** Average IQ scores rose substantially in many countries during the 20th century, too quickly to be genetic. This points to nutrition, schooling, health, cognitive demands, and social changes.
- **Education effects:** More schooling is associated with higher performance on cognitive tests.
- **Early deprivation:** Severe neglect, malnutrition, lead exposure, poor prenatal conditions, and chronic stress can reduce cognitive development.
- **Interventions:** Some early-childhood programs improve cognitive and educational outcomes, though IQ effects vary in persistence.

## Heritability can depend on social conditions

In environments where basic needs are met and opportunities are relatively equal, genetic differences may account for a larger share of variation. In harsher or more unequal environments, environmental constraints may suppress potential and reduce heritability estimates.

There is evidence for this kind of socioeconomic moderation, especially in some U.S. samples, though findings vary across countries and studies.

## Why heritability increases with age

One reason IQ heritability appears to rise from childhood to adulthood is **gene–environment correlation**. As people grow older, they increasingly select, shape, and evoke environments that fit their dispositions and abilities.

For example, a child with a genetic tendency toward verbal ability may read more, seek advanced classes, receive more encouragement, and develop further skills. Genes influence not only biology directly, but also the environments people experience.

## Bottom line

The evidence strongly indicates that human intelligence is **partly heritable**, with adult heritability often estimated around **50–80%** in studied populations. But intelligence is also strongly shaped by environment, education, health, culture, and opportunity.

The best summary is:

> Genetic differences contribute substantially to individual differences in intelligence, especially in adulthood, but intelligence is not genetically predetermined, and environmental conditions can meaningfully affect cognitive development.


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

> **In many contemporary, high-income populations, genetic differences account for roughly 50–80% of individual differences in adult IQ**, with lower estimates in childhood and in more deprived environments.

“Heritable” here means **the proportion of variation among people in a particular population and environment that is associated with genetic differences**. It does **not** mean that an individual’s intelligence is 50–80% “caused by genes,” nor that intelligence cannot be changed by environment.

## Approximate heritability by age

Findings vary by study and population, but a common pattern is:

| Age/stage | Approximate heritability of IQ/general cognitive ability |
|---|---:|
| Early childhood | ~20–40% |
| Middle childhood | ~40–50% |
| Adolescence | ~50–70% |
| Adulthood | ~60–80% |

Heritability tends to **increase with age**, partly because people increasingly select, create, and respond to environments that fit their genetic tendencies—known as **gene–environment correlation**.

## What evidence supports this?

### 1. Twin studies

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

Typical finding:

- Identical twins are much more similar in IQ than fraternal twins.
- Identical twins reared apart are still quite similar, though not identical.
- Fraternal twins and ordinary siblings are less similar.

This pattern strongly suggests a genetic contribution.

### 2. Adoption studies

Adoption studies compare children to their biological and adoptive relatives.

Common findings:

- In childhood, adopted children often resemble both adoptive and biological families to some extent.
- By adulthood, resemblance to biological relatives tends to increase.
- Resemblance to adoptive siblings often declines with age.

This suggests that shared family environment matters, especially early, but genetic influences become more apparent over development.

### 3. Genome-wide association studies

Modern genetic studies scan millions of DNA variants across very large samples.

They show that intelligence is **highly polygenic**:

- No single “intelligence gene” has a large effect.
- Thousands of variants each have tiny effects.
- Polygenic scores can predict some variation in cognitive performance, but far from all of it.

Current DNA-based methods typically explain **less variance than twin-study estimates**, often on the order of perhaps **10% or less for direct prediction**, depending on the trait, sample, and method. SNP-based heritability estimates are higher than prediction accuracy but still often below twin-study estimates. This “missing heritability” is expected for complex traits influenced by many variants, rare variants, gene–environment interplay, measurement limits, and imperfect genomic tagging.

## Environment still matters a lot

High heritability does not mean environment is unimportant. Environmental influences known to affect cognitive development include:

- schooling quality and duration
- nutrition
- prenatal health
- exposure to lead or other toxins
- infectious disease burden
- poverty and stress
- sleep
- parenting and language exposure
- social stability and safety

The **Flynn effect**—large IQ-score gains across many countries during the 20th century—is a major demonstration that average cognitive test performance can shift substantially for environmental reasons, even when within-population heritability is high.

## Important caveats

1. **Heritability is population-specific.**
   An estimate from one country, time period, or social class may not apply elsewhere.

2. **Heritability is not immutability.**
   Highly heritable traits can still be changed by environmental intervention. For example, height is highly heritable, but nutrition strongly affects average height.

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

4. **“Intelligence” is not a single simple trait.**
   IQ and g capture important cognitive abilities and predict many life outcomes, but they do not exhaust all forms of reasoning, creativity, wisdom, motivation, or expertise.

## Bottom line

The evidence from twins, adoption studies, and molecular genetics converges on the conclusion that **human intelligence is moderately to highly heritable**, especially in adulthood. A reasonable adult estimate in many modern affluent societies is around **60–80%**, while childhood estimates are lower. But intelligence is also meaningfully shaped by environment, development, education, health, and social conditions, and high heritability does not imply genetic determinism.


text:
Human intelligence—usually measured as general cognitive ability or IQ—is **substantially heritable**, but not fixed or genetically determined.

## Short answer

In contemporary high-income populations, the heritability of IQ is commonly estimated at roughly:

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

That means that, within a given population and environment, a large share of differences between people in measured cognitive ability is associated with genetic differences. It does **not** mean that 60% of a person’s intelligence is “caused by genes,” nor that intelligence cannot be changed by environment.

## Main lines of evidence

### 1. Twin studies

Identical twins share nearly all their DNA; fraternal twins share about half, on average. Across many studies:

- Identical twins’ IQ scores are much more similar than fraternal twins’.
- Identical twins reared apart are still quite similar, though not identical.
- Heritability estimates tend to rise with age.

Typical pattern:

- Monozygotic identical-twin IQ correlations: often around **0.7–0.85** in adulthood.
- Dizygotic fraternal-twin correlations: often around **0.4–0.6**.

This gap is a major reason researchers infer substantial genetic influence.

### 2. Adoption studies

Adoption studies compare children with their biological and adoptive relatives.

Findings generally show:

- In early childhood, adopted children resemble their adoptive families somewhat.
- As they grow older, their cognitive scores tend to resemble their biological relatives more strongly.
- Adoptive siblings become less similar in IQ by adulthood than biological siblings.

This suggests that shared family environment matters more in childhood, while genetic differences become more predictive later.

### 3. Molecular genetic evidence

Modern genome-wide association studies find that intelligence is highly polygenic: influenced by thousands of genetic variants, each with tiny effects.

Evidence includes:

- Common genetic variants collectively explain a meaningful fraction of IQ variation, often estimated around **20–30%** for measured cognitive ability, depending on the sample and method.
- Polygenic scores can predict some differences in educational attainment and cognitive performance, though imperfectly.
- Within-family genetic studies still find predictive effects, though often smaller than population-level estimates, showing that some earlier estimates include environmental and demographic confounding.

This confirms that genetic influence is real, but highly distributed and probabilistic.

## Important caveats

### Heritability is population-specific

A heritability estimate applies to a particular population at a particular time. If environments become more equal, genetic differences may explain more of the remaining variation. If environments are highly unequal or deprived, environmental effects may be larger.

So “IQ is 60% heritable” does not mean this is a universal biological constant.

### Heritable does not mean unchangeable

Height is highly heritable, but nutrition and disease strongly affect average height. Similarly, intelligence is affected by:

- Education
- Nutrition
- Childhood illness
- Lead exposure and toxins
- Stress and deprivation
- Sleep
- Prenatal environment
- Quality of schooling
- Cognitive stimulation

The Flynn effect—the large rise in IQ scores across generations during the 20th century—shows that population averages can change substantially for environmental reasons.

### Heritability does not explain group differences

Heritability within a population does not imply that differences between populations or social groups are genetic. A trait can be highly heritable within groups while group differences are caused by environment.

### Genes and environments are intertwined

Genetic effects often operate through environmental pathways. For example, a child with certain predispositions may seek more intellectually stimulating activities, receive different responses from adults, or choose different peer groups. This is called gene–environment correlation.

## Bottom line

The evidence strongly shows that **genetic differences contribute substantially to individual differences in human intelligence**, especially by adulthood. A reasonable summary is that adult IQ in modern developed populations is about **half to two-thirds heritable**, with estimates often ranging from **50% to 80%**.

But intelligence is also meaningfully shaped by environment, and heritability does not mean inevitability, immutability, or genetic explanation of social group differences.


text:
Human intelligence, as measured by IQ and related cognitive tests, is **substantially heritable**, but it is **not genetically fixed** and heritability does **not** mean “unchangeable.”

## Short answer

In modern, industrialized populations:

- **Childhood IQ heritability:** roughly **30–50%**
- **Adolescent IQ heritability:** roughly **40–60%**
- **Adult IQ heritability:** often **50–80%**, with many estimates around **60–70%**

The rest of the variation is due to **environmental influences**, measurement error, developmental randomness, and gene–environment interplay.

## What “heritability” means

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

It does **not** mean:

- that a person’s intelligence is 60% genetic and 40% environmental;
- that IQ cannot be changed;
- that group differences are genetic;
- that environment does not matter.

For example, height is highly heritable, but average height has changed substantially across generations because of nutrition, health, and living conditions. Intelligence shows a similar point: IQ scores rose dramatically during the 20th century in many countries, the so-called **Flynn effect**, despite high heritability.

## Main evidence

### 1. Twin studies

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

Typical findings:

- Identical twins raised together: correlations often around **0.75–0.85**
- Fraternal twins raised together: correlations often around **0.40–0.60**
- Identical twins raised apart: still substantially similar, often around **0.60–0.75**

This pattern supports a strong genetic contribution.

### 2. Adoption studies

Adoption studies help separate genetic and family-environment effects.

A common finding is:

- In childhood, adopted children resemble both their adoptive families and biological relatives.
- As they age, resemblance to biological relatives tends to increase.
- Resemblance to adoptive siblings and adoptive parents often decreases by adulthood.

This suggests that shared family environment matters more in childhood, while genetic influences become more apparent with age.

### 3. Molecular genetics

Modern genome-wide studies also support heritability.

Researchers can now estimate how much variation in intelligence is associated with measured DNA variants across the genome. These estimates are usually lower than twin-study estimates, often around **20–30%** for common SNP-based heritability, because current methods do not capture all genetic variation.

Genome-wide association studies show that intelligence is **highly polygenic**: thousands of genetic variants each have tiny effects. There is no single “intelligence gene.”

Polygenic scores can predict some variation in cognitive performance, especially in European-ancestry samples, but their predictive power is still limited and transfers poorly across ancestries because of differences in datasets, environments, and population history.

## Heritability increases with age

One robust finding is that the heritability of intelligence tends to rise from childhood into adulthood. This is sometimes called the **Wilson effect**.

Possible reasons include:

- people increasingly select environments that fit their genetic tendencies;
- parents initially shape children’s environments, but individuals gain more autonomy with age;
- genetic influences on motivation, curiosity, school engagement, and peer selection compound over time.

So genes may influence intelligence partly by influencing the kinds of environments people seek and create.

## Environment still matters a lot

Environmental influences on intelligence include:

- prenatal health;
- nutrition;
- childhood disease burden;
- lead and other toxins;
- schooling quality and duration;
- family stress;
- cognitive stimulation;
- sleep;
- socioeconomic conditions;
- trauma and neglect.

Severe deprivation can have large negative effects. Conversely, improvements in education, health, and nutrition can raise cognitive performance at the population level.

Also, heritability can be lower in environments where poverty, malnutrition, poor schooling, or disease strongly constrain development. In more equal and affluent environments, genetic differences may account for a larger share of remaining variation.

## Important caveats

Heritability estimates are:

1. **Population-specific**
   A heritability estimate from one country, era, or socioeconomic context may not apply elsewhere.

2. **Not about individuals**
   You cannot say “this person’s IQ is 70% genetic.”

3. **Not evidence about group differences**
   High heritability within a population does not imply that average differences between populations are genetic.

4. **Compatible with environmental change**
   Traits can be highly heritable and still strongly affected by environment.

5. **Affected by assumptions**
   Twin and adoption studies rely on assumptions about environments, assortative mating, and representativeness. Molecular genetic studies avoid some of these issues but have their own limitations.

## Bottom line

The evidence from twin, adoption, family, and DNA-based studies converges on the conclusion that **genetic differences make a substantial contribution to individual differences in human intelligence**, especially in adulthood. A reasonable summary is that adult intelligence is often estimated to be around **50–70% heritable** in contemporary developed populations.

But intelligence is also meaningfully shaped by environment, development, education, health, and social conditions. Heritability describes variation within a population; it does not imply genetic determinism or that intelligence cannot be improved.


text:
Human intelligence, as measured by IQ tests and related cognitive batteries, is **substantially heritable**, but it is also strongly shaped by environment. The best summary is:

> **In contemporary, well-studied populations, genetic differences account for roughly 40–80% of variation in adult intelligence, with lower estimates in childhood and higher estimates in adulthood.**

That does **not** mean intelligence is fixed, nor that “80% of a person’s intelligence is genetic.” Heritability is about **variation within a population under particular environmental conditions**.

## What heritability estimates look like

Approximate ranges from behavioral genetics:

- **Early childhood:** ~20–40% heritable
- **Adolescence:** ~40–60%
- **Adulthood:** ~50–80%

A common broad estimate for adult IQ is around **60% heritable**, though estimates vary by sample, age, country, socioeconomic conditions, and methods.

The increase with age may seem surprising, but one explanation is **gene–environment correlation**: as people grow older, they increasingly choose, create, or remain in environments that match their genetic tendencies—books, schooling, peer groups, occupations, hobbies, etc.

## What the evidence shows

### 1. Twin studies

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

Typical finding:

- **Identical twins** have much more similar IQ scores than **fraternal twins**.
- Identical twins reared apart are still substantially similar in IQ.

This supports a genetic contribution.

However, twin studies rely on assumptions, such as the “equal environments assumption,” and can be affected by assortative mating—people with similar education or cognitive ability tending to partner—which can complicate estimates.

### 2. Adoption studies

Adoption studies compare children with their biological and adoptive relatives.

Typical finding:

- In childhood, adopted children’s IQ scores correlate with both adoptive-family environment and biological relatives.
- By adulthood, adopted individuals’ IQ scores tend to resemble their biological relatives more than their adoptive relatives.

This suggests both environmental influence, especially early in life, and increasing genetic influence over development.

### 3. Molecular genetic studies

Modern genome-wide association studies, or GWAS, scan DNA across very large samples.

Findings:

- Intelligence is **highly polygenic**: thousands of genetic variants contribute, each with tiny effects.
- Common genetic variants captured by SNP arrays explain a meaningful but incomplete share of IQ variation, often around **20–30%** depending on the study and trait.
- Polygenic scores can predict some differences in cognitive performance and educational attainment, but they are far from deterministic and currently have limited individual predictive power.

GWAS evidence confirms that genetic influence is real, dispersed across the genome, and not due to a few “intelligence genes.”

## Environment still matters a lot

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

- prenatal health and nutrition
- lead exposure and pollution
- childhood illness
- schooling quality
- family stress and poverty
- sleep, safety, and stimulation
- access to books, language, and learning opportunities

The **Flynn effect**—large IQ score increases across generations in many countries—also shows that environmental changes can shift population-level cognitive test performance substantially over time.

## Key caveats

### Heritability is population-specific

A heritability estimate applies to a particular population at a particular time. If environments become more equal, heritability may rise because environmental variation contributes less. If environments are harsh or unequal, environmental effects may be larger.

### Heritability does not imply immutability

Height is highly heritable, but nutrition can dramatically affect average height. Similarly, intelligence can be heritable while still being influenced by education, health, and social conditions.

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

Evidence that IQ variation within a population is partly genetic does **not** by itself explain average differences between socioeconomic, ethnic, national, or racial groups. Group differences can be caused by environmental, historical, cultural, economic, or testing-related factors, and heritability estimates within groups cannot be directly used to infer causes of between-group differences.

## Bottom line

Human intelligence is **moderately to highly heritable**, especially in adulthood, with typical estimates around **50–70%** in many modern studies. The evidence comes from twin, adoption, family, and molecular genetic research. But intelligence is also meaningfully influenced by environment, development, education, health, and social conditions. The genetic architecture is highly polygenic, probabilistic, and not deterministic.


text:
Human intelligence—as measured by IQ tests and related cognitive test batteries—is **substantially heritable**, but it is **not genetically fixed**, and heritability estimates depend on age, population, environment, and measurement.

## Short answer

In modern high-income populations, the heritability of general cognitive ability is often estimated at roughly:

- **Childhood:** ~20–40%
- **Adolescence:** ~40–60%
- **Adulthood:** ~50–80%

A common summary is that **about half or more of adult variation in intelligence within a population is associated with genetic differences**, while the rest reflects environmental influences, developmental factors, measurement error, and gene–environment interplay.

Importantly, **heritability does not mean immutability**. Nutrition, schooling, disease burden, stress, lead exposure, family resources, and social conditions can strongly affect cognitive development.

---

## What “heritability” means

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

If IQ has heritability of 60%, that does **not** mean:

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

It means that, in the studied population, **60% of the observed differences among individuals** are statistically associated with genetic differences.

If environments become more equal, heritability can rise because environmental variation contributes less. If environments are harsh or unequal, environmental effects may account for more variation.

---

## Main lines of evidence

### 1. Twin studies

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

Findings are highly consistent:

- **Identical twins are much more similar in IQ than fraternal twins.**
- Identical twins reared apart are still substantially similar.
- Heritability estimates tend to increase with age.

A typical pattern:

| Age | Approximate heritability |
|---|---:|
| Early childhood | 20–40% |
| Middle childhood/adolescence | 40–60% |
| Adulthood | 50–80% |

The increase with age may reflect people increasingly selecting and shaping environments that fit their genetic tendencies—for example, choosing books, friends, schools, hobbies, or occupations that reinforce cognitive strengths.

---

### 2. Adoption studies

Adoption studies compare children with their biological and adoptive relatives.

Typical findings:

- In childhood, adopted children often resemble both adoptive and biological parents to some degree.
- As children grow older, resemblance to **biological parents** tends to increase.
- Resemblance to **adoptive siblings or adoptive parents** for IQ often weakens by adulthood.

This supports genetic influence, while also showing that the family environment matters, especially earlier in development.

---

### 3. Molecular genetic studies

Modern genome-wide association studies, or GWAS, examine millions of genetic variants across many people.

They show that cognitive ability is **highly polygenic**:

- No single “intelligence gene” exists.
- Thousands of genetic variants each have tiny effects.
- Polygenic scores can predict some variation in cognitive test scores and educational attainment, but far from all of it.

Current DNA-based methods generally explain less variance than twin studies suggest. For example:

- SNP-based heritability estimates for cognitive ability are often around **20–30%**, depending on sample and method.
- Polygenic scores may predict a meaningful but still limited fraction of individual differences.

The gap between twin-study heritability and measured DNA prediction is sometimes called “missing heritability,” likely due to many tiny effects, rare variants, non-additive effects, measurement limits, and gene–environment complexity.

---

## Environmental evidence is also strong

Even with substantial heritability, environment clearly matters.

Evidence comes from:

- **Flynn effect:** IQ scores rose substantially across many countries during the 20th century, too quickly to be genetic. Likely contributors include schooling, nutrition, health, smaller families, cognitively demanding environments, and test familiarity.
- **Education:** Additional schooling generally improves measured cognitive performance.
- **Nutrition and disease:** Malnutrition, iodine deficiency, infectious disease burden, and prenatal insults can reduce cognitive development.
- **Toxins:** Lead exposure and some pollutants are associated with lower cognitive outcomes.
- **Severe deprivation:** Institutional neglect, extreme poverty, or chronic stress can impair development.
- **Adoption from deprived to enriched environments:** Children adopted into more supportive environments often show cognitive gains, especially when adoption occurs early.

So the evidence supports both claims:

1. **Genes matter a lot for individual differences.**
2. **Environments can substantially raise or lower cognitive outcomes.**

---

## Why heritability rises with age

One notable finding is that IQ heritability often increases from childhood to adulthood. This may seem counterintuitive, but several mechanisms can explain it:

- **Gene–environment correlation:** People select or evoke environments correlated with their genetic tendencies.
- **Increasing autonomy:** Adults choose activities, peers, jobs, and learning environments more than children do.
- **Amplification over time:** Small early differences can compound through practice and opportunity.
- **Reduced shared family influence:** Siblings often become less similar as they leave the shared home environment.

This does not mean childhood environment is irrelevant. Early conditions can influence developmental trajectories, health, motivation, schooling, and opportunity.

---

## Caveats

Heritability estimates are not universal constants.

They vary by:

- country;
- historical period;
- socioeconomic conditions;
- age;
- test type;
- range of environments studied.

For example, in populations where children face large differences in nutrition, schooling, disease, or safety, environmental variance may be larger and heritability lower.

Also, heritability within a population does **not** justify conclusions about causes of average differences between populations. A trait can be highly heritable within groups while group differences are entirely or largely environmental. Height is a classic example: it is highly heritable, yet average height has changed dramatically with nutrition and health.

---

## Bottom line

The evidence from twin, adoption, and DNA studies shows that **human intelligence is moderately to highly heritable**, especially in adulthood. A reasonable broad estimate is that **genetic differences account for about 50% or more of adult individual differences in IQ in many studied populations**.

But intelligence is also strongly shaped by environment, development, education, health, and social conditions. Heritability is about sources of variation, not destiny.