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

It works through a few key steps:

1. **Variation exists**
   Individuals in a population are not 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**
   Organisms often produce more young than the environment can support, so there is competition for food, mates, shelter, 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 **natural selection**.

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

Over many generations, this can lead to major changes in a population. If different populations of the same species adapt to different environments and become isolated from one another, they may eventually become separate species.

A simple example: suppose a population of insects includes both light and dark individuals. If the insects live on dark tree bark, birds may more easily see and eat the light insects. Dark insects survive and reproduce more often, so over generations the population becomes mostly dark.

Natural selection does not “try” to make organisms perfect. It simply favors traits that work well enough in a particular environment at a particular time. If the environment changes, different traits may become advantageous.


prompt_id: sci-evolution

provider: openai

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

Here’s the basic idea:

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. New variation can arise through mutation, genetic recombination, and other genetic processes.

3. **More offspring are produced than can survive**
   In nature, resources like food, space, and mates are limited. Not every individual survives long enough to reproduce.

4. **Some traits improve survival or reproduction**
   If a trait gives an individual an advantage in a particular environment, that individual may be more likely to survive, attract mates, or produce offspring.

5. **Helpful traits become more common**
   Because advantageous traits are inherited, individuals with those traits tend to pass them on more often. Over many generations, those traits can become more common in the population.

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 survive and reproduce more often, so over generations the population may become mostly brown.

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

In short: **natural selection is evolution driven by inherited differences in survival and reproduction.**

finish_reason: stop


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

It works through a few key steps:

1. **Variation exists**
   Individuals in a population are not 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 many 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 longer or have more offspring in a particular environment, that trait gives the organism a reproductive advantage.

5. **Helpful traits become more common**
   Because individuals with advantageous traits leave more offspring, the genes associated with those traits tend to increase in frequency over generations.

Over long periods of time, this can lead to major changes in a population. If groups of the same species become separated and evolve in different environments, they may eventually become different species.

A classic example is camouflage. If insects vary in color and birds more easily spot the lighter ones, darker insects may survive and reproduce more often. Over many generations, the population may become mostly dark-colored.

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.


text:
Evolution by natural selection is the process by which populations of living things change over generations because some 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. If a trait has a genetic basis, 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 improve survival or reproduction**
   Individuals with traits that give them an advantage in a particular environment are more likely to survive long enough to reproduce, or to produce more offspring.

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

For example, imagine a population of beetles where some are green and some are brown. If birds can see green beetles more easily on dark soil, brown beetles may survive more often. Over many generations, more beetles in the population may become brown.

Importantly, natural selection does **not** mean organisms “try” to evolve. Individuals do not change because they need to. Instead, populations change over generations because certain inherited traits lead to greater reproductive success.

Over long periods, natural selection can lead to adaptations, new species, and the diversity of life we see today.


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

It works through a few key steps:

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

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

3. **More offspring are produced than can survive**
   In most environments, resources such as food, space, mates, and shelter are limited. Not every individual survives long enough to reproduce.

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

5. **Helpful traits become more common**
   Because those individuals reproduce more successfully, the genes associated with advantageous traits tend to increase in frequency in the population over generations.

Over long periods of time, these changes can produce major adaptations and sometimes even new species.

A classic example is **peppered moths** in industrial England. Before heavy pollution, light-colored moths were better camouflaged on pale tree bark. When pollution darkened the trees, dark-colored moths were harder for birds to see, so they survived and reproduced more. As a result, dark moths became more common.

A few important points:

- Natural selection acts on individuals, but evolution happens in populations.
- Mutations are random with respect to need, but natural selection is not random: traits that improve reproductive success are more likely to spread.
- Evolution does not aim for perfection. It works with existing variation and environmental conditions.
- A trait that is useful in one environment may be harmful or neutral in another.

In short, **natural selection is the non-random spread of inherited traits that improve reproductive success in a particular environment**.


text:
Evolution by natural selection is the process by which populations of living organisms change over generations because individuals with certain inherited traits 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 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 improve survival or reproduction**
   - Individuals with traits that are better suited to their environment are more likely to survive and have offspring.
   - These helpful traits are called **adaptations**.

5. **Helpful traits become more common**
   - Because successful individuals pass on their genes, the traits that helped them survive or reproduce become more frequent in the population over generations.

Over long periods of time, this can lead to major changes in a species, and sometimes to the formation of new species.

A simple example:
If a population of beetles includes both green and brown beetles, and birds can more easily see and eat the green beetles, brown beetles may survive more often. If color is inherited, brown beetles will leave more offspring. Over generations, the population may become mostly brown.

Natural selection does not mean organisms “try” to evolve. Instead, individuals with beneficial inherited traits tend to leave more descendants, causing populations to change over time.


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.

The basic steps are:

1. **Variation exists**
   - Individuals in a population are not 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 safety.

4. **Some traits improve survival or reproduction**
   - If a trait helps an organism survive or have more offspring in a particular environment, that trait gives a reproductive advantage.
   - For example, a better-camouflaged insect may be less likely to be eaten.

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

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

A simple example:
If a population of rabbits lives in a snowy environment, rabbits with whiter fur may be harder for predators to see. If fur color is inherited, white rabbits may survive and reproduce more often. Over generations, white fur may become more common in that rabbit population.

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


text:
Evolution by natural selection is the process by which populations of living 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, resistance to disease, or ability to find food.

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, 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 **natural selection**.

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

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

For example, if a population of insects includes some that are naturally resistant to a pesticide, those resistant insects are more likely to survive and reproduce. Over time, the population may become mostly pesticide-resistant.

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


text:
Evolution by natural selection is the process by which populations of living things change over generations because individuals with traits that help them survive and reproduce tend to pass those traits on more often.

The basic steps are:

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

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

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

4. **Some traits improve survival or reproduction**
   If a trait helps an organism survive longer or have more offspring, that organism is more likely to pass on the genes related to that trait.

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 see green insects more easily on tree bark, brown insects may survive more often. The brown insects then reproduce more, passing on genes for brown coloration. Over generations, the population may become mostly brown.

Natural selection does **not** mean individuals evolve during their lifetimes. Instead, **populations evolve over generations**. It also does not aim for “perfect” organisms; it simply favors traits that work well enough in a particular environment. If the environment changes, different traits may become advantageous.

In short: **natural selection is the differential survival and reproduction of individuals due to inherited variation, leading populations to change over time.**


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

It works through a few key steps:

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

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 better suited to their environment are more likely to survive, find mates, and produce offspring.

5. **Helpful traits become more common**
   Because those individuals reproduce more successfully, their traits are passed on more often. Over many generations, the population changes.

For example, imagine a population of beetles where some are green and some are brown. If birds can see green beetles more easily on dark soil, they may eat more green beetles. Brown beetles survive and reproduce more often, so over generations, brown coloring becomes more common in the population.

Natural selection does **not** mean organisms choose to change or that evolution has a goal. Mutations and genetic variation arise without regard to what is “needed.” The environment simply favors some traits over others.

Over long periods of time, natural selection can lead to major changes, including adaptations such as camouflage, antibiotic resistance in bacteria, or the evolution 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, resistance to disease, behavior, and many other traits.

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

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

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

5. **Helpful traits become more common**
   - Because individuals with useful traits leave more offspring, the genes for those traits become more common in the population over generations.

Over long periods of time, this can lead to major changes in a species, and eventually, new species can evolve.

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

Natural selection does not mean organisms “try” to evolve. Instead, evolution happens because individuals with certain inherited traits happen to survive and reproduce more successfully in a particular environment.


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

It works through four main ideas:

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

2. **Heritability**
   Some of these differences are genetic and can be passed from parents to offspring.

3. **Differential survival and reproduction**
   In a given environment, certain traits may give individuals an advantage. Those individuals are more likely to survive, find mates, and produce offspring.

4. **Change over generations**
   Because advantageous traits are passed on more often, they become more common in the population over time. Less helpful traits may become less common.

For example, if a population of insects is exposed to a pesticide, most may die. But if a few have a genetic trait that makes them resistant, they survive and reproduce. Their offspring inherit the resistance, so over generations the population becomes more pesticide-resistant.

Natural selection does **not** mean organisms choose to change, and it does not work toward a goal. Instead, the environment “selects” traits that happen to improve survival and reproduction under current conditions.

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


text:
Evolution by natural selection is the process by which populations of organisms change over generations because individuals with traits that help them survive and reproduce tend to pass those traits on more often.

It works through a few key steps:

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

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

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

4. **Some traits improve survival or reproduction**
   Individuals with advantageous traits are more likely to survive, find mates, and produce offspring. For example, a moth whose color makes it harder for birds to see may survive longer.

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

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

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

Natural selection does not involve organisms “trying” to evolve. It is not goal-directed. Instead, it happens because inherited traits affect survival and reproduction in a particular environment. A trait that is useful in one environment may be useless or harmful 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 more successfully than others.

The basic idea has four main parts:

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. New variation can also arise through mutation and genetic recombination.

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

4. **Individuals with advantageous traits reproduce more**
   If a trait helps an organism survive or reproduce in a particular environment, individuals with that trait tend to leave more offspring. Over time, the genes associated with that trait become more common in the population.

For example, if a population of insects is exposed to a pesticide, most may die, but a few may have a genetic resistance. Those resistant insects survive and reproduce, passing resistance to their offspring. After many generations, the population may become mostly pesticide-resistant.

A key point is that **individual organisms do not evolve during their lifetimes**. Instead, **populations evolve** as inherited traits become more or less common across generations.

Also, natural selection is not goal-directed. Mutations arise largely at random, but selection is not random: traits that improve survival or reproduction in a given environment are more likely to spread. If the environment changes, different traits may become advantageous.


text:
Evolution by natural selection is the process by which populations of living organisms change over generations because individuals with certain inherited traits survive and reproduce more successfully 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**
   In nature, 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 help them survive or reproduce in a particular environment are more likely to pass those traits on.

5. **Helpful traits become more common**
   Over many generations, advantageous inherited traits increase in frequency within 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, the darker insects may survive better. If color is inherited, dark insects will leave more offspring, and over generations the population may become mostly dark.

Natural selection does not act with a goal or plan. It simply favors traits that work well in a given 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 some inherited traits help organisms survive and reproduce better than others.

It works through a few key steps:

1. **Variation exists**
   Individuals in a population are not all identical. 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 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. This is called higher “fitness.”

5. **Helpful traits become more common**
   Because successful individuals pass their traits on, 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 separated and evolve differently for long enough, they may eventually become separate species.

A simple 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 brown tree bark, brown insects may survive and reproduce more often. After many generations, the population may become mostly brown.

Natural selection does not “try” to create perfect organisms. It simply favors traits that work well in a particular environment at a particular time. If the environment changes, different traits may become advantageous.


text:
Evolution by natural selection is the process by which populations of organisms change over generations because 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 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 make them better suited to their environment are more likely to survive and have offspring. This is called **fitness**—not necessarily being strongest, but being better at surviving and reproducing in a particular environment.

5. **Helpful traits become more common**
   Because successful individuals pass on their genes, advantageous traits tend to increase in frequency over generations. Harmful traits may become less common.

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

For example, if a population of insects lives on dark tree bark, darker-colored insects may be harder for birds to see. Those insects survive more often and reproduce more, so genes for darker color become more common in the population.

Natural selection does not involve organisms “trying” to evolve. Instead, the environment favors certain inherited variations that already exist or arise through mutation.


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

It works through a few key steps:

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

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

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

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

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

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

A simple 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 survive more often and produce more offspring. After many generations, the population may become mostly brown.

Natural selection does not “try” to make organisms perfect. It simply favors traits that are useful in a particular environment at a particular time. If the environment changes, different traits may become advantageous.


text:
Evolution by natural selection is a process that explains how populations of living things change over generations.

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, space, mates, and other resources.

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

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

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

Natural selection does **not** mean organisms choose to change. Instead, individuals with certain inherited traits happen to be better suited to their environment. Over long periods, this can lead to major changes and even the formation of new species.


prompt_id: sci-iq-heritability

text:
Human intelligence, as measured by IQ tests and related cognitive batteries, is **substantially heritable**, but not genetically determined in a simple or fixed way.

## Short answer

In modern behavioral-genetic research, **heritability of intelligence is usually estimated at roughly:**

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

These numbers mean that, **within a given population and environment**, genetic differences account for that proportion of the observed variation in cognitive test scores. They do **not** mean that an individual’s intelligence is “60% genetic,” nor that intelligence cannot be changed.

---

## What the evidence shows

### 1. Twin studies

Twin studies compare identical twins, who share ~100% of their segregating DNA, with fraternal twins, who share ~50% on average.

A common finding is:

- Identical twins are more similar in IQ than fraternal twins.
- Identical twins reared apart are still substantially similar.
- Fraternal twins and siblings are less similar, even when raised together.

This pattern strongly suggests a genetic contribution.

Typical correlations, approximately:

- Identical twins raised together: **~0.75–0.85**
- Identical twins raised apart: **~0.65–0.75**
- Fraternal twins/siblings: **~0.35–0.50**
- Adoptive siblings raised together: often much lower in adulthood

These are not exact constants, but they illustrate the general pattern.

---

### 2. Adoption studies

Adoption studies find that adopted children’s IQs are influenced by their adoptive homes, especially in childhood. But as children grow older, their cognitive scores tend to become more similar to those of their biological relatives than to those of their adoptive relatives.

This suggests both:

- **Environment matters**, especially early on.
- **Genetic influence becomes more apparent with age.**

One explanation is that as people mature, they increasingly select and shape environments that fit their genetic tendencies — for example, choosing certain hobbies, peer groups, books, courses, or occupations.

---

### 3. Molecular genetics and GWAS

Modern genome-wide association studies show that intelligence and educational attainment are **highly polygenic**: influenced by thousands of genetic variants, each with a tiny effect.

Important findings:

- No single “intelligence gene” exists.
- Polygenic scores can predict some variation in cognitive performance, but far from all of it.
- SNP-based heritability estimates are usually lower than twin-study estimates, often around **20–30%** for cognitive ability, because current methods capture only part of genetic influence.

This gap is sometimes called “missing heritability,” though it has narrowed as sample sizes and methods improve.

---

## Heritability increases with age

One of the most robust findings is that IQ heritability tends to rise from childhood into adulthood.

Approximate pattern:

| Age period | Heritability estimate |
|---|---:|
| Early childhood | ~20–40% |
| Middle childhood | ~30–50% |
| Adolescence | ~40–60% |
| Adulthood | ~50–80% |

At the same time, the effect of the **shared family environment** — factors that make siblings more alike, such as household income, parenting style, neighborhood, and schooling — tends to be stronger in childhood and weaker in adulthood.

However, that does not mean environment stops mattering. Rather, environmental effects in adulthood are often more individualized: peers, education, illness, motivation, nutrition, occupation, stress, and personal experiences.

---

## What heritability does and does not mean

### Heritability does mean:

A heritability estimate tells us how much of the **variation among people in a particular population at a particular time** is statistically associated with genetic differences.

For example, if adult IQ heritability is estimated at 60%, that means about 60% of the observed differences in IQ within that studied population are associated with genetic variation.

### Heritability does not mean:

- Intelligence is fixed.
- Environment is unimportant.
- An individual’s IQ is “60% genetic.”
- Genetic differences explain differences between social groups.
- Education, nutrition, health, and social conditions do not matter.

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

---

## Environment still matters a lot

Evidence shows cognitive development is affected by many environmental factors, including:

- prenatal health
- nutrition
- exposure to toxins such as lead
- quality of schooling
- chronic stress
- childhood disease burden
- sleep
- family resources
- language exposure
- educational opportunities

The **Flynn effect** is especially important: average IQ scores rose substantially in many countries during the 20th century, far too quickly to be caused by genetic evolution. That shows large-scale environmental changes can shift cognitive test performance.

---

## Heritability is population-specific

Heritability estimates depend on the environment being studied.

In a society where everyone has similar nutrition, schooling, and healthcare, remaining differences may be more strongly associated with genetic variation.

In a society with large deprivation or unequal opportunity, environmental differences may explain more variation.

So heritability is not a universal constant.

---

## Group differences cannot be inferred from within-group heritability

A crucial point: high heritability within a group does **not** imply that average differences between groups are genetic.

For example, height is highly heritable within populations, but average height differences between populations can be strongly affected by nutrition, disease, and living conditions.

The same logic applies to intelligence. Heritability estimates within populations do not by themselves explain differences between socioeconomic, ethnic, national, or racial groups.

---

## Bottom line

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

> In contemporary developed populations, genetic differences explain roughly half or more of individual differences in measured intelligence, with estimates often around 50–70% in adults.

But intelligence is also clearly shaped by environment, development, education, health, culture, and opportunity. Heritability is about variation within a population; it does not mean intelligence is biologically fixed or that environmental improvements cannot raise cognitive performance.


text:
Human intelligence—as usually measured by IQ tests and related cognitive batteries—is **substantially heritable**, but heritability is often misunderstood.

## Short answer

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

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

A common summary is that **about half or more of individual differences in adult intelligence within a given population are associated with genetic differences**.

But this does **not** mean intelligence is fixed, that environment is unimportant, or that “50% of your intelligence comes from genes.” Heritability is a population-level statistic about variation under particular environmental conditions.

---

## What “heritability” means

Heritability asks:

> Of the observed differences among people in a particular population, how much of that variation is statistically associated with genetic variation?

So if IQ has high heritability in a population, that means genetic differences help explain why people in that population differ from one another.

It does **not** mean:

- a person’s IQ is genetically predetermined;
- environment does not matter;
- the same estimate applies in every country, class, or historical period;
- genetic causes explain differences between social groups;
- intelligence cannot be improved.

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

---

## Main evidence

### 1. Twin studies

Twin studies compare:

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

Identical twins tend to be much more similar in IQ than fraternal twins. This holds especially strongly in adolescence and adulthood.

Typical findings:

- Identical twins reared together correlate around **0.80 or higher** in IQ.
- Fraternal twins reared together correlate more like **0.40–0.60**.
- Identical twins reared apart are still substantially similar, often correlating around **0.60–0.75**, though these studies are rarer and have limitations.

This pattern strongly suggests genetic influence.

### 2. Adoption studies

Adoption studies compare children with their biological and adoptive relatives.

Findings generally show:

- In childhood, adopted children’s IQs are influenced by the adoptive family environment.
- As they grow older, their IQs tend to become more similar to their biological relatives.
- The effect of the shared family environment often declines by adulthood.

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

### 3. Molecular genetic studies

Genome-wide association studies, or GWAS, examine millions of DNA variants across huge samples.

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

Current molecular findings show:

- Common genetic variants explain a meaningful portion of variation in cognitive ability.
- SNP-based heritability estimates are often around **20–30%**, lower than twin-study estimates.
- Polygenic scores can predict some variance in cognitive performance and educational attainment, but they are far from deterministic.

The gap between twin-based heritability and SNP-based estimates is often called “missing heritability,” and may reflect rare variants, non-additive effects, imperfect measurement, and other complexities.

---

## Heritability increases with age

One of the most consistent findings is that intelligence becomes **more heritable with age**.

This may seem surprising, but it likely reflects gene–environment correlation:

- Children do not fully choose their environments.
- As people age, they increasingly select, shape, and evoke environments that fit their interests and abilities.
- A child with a genetic inclination toward verbal ability may read more, seek challenging classes, and receive more encouragement, amplifying initial differences.

So genes may influence not only brain development directly, but also the environments people experience.

---

## Environment still matters a lot

Even when heritability is high, environment can have large effects.

Important environmental influences include:

- prenatal health;
- nutrition;
- exposure to toxins such as lead;
- disease burden;
- stress and trauma;
- parenting and early language exposure;
- schooling quality;
- socioeconomic deprivation;
- cultural and historical conditions.

The **Flynn effect** is especially important: IQ scores rose substantially across many countries during the 20th century, too quickly to be genetic. This shows that environments can shift population averages dramatically.

---

## Shared vs non-shared environment

Behavioral genetic studies often separate environmental influence into:

### Shared environment

Factors that make siblings similar, such as:

- family income;
- neighborhood;
- parental education;
- household resources.

Shared environment tends to matter more in childhood and less in adulthood for IQ.

### Non-shared environment

Factors that make siblings different, such as:

- different peer groups;
- different teachers;
- illness or injury;
- idiosyncratic experiences;
- measurement error.

Non-shared environmental effects remain important throughout life.

---

## Important caveats

### Heritability depends on the population

A heritability estimate from one society does not automatically apply to another. If everyone has similar schooling and nutrition, genetic differences may explain more of the remaining variation. If environments vary greatly, environmental differences may explain more.

### High heritability does not imply immutability

Phenylketonuria, or PKU, is genetically caused but can be treated environmentally through diet. Likewise, high heritability does not mean an outcome cannot be changed.

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

Evidence that intelligence is heritable within a population does not by itself explain average differences between populations or socially defined groups. Group differences can arise from environmental, historical, socioeconomic, educational, health, and discriminatory factors. Genetic claims about group differences require separate evidence and are not established by ordinary heritability estimates.

### IQ is not all of intelligence

IQ tests measure important cognitive abilities—reasoning, working memory, processing speed, verbal and spatial skills—and predict many outcomes. But they do not capture every valuable mental trait, such as creativity, wisdom, curiosity, motivation, emotional understanding, or practical judgment.

---

## Bottom line

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

At the same time, intelligence is also strongly shaped by environment, development, education, health, and social conditions. Heritability is not destiny; it describes sources of variation in a population, not the limits of an individual.


text:
Human intelligence, as measured by IQ-type tests and the general cognitive ability factor “g,” is **substantially heritable**, but **not genetically fixed**. The best evidence suggests that, within a given population and environment, genetic differences explain a meaningful share of individual differences in cognitive ability—especially in adulthood—while environment also matters a great deal.

## What “heritable” means

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

It does **not** mean:

- “X% of a person’s intelligence is genetic”
- intelligence cannot be changed
- environments are unimportant
- differences between social groups are genetic

A trait can be highly heritable and still be strongly affected by environment. For example, height is highly heritable, yet nutrition and disease can shift average height substantially.

## Approximate heritability estimates

Across behavioral-genetic studies, the heritability of intelligence is usually estimated roughly as:

- **Early childhood:** around **20–40%**
- **Adolescence:** around **40–60%**
- **Adulthood:** around **60–80%** in many high-income populations

These are broad estimates, not constants. Heritability varies by age, society, socioeconomic conditions, test quality, and statistical method.

A common finding is that heritability of cognitive ability **increases with age**. One explanation is that as people grow older, they increasingly select, shape, and respond to environments partly influenced by their genetic tendencies—such as reading habits, peer groups, educational choices, and interests.

## Main lines of evidence

### 1. Twin studies

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

A typical finding is:

- 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 supports a genetic contribution.

However, twin studies rely on assumptions, such as the “equal environments” assumption: that identical and fraternal twins experience equally similar environments relevant to intelligence. Researchers debate details, but the broad conclusion of substantial heritability has held up across many designs.

### 2. Adoption studies

Adoption studies compare adopted children with their biological and adoptive relatives.

Typical findings:

- In childhood, adopted children’s IQs often resemble both adoptive-family environment and biological relatives.
- By adulthood, resemblance to biological relatives tends to increase.
- Resemblance to adoptive siblings often decreases with age.

This suggests that shared family environment affects cognitive development, especially earlier in life, but genetic differences become more predictive over time.

### 3. Molecular genetic studies

Modern genome-wide association studies, or GWAS, examine many genetic variants across the genome.

Findings include:

- Many thousands of genetic variants are associated with cognitive ability or educational attainment.
- Each variant has a tiny effect.
- Intelligence is highly polygenic: no single “intelligence gene” explains much.
- Polygenic scores can predict some variation in cognitive test performance and education, but far from all of it.

Current DNA-based methods typically explain less variance than twin studies do. For cognitive ability, common genetic variants captured by current genotyping may explain perhaps **20–30% or more** of variation depending on the sample and method, while polygenic scores usually predict a smaller portion, often in the single digits to low teens. Estimates continue to change as sample sizes grow.

This gap between twin-study heritability and measured-DNA prediction is sometimes called “missing heritability.” It may reflect rare variants, structural variants, gene–environment interaction, imperfect measurement, and limits of current methods.

## Environmental evidence is also strong

Genetic influence does not mean environment is weak. Environmental effects on intelligence are well established.

Important influences include:

- prenatal health
- nutrition
- exposure to toxins such as lead
- childhood illness
- quality and duration of schooling
- family stress and poverty
- language exposure and cognitive stimulation
- sleep, safety, and emotional security
- social and educational opportunities

The **Flynn effect** is especially important: average IQ scores rose substantially in many countries during the 20th century. Genetic evolution cannot explain such rapid gains. Environmental changes—education, nutrition, health, smaller families, more cognitively demanding environments—likely played major roles.

Also, heritability can be lower in environments where deprivation is severe. If children differ greatly in nutrition, safety, schooling, or disease exposure, environmental variation can explain more of the differences in cognitive outcomes.

## Shared vs non-shared environment

Behavioral genetics often separates environment into:

- **Shared environment:** factors that make siblings similar, such as family socioeconomic status, neighborhood, parenting style, school access.
- **Non-shared environment:** factors that make siblings different, such as different peers, illnesses, teachers, birth complications, random developmental events, and measurement error.

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

## Important caveats

### Heritability is population-specific

A heritability estimate from one country, era, or social class does not automatically apply elsewhere.

For example, if everyone in a population had equally excellent nutrition and schooling, remaining differences might appear more heritable. If environments were highly unequal, environmental causes could explain more variation.

### Heritability does not imply immutability

High heritability does not mean a trait cannot be changed. Reading ability, height, and many medical conditions have genetic influences but can be affected by interventions.

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

Even if intelligence is substantially heritable within a population, that does not show that average differences between ethnic, racial, national, or socioeconomic groups are genetic. Group differences can be produced by environmental differences even when within-group heritability is high.

### Intelligence is not one simple thing

IQ tests measure important cognitive abilities and predict many outcomes, including academic achievement and job performance. But intelligence is multidimensional, and tests do not capture every valuable mental ability, such as creativity, wisdom, practical judgment, motivation, emotional skill, or expertise.

## Bottom line

The evidence shows that **human intelligence is moderately to highly heritable**, especially in adulthood, with estimates often around **50–80%** in modern high-income societies. This conclusion is supported by twin, adoption, family, and molecular genetic research.

But intelligence is also **strongly shaped by environment**, especially early development, education, health, and social opportunity. Heritability is not destiny, and it does not imply that environmental interventions are unimportant. The most accurate summary is:

> Genetic differences contribute substantially to individual differences in intelligence, but cognitive development emerges from continuous interaction between genes, environments, and life experiences.


text:
Human intelligence, as measured by IQ tests and related cognitive batteries, is **substantially heritable**, but **not genetically fixed**. The best evidence suggests that in many contemporary, well-nourished populations, genetic differences account for roughly:

- **~20–40% of IQ variation in childhood**
- **~50–70% in adolescence and adulthood**
- Sometimes estimates approach **~80% in older adult samples**, though this depends on methods and population

These numbers refer to **variation among people within a given population and environment**, not to the “percent of a person’s intelligence caused by genes.”

## What “heritability” means

A heritability estimate of, say, 60% means that **60% of the observed differences between individuals in that population** are statistically associated with genetic differences. It does **not** mean:

- intelligence is 60% genetic and 40% environmental for an individual;
- IQ cannot be changed;
- environments are unimportant;
- the same number applies to all societies or historical periods.

Heritability can be high even for traits strongly affected by environment. For example, height is highly heritable in well-nourished countries, but average height can rise dramatically with better nutrition and health.

## Main lines of evidence

### 1. Twin studies

Twin studies are the classic evidence base.

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

Identical twins tend to be much more similar in IQ than fraternal twins. Typical correlations are roughly:

- Identical twins raised together: **~0.80 or higher**
- Fraternal twins raised together: **~0.40–0.60**
- Identical twins raised apart: often **~0.60–0.75**, though samples are smaller and not perfect

This pattern strongly suggests a genetic contribution.

### 2. Adoption studies

Adoption studies show that adopted children’s IQs tend to resemble:

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

By adulthood, resemblance to the adoptive family often becomes small, while resemblance to biological relatives remains stronger. This supports the idea that genetic influence on cognitive differences increases with age.

### 3. Molecular genetics and GWAS

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

Modern studies can estimate “SNP heritability,” the proportion of variance captured by common measured genetic variants. For cognitive ability, estimates are often around **20–30%**, lower than twin-study estimates because GWAS does not capture all genetic variation, rare variants, structural variants, or all non-additive effects.

Polygenic scores can predict some differences in cognitive performance and educational outcomes, but their predictive power is still limited—often in the **single digits to low teens percent of variance**, depending on sample, ancestry, age, and measure. They are much less informative for individuals than for group-level statistical research.

## Why heritability increases with age

A well-known finding is that IQ heritability tends to rise from childhood to adulthood. Possible reasons include:

1. **Gene–environment correlation**: people increasingly select, shape, and evoke environments that fit their genetically influenced tendencies.
   For example, a child who enjoys reading may seek books, classes, and peers that further develop verbal ability.

2. **Greater personal autonomy**: adults have more control over education, work, hobbies, and social environment.

3. **Shared family environment fades**: the effects of growing up in the same household often become less important over time for cognitive differences.

## Environmental effects are still important

Even if intelligence is moderately to highly heritable, environment matters a great deal. Influences include:

- quality and duration of education;
- nutrition;
- prenatal health;
- exposure to lead and other toxins;
- infectious disease burden;
- sleep;
- stress and poverty;
- parenting and language exposure;
- access to books, schools, and cognitively stimulating environments.

The **Flynn effect**—large IQ-score gains across many countries during the 20th century—shows that population-level cognitive test performance can change substantially over time, too quickly to be due to genetic evolution. That is strong evidence for powerful environmental effects.

## Important caveats

1. **Heritability is population-specific.**
   A heritability estimate from middle-class families in a wealthy country may not apply to children facing malnutrition, poor schooling, or severe deprivation.

2. **High heritability does not imply immutability.**
   A trait can be highly heritable and still responsive to intervention.

3. **Heritability within a population does not explain differences between populations.**
   If IQ differences exist between groups or countries, heritability estimates within groups do not by themselves tell us why those differences exist.

4. **IQ is not all of intelligence.**
   IQ tests measure important cognitive abilities—reasoning, working memory, processing speed, verbal and spatial ability—but they do not capture every form of skill, wisdom, creativity, motivation, or practical competence.

## Bottom line

The evidence from twins, adoption studies, and modern genomics converges on the conclusion that **human intelligence is substantially heritable**, especially by adulthood, with typical adult estimates around **50–70%** in many studied populations. But intelligence is also meaningfully shaped by environment, development, education, health, and social conditions. Genes influence cognitive potential and tendencies; environments help determine how those potentials are developed and expressed.


text:
Human intelligence, as measured by cognitive test scores/IQ-like batteries, is **substantially heritable**, but **not genetically fixed**. The best summary is:

- In contemporary high-income populations, **heritability of general cognitive ability is often estimated around 50–80% in adulthood**.
- In childhood it is usually lower, roughly **20–50%**, and it tends to **increase with age**.
- Environment still matters a great deal: nutrition, disease burden, schooling, family resources, stress, toxins, sleep, and social context can all affect cognitive development and test performance.
- “Heritable” does **not** mean “unchangeable,” and it does **not** imply that group differences are genetic.

## What “heritability” means

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

So if adult IQ heritability is estimated at 60%, that means: among people in that studied population, about 60% of the observed differences in scores are associated with genetic variation. It does **not** mean an individual’s intelligence is “60% genetic and 40% environmental.”

Heritability can change across environments. For example, if everyone has similar access to schooling, nutrition, and healthcare, environmental variation is reduced, and genetic differences may explain a larger share of the remaining variation. In harsher or more unequal environments, environmental effects can be larger.

## Main evidence

### 1. Twin studies

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

A consistent finding is:

- Identical twins’ cognitive scores are more similar than fraternal twins’.
- This holds across many cohorts and countries.
- The similarity is especially high for broad “general intelligence” or “g” scores.

Typical estimates from twin studies:

- Early childhood: modest genetic influence.
- Adolescence: moderate to substantial genetic influence.
- Adulthood: often high, around **50–80%**.

Twin studies also show that the **shared family environment** — factors making siblings in the same home more similar — tends to be more important in childhood and less important in adulthood, while **non-shared environment** and individual experiences remain relevant.

### 2. Adoption studies

Adoption studies compare children with their biological and adoptive relatives.

Findings generally show:

- Adopted children’s cognitive scores are influenced by the adoptive environment, especially in childhood.
- As adoptees age, their scores tend to become more similar to those of their biological relatives.
- Moving from deprived environments into more enriched homes can substantially improve cognitive outcomes.

This supports both genetic influence and environmental influence.

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

Modern genetic studies examine millions of DNA variants across very large samples. They find that cognitive ability is **highly polygenic**: influenced by thousands of genetic variants, each with very tiny effects.

Important points:

- No single “intelligence gene” exists.
- Polygenic scores can predict some variation in educational attainment and cognitive test performance.
- Current polygenic scores explain a **minority** of the total variance, often roughly in the single digits to low teens depending on trait, sample, and method.
- SNP-based heritability estimates — the amount attributable to common measured variants — are lower than twin-study estimates, often around **20–30%** for cognitive traits, because they do not capture all genetic variation and because measurement differs.

GWAS confirms that genetic differences contribute to cognitive variation, but also shows the architecture is complex and probabilistic.

## Why heritability increases with age

A striking finding is that heritability of intelligence tends to rise from childhood into adulthood. Several explanations are likely:

1. **Gene–environment correlation**: as people grow, they increasingly select, shape, and evoke environments that match their dispositions. For example, a child who enjoys reading may seek out more books and challenging classes.

2. **Increasing autonomy**: adults have more control over education, work, hobbies, and peer groups.

3. **Cumulative developmental effects**: small genetic tendencies can compound over time through repeated choices and feedback.

This does not mean early environments are unimportant. Quite the opposite: early environments can influence which abilities are developed and which opportunities are available.

## Environmental evidence is also strong

Even highly heritable traits can be environmentally affected. Intelligence-related outcomes are influenced by:

- Education quality and years of schooling.
- Childhood nutrition.
- Lead exposure and other toxins.
- Prenatal environment.
- Infectious disease burden.
- Sleep and stress.
- Socioeconomic deprivation.
- Family instability or neglect.
- Access to cognitively stimulating materials and activities.

The **Flynn effect** — large gains in average IQ test scores over the 20th century in many countries — is strong evidence that environments can shift population averages substantially over time. Genetic change cannot explain such rapid generational increases.

Similarly, interventions and natural experiments show that schooling has positive effects on measured cognitive performance.

## Important limitations

Heritability estimates are context-dependent. They do not automatically apply across:

- Different countries.
- Different historical periods.
- Different socioeconomic conditions.
- Different testing regimes.
- Different age groups.

Also, IQ tests measure important cognitive skills but not all valuable mental capacities. Creativity, wisdom, motivation, emotional regulation, practical judgment, curiosity, and domain-specific expertise are not fully captured by IQ.

## Bottom line

The evidence from twin, adoption, family, and DNA-based studies shows that human intelligence is **meaningfully and substantially heritable**, especially in adulthood. A reasonable broad estimate is that genetic differences account for about **half or more of variation in adult cognitive test scores** in many modern high-income populations.

But intelligence is also **strongly shaped by environment**, especially early development, education, health, and opportunity. Heritability is about variation within a population; it does not mean intelligence is genetically predetermined, immune to intervention, or that differences between social groups are genetic in origin.


text:
Human intelligence, usually measured by IQ tests or a general cognitive ability factor “g,” is **substantially heritable**, but heritability is **not 100%**, is **age- and environment-dependent**, and does **not mean intelligence is fixed or unaffected by environment**.

## Short answer

In modern behavioral-genetic research, the heritability of intelligence is roughly:

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

These numbers mean that, within a given population at a given time, a sizeable share of differences among people in measured cognitive ability is statistically associated with genetic differences.

## 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. Across many studies:

- **Identical twins’ IQ scores are much more similar** than fraternal twins’ scores.
- This remains true even when twins are raised apart, though such studies are rarer and have limitations.
- The similarity between identical twins tends to increase with age.

This pattern is one of the strongest classic lines of evidence that genetic differences contribute meaningfully to individual differences in intelligence.

### 2. Adoption studies support the same conclusion

Adoption studies often find that:

- Young adopted children resemble both adoptive and biological relatives to some extent.
- As children grow older, their IQs tend to become **more similar to their biological parents** and **less similar to their adoptive siblings or adoptive parents**.

This does not mean adoptive environments are unimportant. Rather, it suggests that as people mature, genetic propensities increasingly shape the environments they seek, evoke, and sustain.

### 3. DNA-based studies confirm heritability, but explain less than twin studies

Modern genome-wide association studies, or GWAS, examine measured DNA directly. They find that intelligence and educational attainment are influenced by **thousands of genetic variants**, each with extremely small effects.

DNA-based estimates generally find:

- **SNP heritability** of intelligence around **20–30%**, depending on the sample and method.
- Polygenic scores can predict a modest but meaningful fraction of variation in cognitive performance or educational outcomes.

These estimates are lower than twin-study estimates because common measured genetic variants do not capture all genetic effects, rare variants, structural variants, gene–gene interactions, and some indirect genetic effects.

## Important caveats

### Heritability does not mean immutability

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

Similarly, intelligence is affected by:

- education quality
- nutrition
- toxins such as lead exposure
- disease burden
- family stress
- sleep
- early childhood stimulation
- socioeconomic conditions

The **Flynn effect**—large IQ gains across generations in many countries during the 20th century—is strong evidence that environmental changes can shift average cognitive test performance substantially.

### Heritability is about variation within a population

A heritability estimate does **not** tell us why two groups differ on average. It only describes sources of variation **within a particular population, in a particular environment, at a particular time**.

For example, if a population has relatively equal schooling and nutrition, genetic differences may account for more of the remaining variation. In a population with large environmental deprivation, environmental differences may account for more.

### Shared environment matters more in childhood

Family-wide environmental factors—such as household resources, parenting style, neighborhood, and school access—often show stronger effects in childhood. Their measured influence tends to decline by adulthood in many studies, while genetic influence rises.

That said, this does not imply childhood environment is unimportant. Early environment can affect development, health, education, motivation, and opportunities in ways not fully captured by simple IQ heritability estimates.

### Gene–environment correlation matters

Genes and environments are not independent. Genetic tendencies can influence the environments people experience. For example, a child with early verbal ability may receive more books, attention, advanced classes, or encouragement. This can amplify initial differences.

So “genetic influence” does not mean a simple biological destiny. It often operates through developmental feedback loops involving family, school, culture, and personal choices.

## Bottom line

The best evidence indicates that **human intelligence is moderately to highly heritable**, especially in adulthood, with typical adult estimates around **50–80%** from twin and family studies and lower but still substantial estimates from DNA-based methods.

At the same time, intelligence is **strongly shaped by environment**, is **polygenic**, and is **not genetically predetermined**. Heritability describes statistical variation within populations, not individual potential, not the value of a person, and not the causes of average differences between social groups.


text:
Human intelligence (often measured by IQ tests and related “general cognitive ability” measures) is **substantially heritable**, but not genetically fixed. The best evidence suggests that genetic differences explain a sizable share of **individual differences within a given population and environment**, while environment also matters a lot.

## Short answer

- In childhood, heritability of intelligence is often estimated around **30–50%**.
- In adolescence/adulthood, estimates often rise to around **50–80%**, with many summaries putting adult IQ heritability near **60–70%**.
- The remaining variation reflects **environmental influences**, measurement error, developmental randomness, and gene–environment interplay.
- Heritability does **not** mean immutability, and it does **not** explain differences between social groups or populations by itself.

## What the evidence shows

### 1. Twin studies

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

Findings are consistent across many studies:

- **Identical twins are more similar in IQ than fraternal twins.**
- Identical twins reared apart are still substantially similar in cognitive ability.
- The similarity between twins tends to increase from childhood into adulthood.

This pattern supports a strong genetic contribution. However, twin studies rely on assumptions, such as the idea that identical and fraternal twins experience similarly correlated environments, so they are informative but not perfect.

### 2. Adoption studies

Adoption studies compare children with their biological and adoptive relatives.

Typical findings:

- In early childhood, adopted children’s IQs often resemble both adoptive-family environment and biological-family background.
- As children grow older, resemblance to **biological relatives** tends to increase.
- Resemblance to **adoptive siblings or adoptive parents** often decreases by adulthood.

This suggests that genetic influences become more visible over development, partly because people increasingly select and shape environments compatible with their traits.

### 3. Molecular genetics and GWAS

Modern genomic studies look directly at DNA rather than inferring genetics from family resemblance.

Findings:

- Intelligence is **highly polygenic**: influenced by thousands of genetic variants, each with tiny effects.
- Common DNA variants captured by genotyping arrays account for a meaningful but incomplete portion of IQ differences, often estimated around **20–30%** for “SNP heritability,” depending on the sample and measure.
- Polygenic scores can predict some variation in educational attainment and cognitive performance, but they are far from deterministic and explain only a minority of differences.

This supports the conclusion that genes matter, while also showing that there is no single “intelligence gene.”

## Why heritability increases with age

One interesting finding is that IQ heritability appears to rise from childhood to adulthood. Possible reasons include:

- Older individuals have more freedom to choose environments that fit their abilities and interests.
- Genetic tendencies can influence which books, peers, classes, hobbies, and occupations people seek out.
- Early shared family environment may matter more in childhood, while individual-specific experiences matter more later.

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

## Environment still matters greatly

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

- Quality and duration of schooling
- Nutrition
- Prenatal health
- Exposure to toxins such as lead
- Infectious disease burden
- Sleep, stress, and trauma
- Family resources and stability
- Cognitive stimulation and language exposure

The **Flynn effect**—large IQ gains across generations in many countries during the 20th century—shows that environmental change can shift average test performance substantially over time, even for a trait that is heritable within a generation.

## What heritability does and does not mean

Heritability is often misunderstood.

It means:

> In a particular population, at a particular time, under particular environmental conditions, a certain proportion of observed individual differences is statistically associated with genetic differences.

It does **not** mean:

- “70% of a person’s intelligence is genetic.”
- Intelligence cannot be changed.
- Environment is unimportant.
- Genetic influences explain differences between countries, races, classes, or historical periods.
- A person’s potential can be accurately predicted from DNA.

For example, height is highly heritable, but average height can rise dramatically with better nutrition and health. Intelligence is similar in that genetic variation matters, but environmental conditions can strongly affect outcomes.

## Bottom line

The evidence from twin, adoption, family, and genomic studies converges on the conclusion that **human intelligence is moderately to highly heritable**, especially in adulthood. A reasonable summary is that **genetic differences explain roughly half or more of individual differences in adult intelligence within many studied populations**, while environmental factors remain powerful and can change both individual development and population averages.


text:
Human intelligence—as usually measured by IQ tests and related cognitive batteries—is **substantially heritable**, but not genetically determined in a simple or fixed way.

## Short answer

In contemporary developed populations, the heritability of measured intelligence is commonly estimated at roughly:

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

These numbers mean that, within a given population and environment, genetic differences account for a sizable share of differences among people in measured cognitive ability. They do **not** mean that 60% of a person’s intelligence is “genetic,” nor that intelligence cannot be changed by environment, education, health, nutrition, or social conditions.

## What “heritability” means

Heritability is a **population-level statistic**: it describes how much of the variation among individuals in a particular population, at a particular time, is associated with genetic variation.

Important caveats:

- High heritability does **not** imply immutability. Height is highly heritable, but nutrition and disease can strongly affect average height.
- Heritability can change across societies. If environments become more equal, genetic differences can explain a larger share of remaining variation.
- Heritability within a group does **not** explain differences between groups or societies.

## Main 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 across many studies:

- Identical twins’ IQ scores are much more similar than fraternal twins’.
- Identical twins reared apart are still highly similar, though not identical.
- Genetic influence tends to increase with age.

A common pattern is that heritability estimates rise from childhood into adulthood. This is sometimes called the **Wilson effect**. One explanation is that as people grow older, they increasingly select, shape, and respond to environments that match their genetic dispositions.

### 2. Adoption studies

Adoption studies provide another line of evidence.

Typical findings:

- Young adopted children often resemble their adoptive families to some degree, reflecting shared environment.
- As adopted children age, their cognitive scores tend to become more similar to those of their biological relatives and less similar to unrelated adoptive siblings.
- By adulthood, the shared family environment often explains relatively little of IQ differences in middle-class samples, though nonshared environment and measurement error still matter.

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

### 3. Molecular genetics

Genome-wide association studies show that intelligence is influenced by **many thousands of genetic variants**, each with tiny effects. There is no “gene for intelligence.”

Molecular methods estimate that common SNPs explain perhaps **20–30%** of variation in intelligence, depending on the sample and method. This is lower than twin-study estimates because common SNP analyses miss rare variants, structural variants, gene–gene interaction, and some other genetic effects.

Polygenic scores can now predict a meaningful but still limited fraction of cognitive and educational outcomes. Their predictive power is strongest in populations similar to the discovery samples, mostly people of European ancestry, and weaker across ancestries because of differences in genetic background, environments, and study representation.

## Environmental evidence

Even though intelligence is heritable, environment clearly matters.

Evidence includes:

- **Education:** Additional schooling tends to improve performance on cognitive tests.
- **Nutrition and health:** Malnutrition, iodine deficiency, prenatal exposures, sleep, and childhood disease can affect cognitive development.
- **Toxins:** Lead exposure and some pollutants are associated with lower cognitive scores.
- **Socioeconomic conditions:** Poverty, stress, instability, and lack of stimulation can affect development.
- **Flynn effect:** Average IQ scores rose substantially across much of the 20th century in many countries, far too quickly to be explained by genetic evolution. This shows that environmental change can shift population averages.

## Shared vs nonshared environment

Behavioral genetic studies often distinguish:

- **Shared environment:** factors making siblings similar, such as family income, neighborhood, parenting style, or school.
- **Nonshared environment:** factors making siblings different, including different peer groups, illnesses, teachers, random developmental events, and differential treatment.

For intelligence:

- Shared environment tends to matter more in childhood.
- Its estimated effect often declines by adulthood in affluent populations.
- Nonshared environment remains important throughout life.

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

## Gene–environment interplay

Genes and environments are not separate, independent forces. They interact and correlate.

Examples:

- A child with a genetic tendency toward curiosity may seek out books and intellectual activities.
- Parents provide both genes and environments, so genetic and environmental influences are partly entangled.
- The effect of genetic differences can be amplified or suppressed by schooling quality, poverty, nutrition, or social opportunity.

This means heritability estimates are not fixed laws of nature.

## Bottom line

The evidence from twin, adoption, and molecular genetic studies shows that **human intelligence is moderately to highly heritable**, especially in adulthood. A reasonable summary is that genetic differences explain around **half or more** of variation in measured intelligence among adults in many modern populations.

But intelligence is also strongly shaped by environment, development, education, health, and opportunity. Heritability does not mean destiny, and it does not tell us how changeable intelligence is or why groups differ in average test scores.


text:
Human intelligence (often measured as general cognitive ability, or **g**) is **substantially heritable**, but it is also strongly influenced by environment, development, education, health, and social conditions.

A good summary is:

> **In modern high-income populations, genetic differences account for roughly 40–80% of variation in measured cognitive ability, with heritability generally increasing from childhood into adulthood.**

But that statement needs careful interpretation.

## What “heritability” means

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

It does **not** mean:

- intelligence is “fixed”;
- environment does not matter;
- an individual’s intelligence is 60% genetic and 40% environmental;
- differences between populations are genetic;
- education, poverty, nutrition, toxins, parenting, or schooling are unimportant.

A trait can be highly heritable and still be environmentally changeable. For example, height is highly heritable, yet nutrition and disease strongly affect height.

## Evidence from twin and adoption studies

The strongest classical evidence comes from twin, sibling, and adoption studies.

### Twin studies

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

Across many studies:

- **Identical twins raised together** are very similar in IQ.
- **Fraternal twins and ordinary siblings** are less similar.
- **Identical twins raised apart** are still substantially similar, though not as similar as those raised together.

Typical estimates:

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

The exact numbers vary by sample, country, socioeconomic range, and method.

### Adoption studies

Adoption studies also support genetic influence. Adopted children’s cognitive scores tend to resemble:

- their **adoptive families** more in childhood;
- their **biological relatives** more as they get older.

This does not mean adoptive environments are irrelevant. Rather, shared family environment appears to have its strongest influence earlier in life, while individual-specific experiences and gene–environment correlations become more important with age.

## Evidence from DNA studies

Modern genomic studies also find genetic influence, but with a more complex picture.

### GWAS findings

Genome-wide association studies show that intelligence and educational attainment are influenced by **many thousands of genetic variants**, each with extremely small effects. There is no single “intelligence gene.”

Polygenic scores derived from DNA can predict some variation in cognitive outcomes and educational attainment, but not nearly all of it.

Roughly:

- Common SNP-based genetic measures explain a meaningful but incomplete portion of variation.
- Polygenic scores for education or cognitive ability may predict on the order of **several percent to perhaps 10–15%** of variance in some samples, depending on the trait and dataset.
- Twin-study heritability estimates are usually higher than what current measured DNA explains, partly because many causal variants are not yet captured well, and because genetic effects are complex.

### Genetic architecture

The evidence suggests intelligence is:

- **highly polygenic**: influenced by many genetic variants;
- affected by both common and rare variants;
- partly overlapping genetically with traits such as educational attainment, brain development, some psychiatric risks, and socioeconomic outcomes;
- influenced by gene–environment correlation, where people partly select or evoke environments related to their genetic tendencies.

## Environment still matters a lot

Even when heritability is high, environment can have large effects.

Important environmental contributors include:

- prenatal health;
- childhood nutrition;
- exposure to lead or other neurotoxins;
- infectious disease burden;
- sleep;
- schooling quality and duration;
- cognitive stimulation;
- stress and trauma;
- poverty and deprivation;
- parental investment;
- neighborhood and peer environments.

Extreme deprivation can severely depress cognitive development. Conversely, interventions such as improved nutrition, reduced lead exposure, better schooling, and early childhood support can improve cognitive outcomes.

The **Flynn effect**—large gains in IQ scores over the 20th century in many countries—also shows that population-level cognitive test performance can change substantially over time, too quickly to be explained by genetic change. Environmental improvements are the likely explanation.

## Heritability can differ by context

Heritability is not a universal constant.

In some studies, especially in the United States, heritability of IQ has appeared lower in children from low-socioeconomic-status backgrounds and higher in more advantaged environments. One interpretation is that poverty and deprivation can suppress genetic potential, while enriched environments allow genetic differences to account for more observed variation.

However, findings differ across countries. In societies with stronger safety nets and less extreme deprivation, socioeconomic moderation of heritability may be smaller.

## What the evidence does *not* show

The evidence does not justify simple genetic determinism.

It does not show that:

- IQ is unchangeable;
- education is futile;
- social policy cannot affect cognitive outcomes;
- individuals can be accurately judged from genetic information;
- group differences in IQ are genetic in origin.

Heritability within a group says little about the causes of average differences between groups. For example, if one population has worse schooling or more toxin exposure, its average scores may differ for environmental reasons even if intelligence is highly heritable within each population.

## Bottom line

Human intelligence is **moderately to highly heritable**, especially by adulthood. Twin, adoption, family, and genomic studies all support a substantial genetic contribution.

But intelligence is also **meaningfully shaped by environment**, and heritability does not imply immutability. The best reading of the evidence is:

> Genetic differences are an important source of individual differences in intelligence, but cognitive development arises from a complex interaction of many genes with many environmental influences across life.


text:
Human intelligence, as measured by IQ tests and related cognitive batteries, is **substantially heritable**, but not genetically fixed. The evidence suggests that **genetic differences explain a meaningful share of individual differences in cognitive ability within a given population**, while environment also matters a great deal.

## Short answer

A reasonable summary of the evidence is:

- **Childhood:** heritability roughly **20–40%**
- **Adolescence:** roughly **40–60%**
- **Adulthood:** often **50–80%**, with many estimates around **50–70%**

These numbers mean that, in a particular population at a particular time, genetic differences account for that proportion of the variation in measured cognitive ability. They do **not** mean that an individual’s intelligence is “50–80% genetic,” nor that intelligence is unchangeable.

## What the evidence shows

### 1. Twin studies

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

A consistent finding is:

- **Identical twins are more similar in IQ than fraternal twins**
- This remains true even when twins are raised apart, though such samples are smaller and more selective

Typical correlations are roughly:

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

These patterns strongly suggest a genetic contribution.

### 2. Adoption studies

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

- Their **adoptive families** more in early childhood
- Their **biological relatives** more strongly as they age

This supports the idea that both family environment and genetics matter, but that genetic influence becomes more visible over development.

### 3. DNA-based evidence

Modern genomic studies also support heritability.

Using measured DNA directly, researchers find that common genetic variants together explain a significant fraction of variation in cognitive performance and educational outcomes.

Important points:

- There is no “intelligence gene.”
- Intelligence is **highly polygenic**: influenced by thousands of genetic variants, each with tiny effects.
- Polygenic scores can predict some variation in cognitive and educational outcomes, but they are far from deterministic and currently work best in populations similar to those used to build them, especially European-ancestry samples.

DNA-based estimates are often lower than twin-study estimates, partly because current methods capture only some kinds of genetic variation.

## Why heritability increases with age

One of the most robust findings is that IQ heritability tends to increase from childhood to adulthood.

Possible reasons include:

1. **Gene–environment correlation**
   As people grow up, they increasingly select and shape environments that fit their interests and abilities. For example, a child with an inclination toward reading may read more, which further strengthens verbal ability.

2. **Greater autonomy**
   Adults have more freedom to choose schools, jobs, hobbies, friends, and intellectual environments.

3. **Shared family environment becomes less influential**
   The effect of growing up in the same household appears larger in childhood and smaller in adulthood.

## Environmental effects are real

High heritability does not mean the environment is unimportant.

Environmental factors known or strongly suspected to affect cognitive development include:

- Prenatal health
- Nutrition
- Exposure to toxins such as lead
- Childhood illness
- Quality of schooling
- Family stress
- Sleep
- Cognitive stimulation
- Poverty and deprivation
- Social and emotional stability

The **Flynn effect**—large IQ gains across many countries during the 20th century—also shows that population-level cognitive performance can change substantially over time for environmental reasons.

## What heritability does not mean

Heritability is often misunderstood.

It does **not** mean:

- Intelligence is fixed
- Education does not matter
- Environment does not matter
- Genetic influence is the same in every society
- Differences between groups are genetic
- An individual’s IQ is determined “X% by genes”

Heritability is about **variation within a population**, not about the causes of average differences between populations or historical groups.

For example, height is highly heritable, but average height has changed dramatically over time because of nutrition and health. Similarly, IQ can be heritable while still being environmentally influenced.

## Bottom line

The evidence from twin, adoption, family, and DNA studies converges on the conclusion that **human intelligence is moderately to highly heritable**, especially by adulthood. A typical adult estimate is around **50–70% heritable**, depending on the sample and method.

But intelligence is also shaped by environment, development, culture, education, health, and opportunity. Genes matter substantially, but they do not operate in isolation, and they do not make cognitive ability fixed or immune to environmental change.