text:
# Evolution by Natural Selection

Evolution by natural selection is the process by which populations of organisms change over generations. It was famously formulated by Charles Darwin and Alfred Russel Wallace in the mid-19th century.

## The Core Mechanism

Natural selection works through four key conditions:

**1. Variation**
Individuals within a population differ from one another in their traits (size, color, behavior, disease resistance, etc.). These differences arise from genetic mutations and the reshuffling of genes during reproduction.

**2. Heritability**
Many of these traits can be passed from parents to offspring through genes.

**3. Differential Survival and Reproduction**
More offspring are produced than the environment can support. Individuals must compete for limited resources like food, mates, and space.

**4. Selection**
Individuals with traits better suited to their environment tend to survive longer and produce more offspring. Their advantageous traits become more common in the next generation.

## A Classic Example: Peppered Moths

Before the Industrial Revolution in England, most peppered moths were light-colored, blending in with lichen-covered trees. As pollution darkened tree bark, dark-colored moths became better camouflaged from predators. Within decades, the dark variety dominated populations in industrial areas—a measurable shift driven by natural selection.

## Important Clarifications

- **It's not random**: Mutations are random, but selection is *not*—it consistently favors traits that improve survival and reproduction in a given environment.
- **It has no goal**: Evolution doesn't aim toward "better" or more complex organisms; it simply favors what works *now*.
- **It acts on populations, not individuals**: A single organism doesn't evolve—populations do, over generations.
- **Environment matters**: A trait advantageous in one environment may be harmful in another.

## The Bigger Picture

Over vast timescales, natural selection (combined with genetic drift, mutation, and migration) can produce dramatic changes—including the formation of entirely new species. It explains the remarkable diversity of life and why organisms appear so well-adapted to their environments.


prompt_id: sci-evolution

provider: anthropic

text:
# Evolution by Natural Selection

Evolution by natural selection is the process by which populations of organisms change over generations. It was developed primarily by Charles Darwin and Alfred Russel Wallace in the mid-1800s.

## The Core Mechanism

Natural selection works through four key conditions:

**1. Variation**
Individuals within a population differ from one another in their traits (size, color, speed, disease resistance, etc.). Much of this variation is heritable, passed from parents to offspring through genes.

**2. Inheritance**
Traits are passed from parents to offspring via DNA. New variations arise through mutations and the shuffling of genes during sexual reproduction.

**3. Differential Survival and Reproduction**
Organisms produce more offspring than the environment can support. This creates competition for limited resources like food, mates, and shelter. Individuals with traits better suited to their environment tend to survive longer and produce more offspring.

**4. Time**
Over many generations, advantageous traits become more common in the population, while disadvantageous traits become rarer. This gradual shift is evolution.

## A Classic Example

Consider a population of beetles where some are green and some are brown, living on brown tree bark:
- Birds spot and eat the green beetles more easily
- Brown beetles survive longer and produce more offspring
- Brown offspring inherit the trait
- Over generations, the population becomes predominantly brown

## Key Points to Understand

- **It's not "survival of the fittest"** in a brute-strength sense—"fitness" means reproductive success in a particular environment
- **It has no goal or direction**—organisms don't evolve toward "perfection," only toward what works in current conditions
- **It acts on individuals but changes populations**—a single beetle doesn't evolve; the population's composition shifts
- **Environments change**, so traits that are advantageous now may not be later

Over millions of years, accumulated changes can produce entirely new species—a process called **speciation**—which explains the incredible diversity of life on Earth.


text:
# Evolution by Natural Selection

Evolution by natural selection is the process by which populations of organisms change over generations. It rests on a few simple observations that, combined, lead to a powerful conclusion.

## The Core Mechanism

Natural selection requires four key conditions:

1. **Variation** — Individuals within a population differ from one another in their traits (size, color, behavior, disease resistance, etc.).

2. **Heritability** — Some of these traits are passed from parents to offspring through genes.

3. **Differential survival and reproduction** — More offspring are produced than can survive, and individuals with certain traits tend to survive and reproduce more successfully in their environment.

4. **Time** — Over many generations, the traits that improve survival and reproduction become more common in the population.

## A Simple Example

Imagine a population of beetles living on dark tree bark:
- Some beetles are green, others are brown (variation).
- Color is inherited from parents (heritability).
- Birds spot green beetles more easily and eat them; brown beetles survive longer and produce more offspring (differential reproduction).
- Over generations, brown beetles become more common.

The population has evolved—not because individual beetles changed color, but because the proportion of brown beetles increased.

## Important Clarifications

- **Individuals don't evolve; populations do.** A single beetle can't change its color, but a population's average color can shift over time.
- **It's not "survival of the fittest" in a brutal sense.** "Fitness" simply means reproductive success in a particular environment. A trait that's beneficial in one setting may be harmful in another.
- **There's no goal or direction.** Natural selection doesn't aim toward "better" or "more complex." It only favors what works *right now* in the current environment.
- **Mutation provides raw material.** Random genetic mutations create new variations; selection then filters them.

## The Bigger Picture

Given enough time, natural selection can produce remarkable adaptations—the camouflage of a stick insect, the echolocation of bats, the human eye. When populations are separated and face different pressures, they can diverge enough to become entirely new species.

First proposed by Charles Darwin and Alfred Russel Wallace in 1858, this mechanism remains one of the most well-supported theories in science, confirmed by evidence from genetics, paleontology, biogeography, and direct observation of evolution in action (such as antibiotic resistance in bacteria).


text:
# Evolution by Natural Selection

Evolution by natural selection is the process by which populations of organisms change over generations. It was most famously articulated by Charles Darwin in *On the Origin of Species* (1859).

## The Core Mechanism

Natural selection works through four key conditions:

**1. Variation**
Individuals within a population differ from one another in their traits (size, color, speed, disease resistance, etc.). Much of this variation arises from random genetic mutations and the shuffling of genes during reproduction.

**2. Inheritance**
Many of these traits are heritable—they can be passed from parents to offspring through genes.

**3. Differential Survival and Reproduction**
More offspring are born than can survive. Individuals with traits better suited to their environment tend to survive longer and produce more offspring. This is often summarized as "survival of the fittest"—where "fitness" means reproductive success, not physical strength.

**4. Accumulation Over Time**
Because successful individuals pass on their advantageous traits, those traits become more common in the population over generations. Over long periods, this can lead to significant changes—even the emergence of new species.

## A Classic Example: Peppered Moths

Before the Industrial Revolution in England, most peppered moths were light-colored, blending in with lichen-covered trees. As pollution darkened tree bark, dark-colored moths (a rare variant) became better camouflaged from predators. Within decades, dark moths dominated industrial areas. When pollution decreased, light moths rebounded.

## Key Points to Remember

- **Natural selection acts on individuals, but populations evolve.** A single organism doesn't evolve in its lifetime.
- **It's not goal-directed.** Evolution doesn't strive toward perfection or complexity—it simply favors what works in a given environment.
- **Environment is crucial.** A trait that's beneficial in one setting may be harmful in another.
- **It requires time.** Major changes typically take many generations.

## Beyond Natural Selection

Natural selection is one mechanism of evolution, but others contribute too, including **genetic drift** (random changes in gene frequencies), **gene flow** (movement of genes between populations), and **mutation** (the ultimate source of new variation).

Together, these forces have shaped the extraordinary diversity of life on Earth over roughly 3.8 billion years.


text:
# Evolution by Natural Selection

Evolution by natural selection is the process by which populations of organisms change over generations. It was famously described by Charles Darwin and Alfred Russel Wallace in the 19th century. It rests on a few simple observations that, taken together, produce a powerful result.

## The Core Mechanism

Natural selection works through four key conditions:

**1. Variation**
Individuals within a population differ from one another. These differences include traits like size, color, speed, disease resistance, or behavior. Much of this variation is heritable, passed from parents to offspring through genes.

**2. Inheritance**
Traits are passed down through DNA. Offspring tend to resemble their parents because they inherit their genetic material, though mutations and genetic recombination introduce new variations.

**3. Differential Survival and Reproduction**
More offspring are produced than the environment can support. Individuals with traits better suited to their environment tend to survive longer and reproduce more successfully. This is sometimes called "survival of the fittest" — where "fitness" means reproductive success, not physical strength.

**4. Accumulation Over Time**
Because successful individuals pass their advantageous traits to more offspring, those traits become more common in the population over generations. Over long timescales, this can lead to significant changes — and eventually to the formation of new species.

## A Simple Example

Imagine a population of beetles living on tree bark:
- Some are green, some are brown (**variation**)
- Color is determined by genes (**inheritance**)
- Birds eat the more visible green beetles more often (**differential survival**)
- Brown beetles survive and reproduce more, so brown becomes increasingly common (**accumulation**)

After many generations, the population may be almost entirely brown.

## Important Clarifications

- **It's not random.** While mutations are random, selection is not — it consistently favors traits that improve reproductive success in a given environment.
- **It has no goal.** Evolution doesn't aim toward "better" or "more complex" organisms; it simply responds to current environmental pressures.
- **It acts on populations, not individuals.** A single organism doesn't evolve in its lifetime; populations change across generations.
- **Environments change.** A trait that's beneficial today may be harmful tomorrow if conditions shift.

## Evidence

Natural selection is supported by fossil records, comparative anatomy, embryology, biogeography, and direct observation (e.g., antibiotic resistance in bacteria, Darwin's finches, peppered moths during the Industrial Revolution). It's a foundational principle of modern biology.


text:
# Evolution by Natural Selection

Evolution by natural selection is the process by which populations of organisms change over generations. It was famously described by Charles Darwin and Alfred Russel Wallace in the mid-1800s, and it rests on a few simple observations.

## The Core Ingredients

**1. Variation**
Individuals within a population are not identical. They differ in traits like size, color, speed, disease resistance, and countless other characteristics. Much of this variation is heritable, meaning it can be passed from parents to offspring through genes.

**2. Overproduction**
Most organisms produce more offspring than can possibly survive. A single oak tree drops thousands of acorns; a frog lays hundreds of eggs. Resources like food, space, and mates are limited.

**3. Differential Survival and Reproduction**
Because of limited resources, individuals must compete (directly or indirectly). Those whose traits give them even a slight advantage—better camouflage, sharper senses, more efficient metabolism—tend to survive longer and produce more offspring.

**4. Inheritance**
The advantageous traits, being heritable, are passed to the next generation in greater numbers than less advantageous traits.

## The Result

Over many generations, beneficial traits become more common in the population, while disadvantageous ones become rarer. The population gradually becomes better adapted to its environment. Given enough time, these changes can produce entirely new species.

## A Classic Example: Peppered Moths

Before the Industrial Revolution in England, most peppered moths were light-colored, blending in with lichen-covered trees. Dark moths were easily spotted by birds and eaten. When pollution killed lichens and darkened tree bark, the light moths suddenly stood out, while dark moths were camouflaged. Within decades, dark moths dominated the population. When pollution decreased, the light form rebounded.

## Key Points to Remember

- Natural selection acts on **individuals**, but **populations** evolve.
- It is **not random**—though mutations (the source of new variation) are random, selection is not.
- It has **no goal or foresight**; it simply favors what works *now* in the current environment.
- "Fitness" means **reproductive success**, not strength or intelligence.

In short: variation + inheritance + competition + time = evolution.


text:
# Evolution by Natural Selection

Evolution by natural selection is the process by which populations of organisms change over generations. It was famously described by Charles Darwin and Alfred Russel Wallace in the 19th century, and it rests on a few simple, observable facts.

## The Core Mechanism

Natural selection works through four key conditions:

**1. Variation**
Individuals within a population differ from one another in their traits—size, color, speed, disease resistance, etc. Much of this variation arises from random genetic mutations and the reshuffling of genes during reproduction.

**2. Inheritance**
Many traits are heritable, meaning they can be passed from parents to offspring through genes.

**3. Differential Survival and Reproduction**
More offspring are produced than can survive, given limited resources like food, space, and mates. Individuals must compete (directly or indirectly) to survive and reproduce.

**4. Selection**
Individuals with traits better suited to their environment tend to survive longer and produce more offspring. Those advantageous traits become more common in the next generation.

Over many generations, this gradual shift in trait frequencies can lead to populations becoming well-adapted to their environments—and eventually to the emergence of entirely new species.

## A Classic Example: Peppered Moths

During the Industrial Revolution in England, soot darkened tree bark. Light-colored peppered moths became easy targets for birds, while rare dark-colored variants were better camouflaged. Within decades, dark moths dominated populations in polluted areas. When pollution decreased, light moths rebounded. This is natural selection observed in real time.

## Important Clarifications

- **It's not random.** Mutations are random, but selection is not—it consistently favors traits that improve survival and reproduction in a given environment.
- **There's no goal or "progress."** Evolution doesn't aim toward complexity or perfection; it simply favors what works *now*, in the current environment.
- **It acts on populations, not individuals.** An individual organism doesn't evolve—populations do, across generations.
- **"Fittest" means best-suited, not strongest.** Fitness in biology refers to reproductive success.

## Why It Matters

Natural selection explains the astonishing diversity of life, the close fit between organisms and their environments, and phenomena from antibiotic resistance in bacteria to the beaks of Darwin's finches. It remains one of the most thoroughly tested and supported theories in science.


text:
# Evolution by Natural Selection

Evolution by natural selection is the process by which populations of organisms change over generations. Charles Darwin and Alfred Russel Wallace independently developed the theory in the mid-1800s. It rests on a few simple observations that, taken together, produce powerful results.

## The Core Mechanism

Natural selection requires four conditions:

1. **Variation** — Individuals within a population differ from one another (in size, color, behavior, immunity, etc.).

2. **Heritability** — Some of that variation is passed from parents to offspring through genes.

3. **Differential survival and reproduction** — More offspring are produced than the environment can support, so individuals compete for resources, mates, and survival.

4. **Selection** — Individuals with traits better suited to their environment tend to survive longer and produce more offspring, passing those traits on.

Over many generations, beneficial traits become more common in the population, while less useful ones fade. The population gradually adapts to its environment.

## A Classic Example: Peppered Moths

Before the Industrial Revolution in England, most peppered moths were light-colored, blending in with lichen-covered trees. A rare dark variant existed but was easily spotted by birds.

When pollution killed the lichens and darkened tree bark, the dark moths suddenly had the advantage — they were camouflaged, while the light ones became easy prey. Within decades, dark moths dominated the population. When air quality improved, the trend reversed.

## Key Points to Remember

- **Individuals don't evolve; populations do.** A single moth doesn't change color — the *proportion* of colors in the population shifts.
- **There's no goal or direction.** Natural selection isn't striving toward perfection; it just favors what works *now* in the current environment.
- **Mutations supply the raw material.** Random genetic changes create new variation, and selection then filters which variations spread.
- **It takes time.** Major changes typically unfold over thousands or millions of generations, though rapid evolution can occur (as in bacteria evolving antibiotic resistance).

## Why It Matters

Natural selection explains the diversity of life, the close fit between organisms and their environments, and phenomena like antibiotic resistance, pesticide resistance, and the emergence of new viral strains. It's one of the most well-supported theories in science, backed by evidence from genetics, fossils, biogeography, and direct observation in the lab and field.


text:
# Evolution by Natural Selection

Evolution by natural selection is the process by which populations of organisms change over generations because individuals with certain traits are more likely to survive and reproduce. It rests on a few simple observations and logical conclusions.

## The Core Mechanism

Natural selection works through four key conditions:

1. **Variation** — Individuals within a population differ from one another in their traits (size, color, speed, disease resistance, etc.).

2. **Heritability** — Many of these traits are passed from parents to offspring through genes.

3. **Differential survival and reproduction** — More offspring are produced than can survive, so individuals compete for limited resources (food, mates, shelter). Some variants are better suited to the current environment than others.

4. **Accumulation over time** — Individuals with advantageous traits tend to leave more offspring. Over many generations, those traits become more common in the population.

## A Simple Example

Imagine a population of beetles living on dark tree bark:
- Some beetles are green, others are brown (variation)
- Beetle color is inherited (heritability)
- Birds easily spot green beetles against the bark and eat them, while brown beetles blend in and survive (differential survival)
- Surviving brown beetles reproduce, passing on the brown color
- After many generations, the population becomes mostly brown

## Important Clarifications

- **It's not random.** Mutations that create variation arise randomly, but selection itself is not random—it favors traits suited to the environment.
- **There's no goal or "progress."** Evolution doesn't aim toward complexity or perfection; it just favors what works in the current environment.
- **Individuals don't evolve—populations do.** A single beetle can't change its color, but the proportion of colors in the population shifts over generations.
- **"Fitness" means reproductive success**, not strength or intelligence. A fit organism is simply one that leaves many surviving offspring.

## Evidence

Natural selection is supported by:
- **Fossil records** showing gradual changes in species over time
- **Direct observation**, such as antibiotic resistance in bacteria or beak changes in Galápagos finches during droughts
- **Genetic evidence** linking species through shared DNA
- **Artificial selection**, where humans have shaped dogs, crops, and livestock using the same principle

Over vast timescales, this process—combined with genetic drift, mutation, and migration—accounts for the extraordinary diversity of life on Earth.


text:
# Evolution by Natural Selection

Evolution by natural selection is the process by which populations of organisms change over generations. It rests on a few simple observations that, taken together, produce powerful results.

## The Core Mechanism

Natural selection requires four key ingredients:

**1. Variation**
Individuals within a population differ from one another—in size, color, speed, disease resistance, behavior, and countless other traits.

**2. Heritability**
Some of this variation is passed from parents to offspring through genes. Offspring tend to resemble their parents more than they resemble random members of the population.

**3. Differential Survival and Reproduction**
Resources are limited, and not all individuals survive or reproduce equally. Some traits give their bearers a better chance of surviving long enough to reproduce in a given environment.

**4. Accumulation Over Generations**
Because advantageous traits are heritable, they become more common in the next generation. Repeated over many generations, this process can produce dramatic changes.

## A Simple Example

Imagine a population of beetles living on dark tree bark:
- Some beetles are darker, some lighter (variation)
- Color is inherited from parents (heritability)
- Birds spot lighter beetles more easily and eat them, while darker beetles survive to reproduce (differential survival)
- Over generations, the population becomes predominantly dark (accumulation)

If the environment changes—say, lichen makes the bark pale—the selection pressure reverses, and lighter beetles will become more common.

## Important Clarifications

- **It's not random.** Mutations that create variation are random, but selection itself is not—it consistently favors traits suited to the environment.
- **It has no goal or foresight.** Evolution doesn't aim toward "better" or "more complex." It only favors what works *right now* in a particular environment.
- **It acts on populations, not individuals.** A single beetle doesn't evolve; the frequency of traits in the population changes.
- **It's gradual but not always slow.** Bacteria can evolve antibiotic resistance in days; large animals typically change over thousands or millions of years.

## The Bigger Picture

Over vast timescales, this simple process—combined with genetic mutation, migration, and occasional chance events—has produced the entire diversity of life on Earth, from bacteria to whales to redwoods, all descended from common ancestors.


text:
# Evolution by Natural Selection

Evolution by natural selection is the process by which populations of organisms change over generations. It was famously described by Charles Darwin and Alfred Russel Wallace in the 19th century. The mechanism rests on a few simple observations and logical conclusions.

## The Core Requirements

Natural selection occurs whenever these four conditions are met:

1. **Variation** — Individuals within a population differ from one another (in size, color, behavior, etc.).
2. **Heritability** — Some of that variation can be passed from parents to offspring through genes.
3. **Differential reproduction** — More offspring are produced than can survive, leading to competition for resources, mates, and survival.
4. **Selection** — Some traits give individuals a better chance of surviving and reproducing in their environment.

## How It Works Step by Step

1. **Random variation arises** through genetic mutations, recombination during sexual reproduction, and gene flow between populations.
2. **Individuals with helpful traits** (for their particular environment) tend to survive longer and produce more offspring.
3. **Those advantageous traits become more common** in the next generation because they're inherited.
4. **Over many generations**, the population's average traits shift, sometimes dramatically.

## A Classic Example

Consider the **peppered moth** in Industrial Revolution England:
- Originally, most moths were light-colored, blending in with lichen-covered trees.
- When pollution darkened tree bark, light moths became easy prey for birds.
- Rare dark-colored moths now had the survival advantage.
- Within decades, dark moths dominated the population.
- When pollution decreased, light moths returned to prevalence.

## Important Clarifications

- **Natural selection is not random.** Mutations are random, but which traits get "selected" depends on the environment.
- **There's no goal or direction.** Evolution doesn't aim toward "better" or more complex organisms—just whatever works in current conditions.
- **It acts on individuals, but populations evolve.** A single organism doesn't evolve; the frequency of traits in a population changes over generations.
- **Time scales vary.** Some changes happen in a few generations (like bacterial antibiotic resistance), while others take millions of years (like the evolution of whales from land mammals).

Over vast time spans, this simple process can produce remarkable biodiversity, from microscopic bacteria to blue whales—all sharing common ancestors and shaped by their unique evolutionary histories.


text:
# Evolution by Natural Selection

Evolution by natural selection is the process by which populations of organisms change over generations. It was famously articulated by Charles Darwin and Alfred Russel Wallace in the mid-1800s.

## The Core Mechanism

Natural selection works through four key conditions:

**1. Variation**
Individuals within a population differ from one another in their traits (size, color, speed, disease resistance, etc.). Much of this variation is heritable, arising from genetic differences.

**2. Inheritance**
Many of these traits are passed from parents to offspring through genes.

**3. Differential Survival and Reproduction**
More offspring are produced than can survive. Individuals with traits better suited to their environment tend to survive longer and produce more offspring.

**4. Accumulation Over Time**
Because successful traits are inherited, they become more common in the next generation. Over many generations, populations gradually change.

## A Simple Example

Imagine a population of beetles living on dark tree bark:
- Some beetles are brown, some are green (variation)
- Color is inherited from parents (inheritance)
- Birds spot and eat green beetles more easily, so brown beetles survive and reproduce more (differential survival)
- Over generations, the population becomes mostly brown (accumulation)

## Key Points to Understand

- **Selection acts on individuals, but populations evolve.** A single beetle doesn't change color—the proportion of colors in the population shifts.
- **It's not "survival of the fittest" in a dramatic sense.** "Fitness" simply means reproductive success in a given environment.
- **It's not goal-directed.** Evolution doesn't aim for "better" or more complex organisms—just those that reproduce successfully in current conditions.
- **The environment determines what's advantageous.** A trait helpful in one environment may be harmful in another.

## Beyond Natural Selection

Natural selection is one mechanism of evolution, alongside others like genetic drift (random changes), mutation (the ultimate source of variation), and gene flow (migration between populations). Together, these processes account for the diversity of life on Earth.


text:
# Evolution by Natural Selection

Evolution by natural selection is the process by which populations of organisms change over generations. It was first clearly articulated by Charles Darwin in 1859. The mechanism rests on a few simple observations and logical conclusions.

## The Core Requirements

For natural selection to occur, four conditions must be met:

1. **Variation** — Individuals in a population differ from one another in their traits (size, color, behavior, disease resistance, etc.).

2. **Heritability** — Some of that variation is passed from parents to offspring through genes.

3. **Differential survival and reproduction** — More offspring are produced than can survive, and individuals with certain traits are more likely to survive and reproduce in a given environment.

4. **Time** — Over many generations, advantageous traits become more common in the population.

## How It Works in Practice

Imagine a population of beetles living on dark tree bark:
- Some beetles are brown, others are green (variation).
- Color is inherited from parents (heritability).
- Birds eat beetles, but green ones stand out more and get eaten more often (differential survival).
- Brown beetles survive longer, reproduce more, and pass on "brown" genes.
- After many generations, the population becomes predominantly brown.

The beetles didn't *try* to become brown — those that already happened to be brown simply left more descendants.

## Key Points Often Misunderstood

- **No goal or direction.** Evolution isn't striving toward "perfection" or complexity. It just favors what works in current conditions.
- **It acts on populations, not individuals.** A single beetle doesn't evolve; the population's gene frequencies shift over time.
- **Mutations supply new variation.** Random genetic changes create the raw material that selection acts upon.
- **Environment defines "fitness."** A trait helpful in one environment may be harmful in another. When conditions change, what gets selected for changes too.

## The Bigger Picture

Over vast time spans, accumulated changes can produce new species, especially when populations become isolated and adapt to different environments. Combined with other mechanisms (genetic drift, mutation, gene flow), natural selection accounts for the diversity of life we see today — from bacteria to blue whales.


text:
# Evolution by Natural Selection

Evolution by natural selection is the process by which populations of organisms change over generations. It was famously described by Charles Darwin and Alfred Russel Wallace in the 19th century. The process rests on a few simple observations:

## The Core Mechanism

**1. Variation**
Individuals within a population differ from one another. These differences appear in traits like size, color, speed, disease resistance, and behavior. Much of this variation is heritable, passed from parents to offspring through genes.

**2. Overproduction**
Most species produce more offspring than can survive to adulthood. Resources like food, water, mates, and shelter are limited.

**3. Competition and Differential Survival**
Because resources are scarce, individuals compete. Those with traits better suited to their environment are more likely to survive and reproduce. This is sometimes called "survival of the fittest"—where "fitness" means reproductive success, not physical strength.

**4. Inheritance**
The advantageous traits get passed to the next generation. Over many generations, helpful traits become more common in the population, while disadvantageous ones become rarer.

## A Classic Example

The peppered moth in England illustrates this nicely:
- Before the Industrial Revolution, most moths were light-colored, blending with lichen-covered trees.
- As pollution darkened tree bark, dark-colored moths (a rare variant) became better camouflaged.
- Birds ate more of the now-visible light moths.
- Dark moths survived and reproduced more, so within decades, dark moths dominated the population.

## Key Points to Understand

- **Evolution acts on populations, not individuals.** An individual doesn't evolve; its descendants' gene frequencies change.
- **There's no goal or direction.** Evolution doesn't aim for "better" or "more complex"—just for what works in the current environment.
- **Mutation supplies new variation.** Random genetic mutations create the raw material that selection acts on.
- **Environment is the filter.** What's advantageous depends entirely on local conditions, which can change over time.

Given enough time, these small generational changes accumulate, eventually producing new species and the immense diversity of life we see today.


text:
# Evolution by Natural Selection

Evolution by natural selection is the process by which populations of organisms change over generations. It was famously articulated by Charles Darwin in *On the Origin of Species* (1859). The mechanism rests on a few simple observations and a logical conclusion.

## The Core Requirements

Natural selection occurs whenever these four conditions are met:

1. **Variation** – Individuals in a population differ from one another (in size, color, behavior, disease resistance, etc.).
2. **Heritability** – Some of those differences are passed from parents to offspring through genes.
3. **Differential survival and reproduction** – More offspring are produced than the environment can support, so individuals compete for resources, mates, and survival.
4. **Selection pressure** – Some traits give their bearers a better chance of surviving and reproducing in a given environment.

## The Logical Result

If certain heritable traits help individuals survive and reproduce more successfully, those traits will become more common in the next generation. Over many generations, this gradual shift in trait frequencies can lead to significant changes in a population — and eventually to the emergence of new species.

## A Classic Example: Peppered Moths

Before the Industrial Revolution in England, most peppered moths were light-colored, blending in with lichen-covered trees. When pollution darkened the tree bark, dark-colored moths (which had always existed as rare variants) became better camouflaged from birds. Within a few decades, dark moths dominated industrial areas. When pollution was reduced, light moths rebounded. The environment "selected" different traits at different times.

## Key Points Often Misunderstood

- **Natural selection isn't random.** Mutation produces random variation, but selection itself is non-random — it consistently favors traits that improve reproductive success.
- **Organisms don't choose to evolve.** A giraffe doesn't stretch its neck to make its offspring taller. Rather, taller-necked individuals happened to leave more offspring.
- **Evolution has no goal or direction.** "Fitness" simply means reproductive success in a particular environment — not strength, intelligence, or complexity.
- **It happens to populations, not individuals.** A single organism doesn't evolve; the genetic makeup of a population shifts over time.

## Beyond Natural Selection

Natural selection is the main driver of adaptation, but other mechanisms also shape evolution, including **genetic drift** (random changes in gene frequencies), **gene flow** (migration between populations), and **mutation** (the ultimate source of new variation).

Together, these processes explain the extraordinary diversity of life on Earth — all unified by descent from common ancestors.


text:
# Evolution by Natural Selection

Evolution by natural selection is the process by which populations of organisms change over generations. It was famously described by Charles Darwin and rests on a few simple but powerful observations.

## The Core Mechanism

Natural selection works through four key conditions:

**1. Variation**
Individuals within a population differ from one another in their traits (size, color, speed, disease resistance, etc.). These differences arise largely from random genetic mutations and the shuffling of genes during reproduction.

**2. Inheritance**
Many of these traits are heritable—they can be passed from parents to offspring through genes.

**3. Differential Survival and Reproduction**
More individuals are born than can survive. Some traits give individuals a better chance of surviving and reproducing in their particular environment. This is often summarized as "survival of the fittest," where "fitness" means reproductive success, not physical strength.

**4. Accumulation Over Time**
Because individuals with advantageous traits leave more offspring, those traits become more common in the next generation. Over many generations, this gradual shift can produce significant changes in a population.

## A Classic Example: The Peppered Moth

Before the Industrial Revolution in England, most peppered moths were light-colored, blending in with lichen-covered trees. When pollution darkened the trees, dark moths became better camouflaged from predators. Within decades, dark moths dominated the population. When pollution decreased, light moths returned to prominence.

## Important Clarifications

- **It's not random.** Mutations are random, but selection itself is non-random—the environment "filters" which traits succeed.
- **It has no goal.** Evolution doesn't aim toward "better" or "more complex" organisms; it simply favors what works in a given environment.
- **It acts on populations, not individuals.** An individual doesn't evolve; populations evolve as gene frequencies shift over generations.
- **It requires time.** Significant change usually takes many generations, though it can be observed quickly in fast-reproducing organisms like bacteria (e.g., antibiotic resistance).

Over millions of years, this simple process has produced the extraordinary diversity of life we see today.


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# Evolution by Natural Selection

Evolution by natural selection is the process by which populations of organisms change over generations. It was famously articulated by Charles Darwin and Alfred Russel Wallace in the mid-1800s. The mechanism relies on a few key ingredients:

## The Core Requirements

1. **Variation** — Individuals within a population differ from one another in their traits (size, color, speed, disease resistance, etc.).

2. **Heritability** — Some of that variation is passed from parents to offspring through genes.

3. **Differential survival and reproduction** — Some traits make individuals more likely to survive and reproduce in their particular environment.

4. **Time** — These small differences accumulate across many generations.

## How It Works in Practice

In any environment, resources like food, mates, and shelter are limited, so not all individuals can survive and reproduce equally. Individuals with traits better suited to their environment tend to leave more offspring. Because those traits are heritable, the next generation contains a higher proportion of individuals with the advantageous traits. Over many generations, the population's characteristics shift.

## A Classic Example

Imagine a population of beetles living on dark tree bark:
- Some beetles are green, some are brown (variation).
- Color is inherited from parents (heritability).
- Birds spot green beetles more easily and eat them, so brown beetles survive and reproduce more often (differential reproduction).
- Over generations, the population becomes increasingly brown.

## Important Clarifications

- **Natural selection acts on individuals, but populations evolve.** A single beetle doesn't evolve; the frequency of traits in the population changes.
- **It's not goal-directed.** Evolution doesn't "try" to produce better organisms — it simply reflects which traits happened to work in a given environment.
- **"Fitness" means reproductive success**, not strength or intelligence per se.
- **Environments change**, so a beneficial trait today may be neutral or harmful tomorrow.

Over very long timescales, this process — combined with mutation, genetic drift, and geographic isolation — can produce entirely new species and the vast diversity of life we see today.


text:
# Evolution by Natural Selection

Evolution by natural selection is the process by which populations of organisms change over generations. It was famously articulated by Charles Darwin and Alfred Russel Wallace in the mid-19th century.

## The Core Mechanism

Natural selection works through four key conditions:

**1. Variation**
Individuals within a population differ from one another. These differences appear in traits like size, color, speed, disease resistance, and behavior.

**2. Heritability**
Some of this variation is genetic and can be passed from parents to offspring through DNA.

**3. Differential Survival and Reproduction**
More individuals are born than can survive. Those with traits better suited to their environment tend to survive longer and produce more offspring.

**4. Accumulation Over Generations**
Beneficial traits become more common in the population over time, while less advantageous traits become rarer.

## A Classic Example: Peppered Moths

Before the Industrial Revolution in England, most peppered moths were light-colored, blending in with lichen-covered trees. When pollution darkened the trees:
- Light moths became easy targets for birds
- Dark-colored moth variants (which already existed in small numbers) survived better
- Within decades, dark moths dominated the population

When pollution decreased, the trend reversed.

## Key Points to Understand

- **Selection acts on individuals, but populations evolve.** A single organism doesn't evolve in its lifetime; the genetic makeup of the group changes across generations.

- **It's not random.** While mutations that create variation are random, selection itself is non-random—it favors traits that improve survival and reproduction in a specific environment.

- **There's no "goal."** Evolution doesn't aim toward perfection or complexity. It simply favors what works *now* in the *current* environment.

- **"Fitness" means reproductive success**, not strength or athleticism. The "fittest" organism is the one that leaves the most viable offspring.

## Beyond Natural Selection

Natural selection is one mechanism of evolution, but others include genetic drift (random changes in allele frequencies), gene flow (migration between populations), and mutation (the ultimate source of new variation).

Together, these processes have produced the vast diversity of life on Earth over billions of years.


text:
# Evolution by Natural Selection

Evolution by natural selection is the process by which populations of organisms change over generations. It rests on a few simple observations and their logical consequence.

## The Core Requirements

Natural selection happens whenever three conditions are met:

1. **Variation** — Individuals in a population differ from one another (in size, color, behavior, disease resistance, etc.).
2. **Heritability** — Some of that variation is passed from parents to offspring through genes.
3. **Differential reproduction** — Some variants lead to more surviving offspring than others, often because of competition for limited resources, predators, mates, or environmental challenges.

When these three conditions exist, the traits that help individuals survive and reproduce become more common in the next generation. Repeated over many generations, this gradually changes the population.

## A Classic Example: Peppered Moths

Before industrialization in England, peppered moths were mostly light-colored, blending in with pale, lichen-covered trees. A rare dark variant existed but was easily spotted and eaten by birds.

When soot from factories darkened the trees, the situation reversed: light moths stood out, dark moths blended in. Within decades, dark moths dominated the population. The moths didn't *choose* to change — predators simply removed the more visible individuals each generation.

## Key Points Often Misunderstood

- **Individuals don't evolve; populations do.** A moth doesn't change color in its lifetime — the *proportion* of colors in the population shifts over generations.
- **It's not "survival of the strongest."** What matters is reproductive success. A peacock's heavy tail makes survival harder but attracts mates.
- **There's no goal or direction.** Evolution doesn't aim toward "better" or "more complex." It just favors whatever works in the current environment.
- **Mutations provide raw material.** Random genetic mutations create new variations; natural selection is the non-random filter that acts on them.

## The Bigger Picture

Given enough time, accumulated changes can produce dramatically different organisms — and when populations become isolated and adapt to different environments, they can diverge into entirely new species. This is how, over billions of years, the diversity of life on Earth arose from common ancestors.

Charles Darwin and Alfred Russel Wallace independently proposed this mechanism in 1858, and it has since been confirmed by evidence from genetics, paleontology, biogeography, and direct observation of evolution in action (such as antibiotic resistance in bacteria).


prompt_id: sci-iq-heritability

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# Heritability of Human Intelligence

## The Short Answer

Heritability estimates for intelligence (typically measured by IQ) range from about **50% to 80% in adults**, with most modern estimates clustering around **60-70%**. Heritability is lower in children (around 20-40%) and increases with age—a counterintuitive finding called the **Wilson Effect**.

## What Heritability Means (and Doesn't)

Heritability is the proportion of *variance* in a trait within a population attributable to genetic differences. Important caveats:

- It's a **population statistic**, not an individual one. Saying IQ is 70% heritable doesn't mean 70% of your intelligence comes from genes.
- It depends on the **environment sampled**. In environments with severe deprivation, heritability is lower because environmental variance dominates.
- It does **not** mean a trait is unchangeable. Height is ~80% heritable but average heights have risen dramatically with nutrition.

## Key Lines of Evidence

**1. Twin studies**
Identical twins reared apart show IQ correlations around 0.75, compared to ~0.85 for those reared together. Fraternal twins correlate around 0.40.

**2. Adoption studies**
Adopted children's IQs correlate more strongly with their biological parents than adoptive parents, especially as they age. By adulthood, the correlation with adoptive parents approaches zero.

**3. Molecular genetics**
- GWAS (genome-wide association studies) have identified thousands of variants associated with cognitive performance and educational attainment.
- Polygenic scores currently predict about **10-15% of variance** in cognitive test scores—far less than twin-study heritability, the so-called "missing heritability" problem.
- SNP-based heritability estimates (e.g., GREML) suggest common variants account for ~20-30%.

## Important Nuances

- **Gene-environment interactions**: Some studies (Turkheimer et al., 2003) suggested heritability is lower in low-socioeconomic-status families, though this finding has been inconsistent across populations.
- **Flynn effect**: Average IQ scores rose substantially during the 20th century, demonstrating large environmental effects on the population mean even while heritability of individual differences remained high.
- **Assortative mating**: People tend to partner with those of similar cognitive ability, which inflates apparent heritability somewhat.
- **Group differences**: High within-group heritability tells us essentially nothing about the causes of between-group differences (Lewontin's classic point).

## Bottom Line

Intelligence is one of the most heritable behavioral traits, but it's also clearly shaped by environment, education, nutrition, and culture. Genetics sets a substantial influence on individual differences within a given environment, but the trait itself is not "fixed"—and our ability to predict individuals from their DNA remains modest.


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# Heritability of Human Intelligence

## The Basic Finding

Intelligence (typically measured as IQ or "g") is one of the most heritable behavioral traits. Heritability estimates in adults typically range from **0.5 to 0.8**, meaning 50-80% of the variation in intelligence within a studied population is attributable to genetic differences.

## Key Evidence

**Twin studies**: Identical twins reared apart show IQ correlations around 0.75, compared to ~0.85 for those reared together. Fraternal twins correlate around 0.4-0.6.

**Adoption studies**: Adopted children's IQs correlate more strongly with their biological parents than adoptive parents, especially as they age.

**The Wilson Effect**: Heritability *increases* with age—from ~20-40% in young children to ~60-80% in adulthood. This is counterintuitive but well-replicated. One explanation: as people gain autonomy, they increasingly select environments matching their genetic predispositions.

**Molecular genetics**: Genome-wide association studies (GWAS) have identified thousands of variants associated with cognitive performance and educational attainment. Current polygenic scores predict ~10-15% of variance—far below twin-based estimates, leaving a "missing heritability" gap.

## Important Caveats

1. **Heritability is population-specific.** It describes variance *within* a population under particular environmental conditions, not the relative importance of genes vs. environment in any absolute sense.

2. **High heritability ≠ immutability.** Height is highly heritable, yet average heights rose dramatically with nutrition improvements. The **Flynn effect**—IQ gains of ~3 points per decade through the 20th century—shows environments matter enormously.

3. **Heritability says nothing about group differences.** Within-group heritability cannot be used to infer that differences *between* groups (e.g., across countries, races, or eras) are genetic. This is a common but serious logical error.

4. **Gene-environment interplay is substantial.** Genes influence which environments people seek, parents provide environments correlated with their genes, and some research (e.g., Turkheimer) suggests heritability is lower in severely deprived environments.

5. **What "intelligence" means matters.** Estimates apply to psychometric g/IQ. Whether this fully captures "intelligence" in a broader sense is contested.

## Bottom Line

The evidence robustly supports that genetic differences explain a substantial portion of IQ variance among individuals in studied (mostly WEIRD) populations. But heritability is often misunderstood: it doesn't make intelligence fixed, doesn't explain group differences, and coexists with major environmental effects.


text:
# Heritability of Human Intelligence

## The Core Finding

Intelligence (typically measured as IQ or general cognitive ability, "g") is one of the most heritable psychological traits. Heritability estimates in adults typically range from **0.5 to 0.8**, meaning 50–80% of the variance in intelligence within a population can be attributed to genetic differences.

## Key Evidence

**Twin studies**
- Identical (monozygotic) twins raised apart show IQ correlations of roughly 0.7–0.75, nearly as high as those raised together.
- Fraternal twins correlate around 0.4, consistent with sharing ~50% of segregating genes.
- The classic Minnesota Study of Twins Reared Apart (Bouchard et al.) provided some of the strongest evidence.

**Adoption studies**
- Adopted children's IQs correlate more strongly with their biological parents than their adoptive parents, especially in adulthood.
- The correlation between adopted siblings raised together approaches zero by adulthood.

**The Wilson Effect**
- Heritability *increases* with age: roughly 0.2 in early childhood, 0.4 in adolescence, and 0.6–0.8 in adulthood. Shared environmental effects fade as people select their own environments.

**Molecular genetic studies**
- GWAS have identified hundreds of variants associated with cognitive performance and educational attainment.
- Each variant has a tiny effect; intelligence is highly polygenic.
- Polygenic scores currently predict ~10–15% of variance in educational attainment and somewhat less for IQ directly.

## Important Caveats

1. **Heritability is population-specific.** It describes variance within a group under particular conditions—not how "genetic" a trait is in some absolute sense.

2. **High heritability ≠ immutability.** Height is ~80% heritable, yet average heights have risen dramatically with nutrition.

3. **Gene-environment interaction.** Some studies (e.g., Turkheimer et al., 2003) suggest heritability is lower in severely deprived environments, though replication has been mixed in non-U.S. samples.

4. **The Flynn Effect.** IQ scores rose ~3 points per decade through much of the 20th century, demonstrating substantial environmental influence on absolute performance.

5. **Between-group differences cannot be inferred from within-group heritability.** This is a well-established statistical point (Lewontin's argument).

6. **"Missing heritability."** Twin studies suggest higher heritability than current molecular methods can account for, an active research puzzle.

## Bottom Line

Genetic differences robustly explain a majority of individual variation in adult intelligence within studied populations, but this coexists with meaningful environmental influences, substantial secular gains, and important limits on what heritability estimates actually tell us.


text:
# Heritability of Human Intelligence

## The Basic Finding

Intelligence (typically measured as IQ or "g," general cognitive ability) is one of the most heritable behavioral traits studied. Heritability estimates in adults typically range from **0.5 to 0.8**, meaning 50–80% of the variation in IQ within studied populations can be attributed to genetic differences.

## Key Evidence

**Twin studies**: Identical twins reared apart show IQ correlations around 0.75, compared to ~0.85 for those reared together. Fraternal twins correlate around 0.4.

**Adoption studies**: Adopted children's IQs correlate more strongly with their biological parents than their adoptive parents, especially as they age.

**Heritability increases with age**: A counterintuitive but robust finding. Childhood heritability is ~0.4, rising to ~0.8 by late adulthood. This is sometimes called the "Wilson effect." Likely mechanisms include people increasingly selecting environments matching their genetic predispositions, and shared family environment effects fading.

**Molecular genetics**: GWAS studies have identified thousands of genetic variants associated with cognitive performance and educational attainment. Polygenic scores currently predict ~10–15% of variance in cognitive outcomes—substantial but far below twin-study heritability estimates (the "missing heritability" problem).

## Important Caveats

1. **Heritability is population-specific**. It describes variance within a specific population under specific conditions, not the strength of genetic determination for any individual.

2. **High heritability doesn't mean unchangeable**. Height is ~80% heritable, yet average heights have risen dramatically with nutrition. The Flynn effect shows average IQ scores rose roughly 3 points per decade through the 20th century—clearly environmental.

3. **Gene-environment interactions matter**. Some research (e.g., Turkheimer 2003) suggests heritability is lower in severely deprived environments, where environmental factors dominate, though this finding has been mixed in replication attempts.

4. **Heritability within groups says nothing about differences between groups**. This is a frequent logical error. Average differences between populations could be entirely environmental even if within-group heritability is high.

5. **"Intelligence" is what IQ tests measure**—a useful but limited construct that predicts educational and occupational outcomes but doesn't capture everything we might mean by the word.

## Bottom Line

Genetic differences substantially influence individual differences in cognitive ability, with effects that strengthen across the lifespan. But heritability estimates describe statistical variance, not destiny, and environmental factors—nutrition, education, health, early childhood experiences—remain critically important and demonstrably alter outcomes.


text:
# Heritability of Human Intelligence

## The Basic Finding

Intelligence (as measured by IQ tests) is one of the most heritable psychological traits. Heritability estimates typically range from **0.50 to 0.80 in adults**, meaning 50-80% of the variation in IQ within a studied population is attributable to genetic differences.

## Key Evidence

**Twin studies:** Identical twins reared apart show IQ correlations around 0.75, compared to ~0.85 for those reared together. Fraternal twins correlate around 0.40.

**Adoption studies:** Adopted children's IQs correlate more strongly with their biological parents than their adoptive parents, especially as they age.

**The Wilson Effect:** Heritability *increases* with age—from roughly 0.20 in infancy to 0.40 in childhood to 0.60-0.80 in adulthood. This counterintuitive finding suggests people increasingly select environments matching their genetic predispositions.

**Molecular genetics:** Genome-wide association studies (GWAS) have identified thousands of genetic variants associated with cognitive ability and educational attainment. Polygenic scores currently predict ~10-15% of variance in cognitive measures—far less than twin studies suggest, the so-called "missing heritability."

## Important Caveats

1. **Heritability is population-specific.** It describes variation *within* a group under particular environmental conditions, not the cause of any individual's intelligence.

2. **Heritability ≠ immutability.** Highly heritable traits can still be influenced by environment (height is ~80% heritable but has risen dramatically with nutrition).

3. **Group differences.** Within-group heritability tells us *nothing* about the causes of differences *between* groups. This is a common misinterpretation.

4. **Gene-environment interactions.** Genes and environments are correlated and interact. Some research (Turkheimer et al.) suggests heritability may be lower in severely deprived environments, though this finding has been mixed in replication.

5. **What IQ measures.** IQ predicts many life outcomes but doesn't capture all aspects of "intelligence" (creativity, practical skills, wisdom).

## Bottom Line

The evidence robustly supports substantial genetic influence on cognitive ability differences among individuals in studied populations. However, environment matters significantly, the genetic architecture is highly polygenic, and heritability estimates are frequently misinterpreted in ways that overstate genetic determinism.


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# Heritability of Human Intelligence

## The Basic Finding

Heritability of intelligence (typically measured by IQ) is one of the most replicated findings in behavioral genetics. Estimates generally fall in the range of **50–80% in adults**, with the figure increasing across the lifespan.

## Key Evidence

**Twin studies**
- Identical twins reared apart show IQ correlations around 0.75
- Identical twins reared together: ~0.85
- Fraternal twins: ~0.40
- These patterns are consistent across many independent studies (Minnesota Study of Twins Reared Apart, Swedish Adoption/Twin Study, etc.)

**Adoption studies**
- Adopted children's IQs correlate more strongly with biological parents than adoptive parents, especially in adulthood
- Shared family environment effects on IQ largely fade by late adolescence

**The Wilson Effect**
- Heritability *increases* with age: roughly 20% in infancy, 40% in childhood, 60% in adolescence, and 70–80% in adulthood
- A counterintuitive finding—genes seem to matter more as people age and have more freedom to seek environments matching their predispositions

**Molecular genetics**
- GWAS studies have identified many SNPs associated with cognitive performance
- Current polygenic scores predict ~10–15% of IQ variance, far less than twin-study heritability ("missing heritability")
- This gap is expected to close as sample sizes grow

## Important Caveats

1. **Heritability is population-specific.** A heritability estimate describes variance *within* a particular population at a particular time. It does not say how much of any individual's intelligence is "due to genes."

2. **High heritability ≠ unchangeable.** Height is ~80% heritable, but average heights have risen dramatically due to nutrition. The Flynn effect (substantial IQ gains across 20th-century generations) shows environment can move means even when within-cohort heritability is high.

3. **Group differences are a separate question.** Within-group heritability says nothing direct about causes of between-group differences. This is a frequently misunderstood point.

4. **Environment still matters.** Even at 70% heritability, 30% of variance comes from non-genetic factors. Severe deprivation, lead exposure, malnutrition, and poor schooling demonstrably reduce cognitive function.

5. **Scarr-Rowe interaction**: Some evidence (mixed in replication) suggests heritability is lower in disadvantaged environments, where environmental constraints play a larger role.

## Bottom Line

Intelligence is substantially heritable, with genetic influence becoming more pronounced in adulthood. Shared environmental effects are surprisingly small after childhood. But heritability is a statistic about variation in populations, not a statement about fixed individual destinies or about what's possible to change through intervention.


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# Heritability of Human Intelligence

## The Short Answer

Heritability of intelligence (measured by IQ or g, general cognitive ability) is substantial and increases with age:
- **Childhood**: ~20–40%
- **Adolescence**: ~40–60%
- **Adulthood**: ~60–80%

This counterintuitive increase with age is called the **Wilson Effect**.

## Key Lines of Evidence

**1. Twin studies**
Identical twins reared apart still show IQ correlations around 0.75 (Minnesota Study of Twins Reared Apart, Bouchard et al.). Identical twins correlate much more strongly than fraternal twins on cognitive measures.

**2. Adoption studies**
Adopted children's IQs resemble their biological parents more than their adoptive parents, especially as they reach adulthood. Adoptive sibling correlations approach zero in adulthood.

**3. Molecular genetic studies (GWAS)**
Large genome-wide association studies (e.g., Savage et al. 2018, N ≈ 270,000; Lee et al. 2018 on educational attainment, N ≈ 1.1 million) have identified hundreds of variants associated with cognitive performance and educational attainment. Polygenic scores currently predict roughly 7–10% of variance in cognitive test scores — far less than heritability estimates, illustrating the "missing heritability" problem.

**4. SNP-based heritability (GCTA)**
Estimates from common variants alone yield heritabilities of ~20–30%, consistent with intelligence being highly polygenic (thousands of variants of tiny effect).

## Important Caveats

- **Heritability is population-specific.** It describes variance in a particular environment, not the fixed influence of genes. In environments of extreme deprivation, heritability drops and shared environment matters more (the **Scarr-Rowe effect**, though replication is mixed).
- **Heritability ≠ immutability.** The Flynn effect (large 20th-century IQ gains) shows environments can shift averages dramatically even when heritability is high.
- **Heritability ≠ causation at the individual level.** A high heritability tells you about variance across people, not how much "of your IQ comes from genes."
- **Group differences.** Within-group heritability says nothing about the causes of between-group differences — a point emphasized by Lewontin and accepted across the field.
- **Gene-environment correlation.** People partly shape their environments based on genetic predispositions, which inflates heritability estimates in complex ways.

## Bottom Line

The evidence that genetic variation contributes substantially to individual differences in cognitive ability is among the most replicated findings in behavioral science. But intelligence is highly polygenic, environmentally sensitive in important ways, and heritability statistics are widely misinterpreted. The honest summary: genes matter a lot for differences between individuals; environments matter a lot for population averages and for individuals in deprived conditions; and we still cannot predict an individual's cognitive ability well from DNA alone.


text:
# Heritability of Human Intelligence

## The Short Answer

Heritability estimates for intelligence (typically measured by IQ) range from about **0.5 to 0.8 in adults**, meaning roughly 50–80% of the variation in IQ scores within a population can be attributed to genetic differences. This makes intelligence one of the most heritable behavioral traits.

## Key Evidence

**Twin studies**: Identical twins reared apart show IQ correlations around 0.75, while fraternal twins correlate around 0.4. The classic Minnesota Study of Twins Reared Apart (Bouchard et al.) was foundational here.

**Adoption studies**: Adopted children's IQs correlate more strongly with their biological parents than their adoptive parents, especially as they age.

**The Wilson Effect**: Heritability *increases* with age—from around 0.2 in infancy to 0.4–0.5 in childhood to 0.6–0.8 in adulthood. Shared family environment effects, conversely, fade to near zero by adulthood.

**Molecular genetics**: Modern GWAS studies have identified thousands of genetic variants associated with cognitive performance and educational attainment. Polygenic scores currently predict ~10–15% of variance in cognitive test scores—far less than twin studies imply, leaving a "missing heritability" gap.

## Important Caveats

1. **Heritability is population-specific**: It describes variance *within* a population under particular conditions, not a fixed property of the trait. It says nothing about between-group differences.

2. **High heritability ≠ unchangeable**: Height is ~80% heritable, yet average height has risen dramatically with nutrition. The Flynn effect (rising IQ scores across 20th-century generations) shows environmental factors substantially shift populations.

3. **Gene-environment interplay**: Heritability estimates may capture gene-environment correlations (e.g., bright children seek stimulating environments). The Scarr-Rowe effect suggests heritability is lower in disadvantaged environments where environmental constraints dominate.

4. **What "intelligence" means matters**: Most evidence concerns *g* (general intelligence) as measured by IQ tests, which predicts academic and occupational outcomes but doesn't capture all cognitive abilities.

5. **No "intelligence gene"**: The trait is highly polygenic—influenced by thousands of variants each with tiny effects.

## Bottom Line

The evidence robustly supports substantial genetic influence on individual differences in intelligence within studied populations, while also showing that environment matters significantly—especially early in life and in disadvantaged conditions. Claims that go beyond this (e.g., about group differences, or about destiny) outrun what the evidence supports.


text:
# Heritability of Human Intelligence

## The Short Answer

Intelligence (measured as IQ or "g," the general factor) is one of the most heritable psychological traits. Heritability estimates typically range from **~40% in young children to ~70-80% in adults**, with most adult estimates clustering around 50-70%.

## Key Evidence

**Twin studies**: Identical twins raised apart show IQ correlations around 0.7-0.75, while fraternal twins show roughly 0.4. This pattern across many studies (including the Minnesota Study of Twins Reared Apart) consistently points to substantial genetic influence.

**Adoption studies**: Adopted children's IQs correlate more strongly with their biological parents than their adoptive parents, especially as they age.

**The Wilson Effect**: Heritability *increases* with age—from ~20-40% in early childhood to 60-80% by adulthood. This counterintuitive finding suggests people increasingly select environments matching their genetic predispositions.

**Molecular genetics**: GWAS studies have identified thousands of genetic variants associated with cognitive ability and educational attainment. Polygenic scores currently predict ~10-15% of variance in cognitive test scores—much less than twin studies suggest, reflecting the "missing heritability" problem.

## Important Caveats

1. **Heritability ≠ immutability**. High heritability doesn't mean intelligence can't be improved. Height is ~80% heritable, yet average height has risen dramatically with nutrition.

2. **Heritability is population-specific**. It measures variance *within* a population under current conditions. In environments with severe deprivation, environmental factors dominate.

3. **Between-group differences cannot be inferred from within-group heritability**. This is a well-established statistical point (Lewontin's famous illustration).

4. **Gene-environment interplay is substantial**. Genes influence which environments people seek out, making clean separation difficult.

5. **The Flynn Effect**: IQ scores rose ~3 points per decade through the 20th century, demonstrating powerful environmental influences on cognitive performance.

## Bottom Line

The evidence strongly supports that genetic differences explain a large portion of *individual* differences in intelligence within studied populations, particularly in adulthood. But this is compatible with significant environmental influence, the importance of education and nutrition, and large changes in average cognitive performance over time.


text:
# Heritability of Human Intelligence

## The Core Finding

Intelligence (typically measured as IQ or general cognitive ability, "g") is one of the most heritable psychological traits. Heritability estimates in adults typically range from **0.5 to 0.8**, meaning 50-80% of the variance in intelligence within studied populations is attributable to genetic differences.

## Key Evidence

**Twin studies**: Identical twins reared apart show IQ correlations around 0.75, compared to ~0.85 for those raised together. Fraternal twins correlate around 0.40. This pattern is highly consistent across dozens of studies.

**Adoption studies**: Adopted children's IQs correlate more strongly with their biological parents than their adoptive parents, especially in adulthood.

**The Wilson Effect**: Heritability *increases* with age—from roughly 0.20 in infancy to 0.40 in childhood to 0.60-0.80 in adulthood. This counterintuitive finding likely reflects people increasingly selecting environments matching their genetic propensities, while shared family environment effects fade.

**Molecular genetics**: Genome-wide association studies (GWAS) have identified thousands of genetic variants associated with cognitive performance and educational attainment. Polygenic scores currently predict ~10-15% of variance in cognitive measures—much less than twin studies suggest, reflecting the "missing heritability" problem.

## Important Caveats

- **Heritability is population-specific.** It describes variance *within* a given population under given conditions, not a fixed property of the trait.
- **High heritability ≠ immutability.** Height is ~80% heritable, yet average heights rose dramatically with nutrition.
- **It says nothing about between-group differences.** A trait can be highly heritable within groups while differences *between* groups are entirely environmental (Lewontin's classic point).
- **Shared environment matters more in childhood** and in disadvantaged conditions. Some studies (notably Turkheimer 2003) found heritability is lower in poor families, though this finding has been inconsistent across replications and populations.
- **"Genes for intelligence" is misleading.** Thousands of variants each contribute tiny effects; there's no single "intelligence gene."

## Scientific Consensus

The high heritability of intelligence is among the most replicated findings in behavioral genetics. The 2014 consensus statement by Plomin and Deary, and subsequent reviews, treat it as well-established. Debate continues about exact estimates, mechanisms, gene-environment interactions, and especially about interpretation and policy implications.


text:
# Heritability of Human Intelligence

## The Short Answer

Intelligence (measured as IQ or general cognitive ability, "g") is one of the most heritable behavioral traits studied. Heritability estimates typically range from **~50% in childhood to ~70-80% in adulthood**.

## Key Evidence

**Twin studies**: Identical twins reared apart show IQ correlations around 0.70-0.75, nearly as high as identical twins raised together. Fraternal twins correlate around 0.40-0.50. The Minnesota Study of Twins Reared Apart (Bouchard et al.) was particularly influential.

**Adoption studies**: Adopted children's IQs correlate more strongly with their biological parents than their adoptive parents, especially as they age. By adulthood, the correlation with adoptive parents approaches zero.

**The Wilson Effect**: Heritability *increases* with age—from ~20-40% in young children to ~60-80% in adults. This is counterintuitive but well-replicated. One explanation is that people increasingly select environments matching their genetic predispositions ("gene-environment correlation").

**Molecular genetics**: GWAS studies (e.g., Savage et al. 2018, with ~270,000 people) have identified hundreds of genetic variants associated with cognitive ability. However, "polygenic scores" currently predict only ~10-15% of variance—far less than twin-study heritability suggests (the "missing heritability" problem).

## Important Caveats

1. **Heritability ≠ immutability**. High heritability doesn't mean traits can't be changed. Height is ~80% heritable, yet average heights rose dramatically with nutrition.

2. **Heritability is population-specific**. It describes variance *within* a population under current conditions. The Flynn effect (rising IQ scores ~3 points/decade through the 20th century) shows environments matter enormously.

3. **Heritability says nothing about group differences**. Within-group heritability cannot be used to infer that differences *between* groups are genetic—a common but serious logical error.

4. **Shared environment effects fade**. Family environment matters in childhood but has surprisingly little measurable effect on adult IQ, while "non-shared environment" (including measurement error, idiosyncratic experiences) remains significant.

5. **Gene-environment interplay** is complex. Genes influence which environments people seek out and how they respond to them.

## Bottom Line

The evidence robustly shows substantial genetic influence on individual differences in intelligence, but this coexists with major environmental effects (education, nutrition, prenatal conditions, lead exposure, etc.). The nature/nurture framing is largely outdated—both matter, and they interact throughout development.


text:
# Heritability of Human Intelligence

## The Short Answer

Intelligence (as measured by IQ tests) is one of the most heritable behavioral traits studied. Heritability estimates typically range from **~50% to 80% in adults**, with the figure increasing with age.

## Key Evidence

**Twin studies** consistently show:
- Identical twins reared apart correlate ~0.70–0.75 on IQ
- Identical twins reared together correlate ~0.85
- Fraternal twins correlate ~0.40–0.50
- Adopted siblings raised together show near-zero correlation by adulthood

**Adoption studies** show adopted children's IQs come to resemble their biological parents more than their adoptive parents as they age.

**The Wilson Effect**: Heritability *increases* across development—roughly 20–40% in childhood, 50–60% in adolescence, and 60–80% in adulthood. Shared family environment effects largely fade by adulthood.

**Molecular genetics**: GWAS studies have identified hundreds of genetic variants associated with cognitive performance and educational attainment. Polygenic scores currently predict ~10–15% of variance in cognitive outcomes, though "SNP heritability" estimates suggest much more is genetic but not yet captured.

## Important Caveats

1. **Heritability ≠ immutability.** Height is highly heritable, yet average heights have risen substantially with nutrition. The Flynn effect showed IQ scores rising ~3 points per decade through the 20th century.

2. **Heritability is population-specific.** It describes variance *within* a population under current conditions, not differences *between* groups or individuals' fixed potential.

3. **Gene-environment interactions matter.** Some research (e.g., Turkheimer et al.) suggests heritability may be lower in severely impoverished environments, though this finding has been mixed in replications.

4. **What's measured matters.** IQ correlates meaningfully with educational, occupational, and health outcomes, but doesn't capture every aspect of "intelligence" in a broader sense.

5. **The shared environment puzzle**: Family environment has surprisingly little long-term effect on IQ in typical Western samples—a counterintuitive finding sometimes called one of behavioral genetics' "laws."

## Bottom Line

The evidence that genetic differences substantially contribute to individual differences in cognitive ability within studied populations is robust and replicated across methods. The remaining variance is largely "non-shared environment" (which includes measurement error, developmental noise, and idiosyncratic experiences)—not, as often assumed, family upbringing or schooling quality in any straightforward way.


text:
# Heritability of Human Intelligence

## The Core Finding

Intelligence (typically measured as IQ or "g," the general factor) is one of the most heritable psychological traits. Heritability estimates—the proportion of variance in a population attributable to genetic differences—typically fall in these ranges:

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

The increase with age is robust and somewhat counterintuitive; it's called the **Wilson Effect**.

## Key Evidence

**Twin studies**: Identical twins reared apart correlate ~0.70–0.75 on IQ, far higher than fraternal twins (~0.30–0.40) or adopted siblings raised together (near zero by adulthood).

**Adoption studies**: Adopted children's IQs correlate more strongly with biological parents than adoptive parents, especially as they age. Shared family environment effects largely fade by adulthood.

**Molecular genetics**: GWAS studies (e.g., Savage et al. 2018, Lee et al. 2018) have identified hundreds of genetic variants associated with cognitive performance and educational attainment. Polygenic scores currently predict ~10–15% of variance in cognitive test scores—much less than twin estimates, partly due to "missing heritability."

## Important Caveats

1. **Heritability ≠ immutability**. High heritability doesn't mean traits can't be changed by environment. Height is ~80% heritable, yet average height has risen substantially with nutrition.

2. **Heritability is population-specific**. In environments where some children are severely deprived, shared environment matters more (the **Scarr-Rowe effect** suggests heritability is lower in low-SES contexts, though replication is mixed).

3. **It says nothing about group differences**. Within-group heritability cannot be used to infer that average differences between groups are genetic in origin (Lewontin's classic argument).

4. **Gene-environment correlation**. Genes influence the environments people seek and create, blurring "nature vs. nurture" distinctions.

5. **The Flynn effect**: IQ scores rose substantially across the 20th century, demonstrating large environmental influence on absolute performance even if rank-order is heritable.

## Bottom Line

The evidence strongly supports substantial genetic influence on individual differences in intelligence within studied populations, growing with age. But heritability is a statistic about variance in specific populations—not a measure of how fixed, fair, or modifiable intelligence is.


text:
# Heritability of Human Intelligence

## The Core Finding

Intelligence (measured as IQ or general cognitive ability, "g") is one of the most heritable behavioral traits studied. Heritability estimates typically range from **0.5 to 0.8 in adults**, meaning roughly 50–80% of the variation in intelligence within studied populations is attributable to genetic differences.

## Key Evidence

**Twin studies**: Identical twins reared apart show IQ correlations around 0.70–0.75, nearly as high as identical twins raised together. Fraternal twins correlate around 0.40.

**Adoption studies**: Adopted children's IQs correlate more strongly with their biological parents than their adoptive parents, especially as they age. By adulthood, the correlation with adoptive parents approaches zero.

**The Wilson Effect**: Heritability *increases* with age—from about 20% in infancy, to 40% in childhood, to 60–80% in adulthood. This counterintuitive finding likely reflects people increasingly selecting environments that match their genetic predispositions.

**Molecular genetics**: Genome-wide association studies (GWAS) have identified thousands of genetic variants associated with cognitive performance and educational attainment. Polygenic scores currently predict around 10–15% of variance in cognitive outcomes—substantial but well below twin-study estimates (the "missing heritability" problem).

## Important Caveats

- **Heritability is population-specific.** It describes variance within a group under particular conditions, not the cause of any individual's intelligence.
- **Heritability ≠ immutability.** Highly heritable traits (like height) still respond to environment—the Flynn Effect shows IQ scores rose substantially across the 20th century.
- **Group differences.** High within-group heritability tells us nothing definitive about causes of *between-group* differences. This is a common misinterpretation.
- **Gene-environment interplay.** Heritability estimates include gene-environment correlations (e.g., parents who pass on "smart genes" also tend to provide stimulating environments).
- **Socioeconomic moderation.** Some studies (especially in the US) find heritability is lower in disadvantaged environments, suggesting environmental constraints can suppress genetic potential—though this finding is debated.

## Bottom Line

The evidence robustly supports that genes substantially influence individual differences in intelligence, with effects strengthening across the lifespan. But "heritable" doesn't mean "fixed," "purely genetic," or "explained by any single gene"—intelligence emerges from complex interactions between thousands of genetic variants and environmental factors.


text:
# Heritability of Human Intelligence

## The Basic Finding

Intelligence (typically measured as IQ or "g," the general factor) is one of the most heritable behavioral traits studied. Heritability estimates from twin, family, and adoption studies converge on roughly:

- **20–40% in early childhood**
- **50–60% in adolescence**
- **70–80% in adulthood**

This increase with age is called the **Wilson effect** and is well-replicated.

## Key Evidence

**Twin studies:** Identical twins reared apart show IQ correlations around 0.75, nearly as high as identical twins raised together. Fraternal twins correlate around 0.38.

**Adoption studies:** Adopted children's IQs correlate increasingly with their biological parents and decreasingly with adoptive parents as they age. By adulthood, correlation with adoptive parents approaches zero.

**Molecular genetics:** Genome-wide association studies (GWAS) have identified hundreds of genetic variants associated with cognitive ability and educational attainment. Polygenic scores currently predict ~10–15% of variance in cognitive test scores — far below twin-based estimates, leaving a "missing heritability" gap.

## Important Caveats

1. **Heritability is population-specific.** It describes variance within a group under specific conditions, not the cause of any individual's intelligence.

2. **Heritability ≠ immutability.** Highly heritable traits (like height) still respond to environment. The **Flynn effect** — large IQ gains across 20th-century generations — proves environment matters substantially.

3. **Gene-environment interaction is real.** Some research (e.g., Turkheimer et al.) suggests heritability is lower in poverty and higher in enriched environments, though this finding is more robust in U.S. samples than European ones.

4. **Between-group differences cannot be inferred from within-group heritability.** This is a frequently misunderstood point — Lewontin's classic argument about plant seeds in different soils illustrates why.

5. **What's measured matters.** IQ tests capture important cognitive abilities that predict school and job performance, but not the entirety of "intelligence" as commonly understood.

## Bottom Line

Intelligence is substantially heritable, with genetic differences explaining a majority of variation among adults in modern populations. But environment, especially in development, also plays a significant role, and the specific causal pathways from genes to cognition remain largely unknown.


text:
# Heritability of Human Intelligence

## The Core Finding

Intelligence (measured by IQ or g, the general factor of cognitive ability) is one of the most heritable behavioral traits studied. Heritability estimates typically range from **0.5 to 0.8 in adults**, meaning roughly 50–80% of variation in intelligence within a population is associated with genetic differences.

## Key Evidence

**Twin studies**: Identical twins reared apart show IQ correlations around 0.70–0.75, far higher than fraternal twins or adoptive siblings. The Minnesota Study of Twins Reared Apart (Bouchard et al.) was foundational here.

**Adoption studies**: Adopted children's IQs correlate more strongly with their biological parents than their adoptive parents, especially as they age. By adulthood, the correlation with adoptive parents drops toward zero.

**The Wilson Effect**: Heritability of IQ *increases* with age—from about 0.20 in infancy to 0.40 in childhood to 0.60–0.80 in adulthood. This counterintuitive pattern (shared family environment matters less over time) is well-replicated.

**Molecular genetics**: Genome-wide association studies (GWAS) on samples of millions have identified thousands of variants associated with cognitive ability and educational attainment. Polygenic scores currently predict ~10–15% of variance in educational attainment, with predictive power steadily improving.

## Important Caveats

- **Heritability is population-specific**, not a fixed property. It can differ across environments. In severely deprived environments, heritability appears lower (the Scarr-Rowe effect, though this has shown mixed replication).
- **High heritability ≠ unchangeable**. Height is ~80% heritable, yet average heights have risen substantially due to nutrition. The Flynn Effect (rising IQ scores across the 20th century) shows environment matters at the population level.
- **Group differences in test scores cannot be inferred from within-group heritability.** This is a longstanding statistical point (Lewontin's argument): within-group genetic variance tells you nothing about causes of between-group differences.
- **"Genetic" doesn't mean "innate" or "deterministic."** Gene-environment correlations and interactions are pervasive. People partly select environments matching their predispositions.
- **Shared environment effects** (same family, same schools) seem small in adulthood, but **non-shared environmental factors** still account for substantial variance.

## Bottom Line

The evidence that genes substantially influence individual differences in intelligence is robust and accepted across mainstream behavioral genetics. The harder questions—which specific variants, through what biological pathways, and how genes and environments interact—remain active areas of research.


text:
# Heritability of Human Intelligence

## The Short Answer

Heritability estimates for intelligence (measured by IQ or general cognitive ability, "g") typically range from **0.5 to 0.8 in adults**, meaning genetic variation accounts for roughly 50–80% of the variation in intelligence within studied populations. This is one of the most replicated findings in behavioral genetics.

## Key Evidence

**Twin studies**: Identical twins reared apart show IQ correlations around 0.70–0.75, far higher than fraternal twins (~0.40). The Minnesota Study of Twins Reared Apart and large meta-analyses (e.g., Polderman et al., 2015, covering ~14.5 million twin pairs) consistently support substantial heritability.

**Adoption studies**: Adopted children's IQs correlate more strongly with their biological parents than their adoptive parents, especially as they age.

**Molecular genetics (GWAS)**: Genome-wide association studies on hundreds of thousands of people have identified thousands of genetic variants associated with cognitive performance and educational attainment. Each variant has a tiny effect; intelligence is highly polygenic. Current polygenic scores predict roughly 10–15% of variance in educational attainment—less than twin studies suggest, the so-called "missing heritability."

## Important Nuances

1. **Heritability increases with age.** It's around 0.20–0.40 in young children but rises to 0.60–0.80 in adulthood (the Wilson effect). Shared family environment effects largely fade by adolescence.

2. **Heritability is population-specific.** It describes variance *within* a group under given conditions—not how "genetic" intelligence is in some absolute sense. In environments with more deprivation, heritability tends to be lower because environment explains more variance (the Scarr-Rowe effect, though replication is mixed).

3. **Heritability ≠ immutability.** The Flynn effect—rising IQ scores across the 20th century—shows large environmental influences on mean scores even when heritability is high. Schooling, nutrition, and iodine supplementation have measurable cognitive effects.

4. **Heritability says nothing about group differences.** A trait can be highly heritable within groups while differences *between* groups are entirely environmental (Lewontin's classic point).

5. **Gene-environment interplay** is substantial. People partly select environments matching their genetic predispositions (active rGE), and parents provide environments correlated with the genes they pass on (passive rGE).

## Bottom Line

Intelligence is substantially heritable—this is well-established. But heritability is a statistic about variance in specific populations, not a measure of genetic determinism. Both genes and environment matter, they interact in complex ways, and high heritability is compatible with significant malleability through environmental change.


text:
# Heritability of Human Intelligence

## The Short Answer

Heritability of intelligence (as measured by IQ tests) is substantial and increases with age:
- **Childhood:** ~20–40%
- **Adolescence:** ~40–60%
- **Adulthood:** ~60–80%

This counterintuitive age pattern—genes mattering *more* over time—is one of the most replicated findings in behavioral genetics.

## Lines of Evidence

**1. Twin studies**
Identical twins reared apart show IQ correlations around 0.70–0.75, despite different environments. The Minnesota Study of Twins Reared Apart (Bouchard et al.) is the landmark work.

**2. Adoption studies**
Adopted children's IQs correlate increasingly with their biological parents and decreasingly with adoptive parents as they age. By adulthood, the correlation with adoptive parents approaches zero.

**3. Family studies**
IQ correlations track genetic relatedness: identical twins > fraternal twins > siblings > half-siblings > cousins.

**4. Molecular genetic studies (GWAS)**
Modern genome-wide studies (e.g., Savage et al. 2018, ~270,000 individuals) have identified hundreds of variants associated with cognitive performance. Current polygenic scores predict ~10–15% of IQ variance—far below the heritability estimates, reflecting the "missing heritability" problem common to complex traits.

## Important Caveats

- **Heritability is not destiny.** It's a population-level statistic about variance, not a measure of how "genetic" any individual's intelligence is.
- **Heritability is context-dependent.** In conditions of severe deprivation, environmental factors dominate (the Turkheimer 2003 finding that heritability is lower in low-SES households is debated but influential).
- **Heritability ≠ unchangeable.** Height is ~80% heritable, yet average height has risen dramatically with nutrition.
- **The Flynn effect** (rising IQ scores across generations) shows environments matter substantially at the population level.
- **Intelligence is polygenic**, involving thousands of variants each with tiny effects—not a few "smart genes."
- **Group differences:** Heritability *within* groups says nothing about causes of differences *between* groups—a frequently misunderstood point (Lewontin's classic argument).

## What's Genuinely Contested

- The exact mechanisms (gene-environment correlation vs. interaction)
- Why heritability rises with age (likely active gene-environment correlation: people increasingly shape environments matching their dispositions)
- How to interpret polygenic scores ethically and scientifically
- Whether g (general intelligence) reflects a unified biological substrate

The basic finding of substantial heritability is among the most robust in psychology; the interpretation and implications remain actively discussed.


text:
# Heritability of Human Intelligence

## The Short Answer

Intelligence (typically measured as IQ or "g," general cognitive ability) is one of the most heritable behavioral traits. In adults, heritability estimates typically range from **0.5 to 0.8**, meaning 50-80% of the variation in intelligence within a population is attributable to genetic differences.

## Key Findings from the Evidence

**1. Heritability increases with age**
This is one of the most counterintuitive and replicated findings. Heritability of IQ rises from roughly:
- 20-40% in early childhood
- 40-50% in adolescence
- 60-80% in adulthood

The "Wilson effect" suggests shared family environment matters more when young, while genetic predispositions express themselves increasingly as people select their own environments.

**2. Twin and adoption studies**
- Identical twins reared apart show IQ correlations around 0.7-0.75
- Adopted siblings raised together show near-zero IQ correlation as adults, despite shared environment
- Biological parents predict adopted children's IQ better than adoptive parents do

**3. Molecular genetic evidence**
- GWAS studies have identified thousands of variants associated with cognitive ability
- Polygenic scores currently predict ~10-15% of variance in educational attainment and cognitive test performance
- "SNP heritability" estimates (~25%) are lower than twin-study estimates, partly because they capture only common variants

## Important Caveats

- **Heritability ≠ immutability.** Height is highly heritable, yet average heights have risen dramatically with nutrition.
- **Heritability is population- and environment-specific.** In environments with severe deprivation, shared environmental factors matter more. Some studies (e.g., Turkheimer 2003) found heritability is lower among low-SES families, though this finding is contested in recent meta-analyses.
- **Heritability says nothing about between-group differences.** A trait can be highly heritable within groups while group differences are entirely environmental.
- **No single "intelligence gene" exists.** Cognitive ability is highly polygenic, influenced by many thousands of variants with tiny individual effects.
- **Gene-environment correlation** complicates interpretation: people partly create environments that match their genetic dispositions.

## Bottom Line

The evidence strongly supports substantial genetic influence on individual differences in intelligence within studied populations, but this coexists with meaningful environmental effects, and heritability estimates do not justify deterministic conclusions about individuals or groups.