Is Intelligence Genetic? What Heritability Does and Does Not Mean
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Quick answer
Intelligence is among the most heritable of behavioural traits, and heritability estimates rise from roughly 20% in infancy to perhaps 80% in later adulthood. Almost everyone who repeats those numbers gets their meaning wrong. Heritability is not the proportion of your intelligence that came from your genes. It is a statistic about a population, in a particular environment, at a particular time — and change the environment and the number changes with it.
What heritability actually is
This is the misunderstanding that ruins almost every discussion of the subject, so it is worth stating slowly.
Heritability is the proportion of the variation in a trait, across a particular population in a particular environment, that is statistically associated with genetic variation in that population. It is a ratio of variances. It is not a measure of how much genes contribute to a trait, and it does not apply to individuals at all.
What a heritability of 60% does not mean
It does not mean 60% of your intelligence comes from your genes and 40% from your upbringing. That sentence has no coherent interpretation. Your intelligence is not divisible into genetic and environmental parts, any more than the area of a rectangle is divisible into a length part and a width part.
What a heritability of 60% means is that, among the people studied, about 60% of the differences between them tracked genetic differences between them. Study a different population, or the same population under different conditions, and you may get a different number — not because the biology changed, but because heritability is partly a description of the environment.
The clearest illustration: if every child in a country received an identical education, the environmental contribution to variation would shrink towards nothing, and heritability would rise towards 100% — while the education would be doing more for children, not less.
The numbers, and the finding that surprises everyone
Plomin and Deary surveyed the genetics of intelligence in Molecular Psychiatry and set out several findings that are specific to intelligence and not shared by most other traits.
| Life stage | Approximate heritability |
|---|---|
| Infancy | ~20% |
| Childhood | ~40% |
| Adolescence | ~55% |
| Adulthood | ~60–70% |
| Later adulthood | up to ~80% |
Heritability increases with age — which is backwards
Intuition says genetic influence should be strongest at birth, when the environment has had least time to act, and weakest in old age after a lifetime of experience. The data show the exact opposite, and this is among the most distinctive findings in the whole field.
The leading explanation is that people increasingly select, modify and evoke environments correlated with their genetic propensities. A child inclined to read is given books, seeks out books, and is treated as a reader. Over decades, a small initial disposition compounds into a large environmental difference — which then registers, statistically, as genetic influence, because it was set in motion by genetic difference.
The mirror image is equally striking. The influence of the shared family environment — everything siblings raised together have in common — is substantial in childhood and falls towards zero by adulthood. Adopted siblings raised in the same home resemble each other in IQ as children and, by adulthood, essentially do not.
Heritability depends on the society you measure it in
In 2003 Turkheimer and colleagues reported that among 7-year-old twins, the heritability of IQ varied sharply with family socioeconomic status: near zero in the most impoverished families, and high in affluent ones. The interpretation was compelling. Genetic potential can only express itself if the environment permits it, and deprivation suppresses expression.
The result became one of the most cited findings in the field. Then Tucker-Drob and Bates meta-analysed every published and unpublished study they could find — 24,926 twin and sibling pairs across the United States, Australia, England, Sweden, Germany and the Netherlands.
What the meta-analysis found
The Gene × socioeconomic status interaction was clearly present in US samples, and negligible or reversed in studies from Western Europe and Australia — countries whose social policies produce more uniform access to education and healthcare.
Read that carefully, because it is the deepest result on this page. Whether genetic differences in intelligence get to express themselves is not a fact about human biology. It is a fact about the society doing the measuring. In a country where poor children attend adequate schools and see doctors, poverty stops suppressing genetic expression, and the interaction disappears.
Heritability, in other words, is partly a summary statistic about how equal a society's environments are. A high heritability is not evidence that intervention is futile. It can be evidence that intervention has already succeeded.
Heritable does not mean unchangeable
This confusion is so common that it deserves its own section, and the counterexamples are decisive.
- Height is more heritable than intelligence — around 80% in developed populations. Average adult height in many countries rose by more than ten centimetres in a century, driven entirely by nutrition and disease control. High heritability did not prevent it.
- IQ scores themselves rose substantially across the twentieth century — the Flynn effect — over a period far too short for the gene pool to have changed.
- Education raises measured intelligence. Ritchie and Tucker-Drob's meta-analysis of more than 600,000 participants found gains of roughly one to five IQ points per additional year of schooling.
- Phenylketonuria is a genetic condition that causes severe intellectual disability, and is prevented entirely by a change of diet. Heritability of the outcome, before the intervention, was effectively total.
The logical point
Heritability describes the causes of variation under conditions that currently obtain. It is silent about what would happen under conditions that do not. A trait can be perfectly heritable and perfectly malleable at the same time.
The genes have not been found
Twin and adoption studies estimate heritability indirectly, by comparing relatives. Molecular genetics attempts something harder: identifying the actual variants and using them to predict.
Intelligence is highly polygenic. There is no gene for it. There are thousands of variants, each contributing a fraction of a fraction of an IQ point. Genome-wide polygenic scores, built by summing those effects, currently predict on the order of 10% of the variance in intelligence — a real and rapidly improving figure, and one far below the 50–80% that twin studies imply.
The gap is known as the missing heritability problem, and it is unresolved. Some of it reflects rare variants that current arrays do not capture, some reflects the fact that twin-study estimates absorb gene-environment correlation, and some may reflect assumptions in the twin design itself. Nobody claims the accounting is finished.
What a polygenic score can and cannot do
A score explaining 10% of variance is a powerful research instrument and a poor tool for saying anything about a particular person. Two individuals with identical polygenic scores will differ in measured intelligence by an amount that dwarfs the score's predictive range.
Within-group heritability says nothing about between-group differences
This is a formal, uncontroversial point of quantitative genetics, and it is worth stating plainly because it is so frequently violated.
Richard Lewontin's illustration remains the clearest. Take a bag of genetically varied seed. Sow half in rich soil and half in depleted soil. Within each plot, the variation in plant height will be almost entirely genetic — heritability approaching 100%, because every plant in that plot shares the same soil. The difference in average height between the two plots will be entirely environmental.
The inference is invalid
High heritability within groups is logically compatible with a difference between groups that is wholly environmental. No estimate of within-group heritability, however large or well-established, licenses any conclusion about the causes of a difference between two groups.
The American Psychological Association's task-force report on intelligence made the same point, and it has not been seriously contested in the three decades since.
Myths and facts about the heritability of intelligence
| Myth | Fact |
|---|---|
| Heritability of 60% means 60% of your IQ is genetic | Heritability describes variation across a population. It does not decompose any individual's intelligence into parts. |
| Genetic influence is strongest in childhood | The reverse. Heritability rises from about 20% in infancy to perhaps 80% in later adulthood. |
| The family you grow up in determines your adult IQ | The shared-environment effect on intelligence is substantial in childhood and falls towards zero by adulthood. |
| High heritability means intervention is futile | Height is more heritable than IQ and rose ten centimetres in a century. Phenylketonuria is fully heritable and fully preventable by diet. |
| Poverty suppresses genetic expression everywhere | It does so in US samples. In Western Europe and Australia the interaction is negligible or reversed. |
| Scientists have found the genes for intelligence | It is highly polygenic. Polygenic scores currently predict around 10% of variance, well short of twin-study heritability. |
| Heritability within groups explains gaps between groups | It does not, as a matter of logic. Lewontin's seed illustration shows the two are independent. |
Frequently asked questions
Is intelligence genetic?+
Genetic variation accounts for a substantial share of the variation in intelligence between people — heritability estimates rise from about 20% in infancy to perhaps 80% in later adulthood. But heritability is a statistic about a population in a particular environment. It does not mean any individual's intelligence is a fixed genetic quantity, and it does not mean intelligence cannot change.
What does heritability actually mean?+
It is the proportion of the variation in a trait, across a particular population in a particular environment, that is statistically associated with genetic variation. It is a ratio of variances and it does not apply to individuals. A heritability of 60% does not mean 60% of your intelligence came from your genes — that sentence has no coherent interpretation.
Why does the heritability of intelligence increase with age?+
The leading explanation is that people increasingly select, modify and evoke environments that correlate with their genetic propensities. A small initial disposition compounds over decades into a large environmental difference, which registers statistically as genetic influence because a genetic difference set it in motion. Meanwhile the shared family environment's effect falls towards zero by adulthood.
Does high heritability mean IQ cannot be changed?+
No. Height is more heritable than intelligence and average adult height rose by more than ten centimetres in a century through nutrition alone. IQ scores rose across the twentieth century faster than the gene pool could change. Phenylketonuria produces severe intellectual disability, is entirely genetic in origin, and is entirely prevented by diet.
Does poverty reduce the heritability of intelligence?+
In the United States, yes. Turkheimer and colleagues (2003) found heritability near zero in the most impoverished families. But Tucker-Drob and Bates's meta-analysis of 24,926 twin and sibling pairs found the interaction clearly present in US samples and negligible or reversed in Western Europe and Australia, where access to education and healthcare is more uniform.
Have scientists found the genes for intelligence?+
There is no gene for intelligence. It is highly polygenic, involving thousands of variants each contributing a tiny fraction of an IQ point. Genome-wide polygenic scores currently predict on the order of 10% of the variance in intelligence — useful for research, and far too imprecise to say anything about an individual.
What is the missing heritability problem?+
Twin studies imply heritability of 50–80%, while polygenic scores built from identified variants predict around 10%. The gap is unexplained. Candidate reasons include rare variants that current genotyping arrays miss, gene-environment correlation absorbed into twin-study estimates, and assumptions built into the twin design itself.
Does heritability explain differences between groups?+
No, and this is a formal point rather than a contested one. Lewontin's illustration makes it clear: sow genetically varied seed in two plots, one rich and one depleted, and within each plot height will be almost entirely heritable while the difference between plots is entirely environmental. Within-group heritability licenses no inference about the causes of between-group differences.
Sources
This guide draws on standard psychometric references and peer-reviewed research:
- 1.Plomin, R., & Deary, I. J. (2015). “Genetics and intelligence differences: Five special findings.” Molecular Psychiatry, 20(1), 98–108.
- 2.Plomin, R., & von Stumm, S. (2018). “The new genetics of intelligence.” Nature Reviews Genetics, 19(3), 148–159.
- 3.Turkheimer, E., Haley, A., Waldron, M., D'Onofrio, B., & Gottesman, I. I. (2003). “Socioeconomic status modifies heritability of IQ in young children.” Psychological Science, 14(6), 623–628.
- 4.Tucker-Drob, E. M., & Bates, T. C. (2016). “Large cross-national differences in gene × socioeconomic status interaction on intelligence.” Psychological Science, 27(2), 138–149.
- 5.Ritchie, S. J., & Tucker-Drob, E. M. (2018). “How much does education improve intelligence? A meta-analysis.” Psychological Science, 29(8), 1358–1369.
- 6.Lewontin, R. C. (1970). “Race and intelligence.” Bulletin of the Atomic Scientists, 26(3), 2–8.
- 7.Neisser, U., et al. (1996). “Intelligence: Knowns and Unknowns.” American Psychologist, 51(2). APA.
- 8.Flynn, J. R. (2007). What Is Intelligence? Beyond the Flynn Effect. Cambridge University Press.
- 9.Deary, I. J. (2020). Intelligence: A Very Short Introduction (2nd ed.). Oxford University Press.
- 10.American Psychological Association (APA)
Sources are provided for further reading. Organization links point to official sites; academic works are cited in full. See our research standards and editorial team.
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