Is IQ Genetic? What the Evidence Says

What heritability means

Heritability is the share of the variation in a trait within a population that is statistically associated with genetic variation. A heritability of 0.6 for adult IQ says that, in the population studied, 60% of the spread in scores goes with genetic differences and 40% with everything else (environment, measurement error, luck). It does not say that 60% of your IQ comes from your genes — that sentence has no meaning, any more than “60% of the area of a rectangle comes from its width”.

Three consequences follow. Heritability can be high while a trait is highly changeable: height is about 80% heritable and average height rose by 10 cm in a century. It depends on the environment the population lives in. And it says nothing about differences between groups that live in different conditions.

The twin and adoption evidence

Identical twins share all their DNA; fraternal twins share half on average. If identical twins are more alike in IQ than fraternal twins raised in the same homes, genes are implicated. They are: correlations for IQ are typically around 0.85 for identical and 0.60 for fraternal twins reared together. Identical twins reared apart, a rarer group, still correlate around 0.75. Adopted children's IQs correlate more with their biological parents than with their adoptive parents by adulthood, although adoption into a better-off home raises IQ by several points on average.

Across studies, the pattern Robert Plomin and others have documented is the Wilson effect: heritability rises with age, from roughly 0.2 in infancy to 0.4 in middle childhood, 0.6 in adolescence and 0.7–0.8 in adulthood, while the shared family environment's contribution falls toward zero. The usual explanation is that as people gain control over their lives, they select environments that fit and amplify their genetic tendencies.

What the genes themselves say

Genome-wide association studies with more than a million participants have now identified thousands of common variants associated with educational attainment and cognitive performance, each with a minuscule effect. A polygenic score built from them explains about 10–15% of the variation in cognitive test scores in European-ancestry samples — much less than twin studies imply, a gap known as “missing heritability”, partly due to rare variants and partly to the limits of the methods. The practical point is the same either way: intelligence is massively polygenic, and no single gene explains more than a fraction of a point.

What environment does

The same literature shows large environmental effects. Schooling raises IQ by roughly 1–5 points per additional year. The Flynn effect — gains of about three points per decade through the twentieth century — happened far too fast to be genetic. Severe deprivation lowers IQ, and remediation in early childhood recovers part of the loss. In low-income families, heritability estimates in some (mainly US) studies are markedly lower than in affluent ones, because environmental differences, not genetic ones, dominate when environments are poor.

Lead exposure, iodine deficiency, prematurity and childhood illness all lower measured intelligence; their removal is a large part of why average scores rose. None of this contradicts the twin studies. High heritability within a well-nourished, well-schooled population and large environmental effects at the margins are both true.

What this means for your own score

Your IQ is the product of a genetic starting point and everything that happened afterwards, and the two cannot be separated in one person. What you can act on is the second part. The gains available from sleep, exercise, education and treating hearing or vision problems are modest for an adult but real; the gains from “brain training” apps are not. Our review is in can you improve your IQ?, and if you want a baseline to compare against later, the free IQ test gives you one in about eight minutes.

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