Is IQ genetic?

Heritability is probably the most misunderstood number in behavioural science. Getting it right first makes everything else straightforward.

What heritability is not

It is not the proportion of your intelligence caused by your genes. That statement does not even parse, and the intuition behind it is where nearly all the trouble starts.

Heritability describes variation within a population, under the conditions that population actually experienced. A heritability of 0.6 means roughly 60 percent of the differences between people in that group, in those circumstances, tracked genetic differences.

Change the circumstances and the figure changes. In a population where everyone receives identical excellent education, the remaining variation would be more genetic, and heritability would rise, without anyone’s genes having changed at all.

The standard illustration: height is highly heritable, and average height rose dramatically through the twentieth century with nutrition. High heritability, large environmental change. The two coexist without contradiction. The same argument applies to the Flynn effect, where raw scores climbed for decades in populations whose genetics did not.

What the evidence shows

Twin studies are the backbone. Identical twins share essentially all their DNA; fraternal twins about half. Identical twins consistently resemble each other more closely on IQ, including in the rarer cases where they were reared apart.

Adoption studies point the same way. Adopted children’s scores correlate more strongly with their biological parents than their adoptive ones by adulthood.

Estimates for adults commonly fall between about 50 and 80 percent, a wide range reflecting genuine methodological disagreement rather than false precision.

The finding that surprises people

Heritability of IQ rises with age.

You would expect the opposite: more years of life, more accumulated environment, less genetic signal. The observed pattern runs the other way. Shared family environment shows a substantial effect in childhood that shrinks towards adulthood.

The usual explanation is gene-environment correlation. As people gain independence they increasingly select environments matching their predispositions. Someone inclined towards reading finds more books, more conversation, more of the environment that develops the inclination further. Genetic differences get amplified by the environments they cause people to choose.

That is a considerably more interesting mechanism than genes simply determining an outcome.

Missing heritability

Here is where the story gets awkward for strong genetic claims.

If IQ is 50 to 80 percent heritable, you would expect genome-wide studies to identify the responsible variants. They have not, at least not to that magnitude. What they find is very large numbers of variants each with minuscule effects, and together these explain far less variance than twin studies imply.

The gap is called missing heritability and it remains unresolved. There is no gene for intelligence, and there is unlikely to be one.

The between-groups error

This one matters and it is a genuine statistical point, not a political one.

Heritability within a group tells you nothing about the causes of differences between groups. Two populations can each show high internal heritability while the gap between them is entirely environmental.

Plant identical seed in rich and poor soil. Within each tray, height differences are almost entirely genetic. The difference between trays is entirely soil. Applying a within-group statistic across groups is invalid regardless of what conclusion it is used to support.

What follows practically

Not much fatalism, which surprises people.

Heritability sets no ceiling on what a change in conditions achieves. Education still has a measured causal effect on scores. Nutrition, health and schooling still matter. The heritability figure simply does not speak to those questions.

Key takeaways

• Heritability describes variation within a population under given conditions, not the share of an individual’s ability caused by genes.
• Adult estimates commonly range from about 50 to 80 percent.
• Heritable does not mean fixed. Height is highly heritable and rose sharply with nutrition.
• Heritability of IQ rises with age, likely because people select environments matching their predispositions.
• Genome-wide studies find many tiny effects and cannot account for twin-study estimates. This is the missing heritability problem.
• Within-group heritability says nothing about the causes of between-group differences.