How Much of IQ Is Hereditary? What Genes Say and What They Do Not

One of the best-replicated findings in behavioral genetics sounds like a paradox: the genetic influence on IQ tends to rise with age. In young children it counts for less than in adults. To see why, we first have to clear up a word almost everyone misuses: heritability.

What heritability means

It is a statistical concept about populations, not a property of an individual. When someone says the heritability of IQ is, say, 50%, that does not mean half your intelligence comes from your parents and half from your environment. It means that, in a particular population at a particular time, about half the differences in scores between people are associated with genetic differences. Change the environment or the population, and the number can change.

Height is the classic example. It has high heritability, and yet average height in many populations rose several inches over a century thanks to better nutrition and health. High heritability does not mean unchangeable.

How it is studied

The most common methods are twin and adoption studies. Identical twins share nearly all of their DNA; fraternal twins share, on average, half of the genes that vary between people. If identical twins resemble each other more in IQ than fraternal twins do, a genetic contribution is inferred. Adoption studies compare adopted children with both their biological and their adoptive families. The best-known work, such as the University of Minnesota's studies of twins reared apart, found that identical twins were quite similar in their scores even after growing up in different homes.

How much is genetic, by age

It depends on age, and the exact figures vary by study. In a large longitudinal twin study in the United Kingdom, Claire Haworth and colleagues reported in 2010 that heritability was about 41% at age 9, 55% at 12, and 66% at 17. The most accepted explanation is that as you grow up you increasingly choose and build environments that suit your inclinations, which amplifies the initial genetic differences. In early childhood, the shared family environment weighs more.

There are also signs that context matters. A 2003 study by Eric Turkheimer and colleagues suggested that in very low-income families the shared environment explained a larger share of IQ and genes a smaller one, while in well-off families the reverse held. Other studies have not reproduced this consistently, so it is a debated finding, not a settled fact.

There is no gene for intelligence

Intelligence is a polygenic trait: thousands of genetic variants, each with a tiny effect, contribute to differences. Genome-wide association studies have built polygenic scores that today explain only a small fraction of individual differences, noticeably less than twin studies suggest, a gap researchers are still trying to explain. These scores cannot predict the IQ of a specific person.

What none of this means

• It is not destiny. Genes and environment interact; nutrition, education, health, and early stimulation shape how potential is expressed.
• It is not individual. Heritability describes populations, and it does not tell you how much of your own intelligence is due to your genes.
• It says nothing about groups. That a trait is heritable within a population does not explain average differences between different populations, which depend on very different conditions.
• It does not make effort pointless. Learning, practicing, and studying still change what you know and what you can do.

An example that makes it clear

Consider phenylketonuria, a genetic condition. It is caused by a gene variant, but if it is caught at birth and the diet is managed, its effects on intellectual development can largely be prevented. In the United States it is part of routine newborn screening, the heel-prick blood test every baby gets. The example shows that a genetic origin does not imply a fixed outcome: the environment can change what genes do. Something similar happens with IQ, in a far less dramatic way and with thousands of small factors at play at once.

Genes matter, the environment matters, and the two are woven together over a lifetime. The useful question is rarely how much is genetic, but which environments let each person develop their best. If you want to know where you stand today, our free IQ test takes only a few minutes; treat it as a snapshot of the moment, not a verdict on who you are.