Is Intelligence Hereditary?

Dr. Russell T. Warne

Sep 10, 2023

One of the great debates in psychology is the extent to which genes or environment matter more for determining a person’s IQ. Since the late 1800s, this has been called the “nature versus nurture” debate. In the 21st century, the debate has pivoted away from “nature or nurture” to the degree to which one is more important than the other.

Both genes and environment matter. Genes provide the blueprint for a person’s development, and without the blueprint, a trait (like high IQ, athleticism, or a mental health challenge) can’t develop. Environmental characteristics determine the extent to which that development will happen. At one extreme, a child raised in an extremely neglectful environment with no access to education or typical human contact will definitely have a low IQ, no matter what their genes are. On the other hand, the best environment in the world will not compensate if a person has genes that make developing high intelligence impossible. To develop any trait to a high level, including intelligence, a person needs the right genes and the right environment. None of this is controversial among scientists.

Because both genes and environment are important, it is legitimate to ask how important genes and environment are for a given trait and which one matters more. For psychological traits (like IQ, depression, and personality), a discipline called “behavioral genetics” explores this issue. By examining how similar people are who share genes and environment, they can estimate the percentage of differences in the trait that are due to genetic differences or environmental differences. The genetic influence is expressed as a proportion ranging from 0 to 1 and is called “heritability.” Heritability and environmental influence must add up to 1, which means that as heritability increases, the relative importance of environmental differences decrease. The reverse is also true.

Heritability of Intelligence

So, what is the heritability of intelligence? That’s really a trick question because heritability varies from environment to environment and from population to population. A population of clones would have 0 heritability for IQ because there are no genetic differences that could cause IQ score differences. Likewise, if every person had a perfectly identical environment, heritability would be 1 (i.e., 100%) because the equalized environments can’t cause IQ differences -- the only thing left would be genetic differences. Neither of those examples are realistic, though.

Most studies of heritability of IQ have occurred in non-neglectful environments in wealthy countries. In this range of environments, heritability tends to average about .50 to .60 (i.e., 50%), with higher heritability in adults and lower heritability in young children. Within the United States, heritability does not vary across the largest racial/ethnic groups, nor does it seem to vary consistently across socioeconomic groups (again, assuming that the environment isn’t extremely bad).

Environmental Influences on Intelligence

Although heritability for IQ can sometimes be very high, it is never 1, which means that environmental influences are never 0. The best evidence for the impact of the environment on IQ comes from adoption studies which show an increase of about 3 IQ points, compared to what would be expected if the adopted child had grown up with their birth family in a wealthy country. So, the types of homes that are allowed to adopt children in wealthy countries are environments that encourage some IQ growth.

There is also strong evidence that schools raise IQ. This is apparent in research on the Flynn effect, which shows that increasing education in the population is accompanied by a higher average performance on IQ tests. (This is not the only cause of the Flynn effect, though.) One excellent review of studies found that increasing education schooling by 1 year increases IQ by about 1 point. That doesn’t necessarily mean that education increases IQ by 1 point per year; it seems likely that earlier years of education might cause a stronger IQ increase, which might level off with each additional year of schooling. (This is hard to study in countries where everyone attends school for several years, though.)

Beyond the recommendation to send children to school (which is happening anyway in wealthy countries), it has been difficult to identify specific environmental influences that raise IQ. It is one thing to know that adopted families are raising IQ, it is quite a different thing to know the specific things that adopted parents do to raise IQ. So far, the search for specific things that adoptive parents do (and which other families could do in their families) has been fruitless.

On the other hand, there are several things that are known to lower IQ: brain trauma, lead poisoning, and exposure to toxins in pregnancy. Not experiencing these things can prevent a decrease in IQ, but that’s different from doing new things to raise IQ.

Genes for IQ

Just like specific environmental influences, there has also been a search for specific genes that influence IQ. This has been much more successful search, and currently there are hundreds of known genetic variants that are associated with differences in IQ. These can be combined to make predictions of IQ that have a very modest level of accuracy, which should improve in the future as research continues. Still, much more is known about specific genes that influence IQ than specific environmental influences.

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