Scientists have investigated this question for more than a century, and the answer is clear: the differences between people on intelligence tests are substantially the result of genetic differences.
But let’s unpack that sentence. We are talking about average differences among people and not about individuals. Any one person’s intelligence might be blown off course from its genetic potential by, for example, an illness in childhood. By genetic, we mean differences passed from one generation to the next via DNA. But we all share 99.5 percent of our three billion DNA base pairs, so only 15 million DNA differences separate us genetically. And we should note that intelligence tests include diverse examinations of cognitive ability and skills learned in school. Intelligence, more appropriately called general cognitive ability, reflects someone’s performance across a broad range of varying tests.
Genes make a substantial difference, but they are not the whole story. They account for about half of all differences in intelligence among people, so half is not caused by genetic differences, which provides strong support for the importance of environmental factors. This estimate of 50 percent reflects the results of twin, adoption and DNA studies. From them, we know, for example, that later in life, children adopted away from their biological parents at birth are just as similar to their biological parents as are children reared by their biological parents. Similarly, we know that adoptive parents and their adopted children do not typically resemble one another in intelligence.
Researchers are now looking for the genes that contribute to intelligence. In the past few years we have learned that many, perhaps thousands, of genes of small effect are involved. Recent studies of hundreds of thousands of individuals have found genes that explain about 5 percent of the differences among people in intelligence. This is a good start, but it is still a long way from 50 percent.
Another particularly interesting recent finding is that the genetic influence on measured intelligence appears to increase over time, from about 20 percent in infancy to 40 percent in childhood to 60 percent in adulthood. One possible explanation may be that children seek experiences that correlate with, and so fully develop, their genetic propensities.
The ability to predict cognitive potential from DNA could prove tremendously useful. Scientists might use DNA to try to map out the developmental pathways linking genes, intelligence, the brain and the mind. In terms of practical implications, we have known for decades about hundreds of rare single-gene and chromosomal disorders, such as Down syndrome, that result in intellectual disability. Finding additional genes that contribute to intellectual disability could help us perhaps prevent or at least ameliorate these cognitive challenges.