The Nature of Intelligence

Today, few scientists–or even members of the wider public–question the existence of a general cognitive ability that is substantially influenced by genetics. In a survey of 2,000 parents and teachers, more than 90 percent accepted that nature (genetics) is at least as important as nurture (environment) in the origins of intelligence (see “Intelligence Explained”).

Research on intelligence has moved beyond the nature-versus-nurture issue to investigate how rather than how much. My team, for example, has discovered that the genetic influence on IQ becomes more pronounced during development. In a study of 11,000 pairs of twins from four countries, we have recently shown that the heritability of IQ increases linearly from childhood (about 40 percent) to adolescence (about 55 percent) to young adulthood (about 65 percent). Why? No one knows, but my guess is that the answer involves what is called genotype-environment correlation: as children grow up, they increasingly select, modify, and even create their own experiences, partly on the basis of their genetic propensities. A child genetically inclined toward high verbal skills might choose to read more, enhancing those skills.

We have also found that the same set of genes affects different mental abilities. Genes that affect verbal abilities, such as vocabulary and verbal fluency, are largely the same genes that affect nonverbal abilities, including spatial visualization and memory.

Although these findings have far-reaching implications for educational policy and practice, the field of education has scarcely begun to take the nature of intelligence seriously. Heritability does not imply immutability. Nonetheless, the pervasiveness of genetic differences suggests that we must reëxamine the role of education. Instead of thinking about it as a way of countering genetic differences among children, education might profit from accepting that children differ genetically in how and how much they learn. Understanding the nature of intelligence is compatible with the current trend toward personalized education.

Finding a genetic influence on intelligence does not mean that we ought to put all our resources into educating the best learners and forget the rest. This finding could be used to argue for devoting more resources to genetically disadvantaged children. The relationship between knowledge and value is complicated, but there is nothing to be gained by pretending human differences do not exist.

Robert Plomin is a professor of behavioral genetics at the Institute Of Psychiatry at King’s College London.