Intelligence Tests Highlight Importance of Genetic Differences

Genetic differences between people account for up to half of the variation in intelligence, according to a study of more than 3,000 individuals.

Intelligence is known to run in families, but no single genes have yet been identified that can be reliably linked to mental ability. Instead, researchers think, many hundreds or thousands of genes could be involved, each with a small influence on a person’s overall intelligence.

“It has been getting clearer and clearer that any genetic contribution to traits on which people differ–like height and weight–comes about from large numbers of gene differences, each with very small effects,” said Prof Ian Deary of the University of Edinburgh, who led the research on intelligence. “We thought that was one possibility for cognitive ability differences, and our results are compatible with that.”

To test his idea, researchers looked at more than half a million locations in the genetic code of 3,511 unrelated adults. Each of these sites is where people are known to have single-letter variations in their DNA, called single nucleotide polymorphisms (SNPs). These variations were correlated with the individuals’ performance in two types of psychometric tests that are established in assessing intelligence: one test measuring recalled knowledge (via vocabulary) and the second measuring problem-solving skills.

They found that 40% of the variation in knowledge (called “crystallised intelligence” by the researchers) and 51% of the variation in problem-solving skills (“fluid-type intelligence”) between individuals could be accounted for by the differences in DNA. The results are published on Tuesday in the journal Molecular Psychiatry.

Previous work on the environmental and genetic contributions to cognitive ability has been based on comparing intelligence in identical and non-identical twins, or studying it in people who were adopted. In the study led by Deary, the conclusions were gleaned from direct testing of people’s DNA. “It is the first to show biologically and unequivocally that human intelligence is highly polygenic [involving lots of genes] and that purely genetic (SNP) information can be used to predict intelligence,” Deary wrote in the journal paper.

Though the researchers now know the proportion of the variation in intelligence that is likely to be a result of genes, they do not know which genes are likely to be most important in determining intelligence. “If they can be found, and if we want to follow them up, to find out some of the mechanisms that underlie successful thinking, our best guess at present is that the number is huge. It could be many thousands,” said Deary. “That could be a limitation to progress using this type of research.”

Dr Simon Underdown, senior lecturer in biological anthropology at Oxford Brookes University, said human intelligence was a “stunning product of our evolution”. He continued: “This paper brilliantly demonstrates that the genetic basis for our intelligence is not the result of a simple mutation in a single gene. Rather, the diverse range of genes that appear to influence our ability to think must have been actively selected for over hundreds of thousands of years. That we display such genetically influenced variation in intelligence across our species further hints at how important cultural, as well as biological, evolution has been to the human story.”

[Editor’s Note: Here is a link to the abstract of the study.]