About half of the differences in human intelligence derive from variations in genes, according to a new study.

It is something of a truism that intelligence runs in families, and previous studies involving twins and adopted children established a correlation between genes and IQ scores. But the new study, published in the journal Molecular Psychology, found that at least a thousand individual genes each exerted a small influence on a person's IQ.

"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 professor 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."

By performing two tests on 3,511 patients -- one using vocabulary to measure recalled knowledge and the second gauging problem-solving skills -- researchers found that 40% of the variation in knowledge and 51% of the variation in problem-solving skills has a genetic root.

The finding re-enforces the notion that intelligence results from a confluence of different factors, both inherited and environmental. The genetic component results from a complex interplay of many genes working together, and scientists are still not certain which genes are at work. And since the test concluded genetics account for only half of the intellectual pie, the nurture factor is still fundamental.

"The findings leave a lot of room for environmental influences and for interactions between people's genes and their environment," Deary said. "It is a start to understanding the relationship between people's thinking skills and outcomes in life and to understanding why some people cognitively age better than others."

The study also suggested that scientists will be unable to isolate specific genes that cause intelligence, given the complexity of a constellation of different genes working in concert. That would stymie attempts to target specific genes in an effort to boost intelligence.

"Based on these data, it seems unlikely that we will identify one or two genes that make a very big difference in intelligence," Dr. Julio Licinio of Australia's National University Canberra told WebMD.