How important is nature versus nurture in determining how smart someone is?

Well, a new study published online Tuesday by the journal Molecular Psychiatry indicates the genes someone is born with do play a important role, but are only part of the story.

In the first study of its kind, Ian Deary of the University of Edinburgh and colleagues conducted a “genome-wide association”analysis to try to gauge the role of genes in intelligence. The researchers analyzed DNA collected from 3,511 unrelated adults to see how much 549,692 common genetic variations known as single-nucleotide polymorphisms were associated with variations in intelligence.

Previous studies involving twins and adopted children have indicated that genes do play an important role in intelligence, perhaps accounting for about half the variation among individuals. But the relative contributions of genetics and environment remain controversial, and it has been unclear how much of a factor genes are. And scientists have been frustrated in their attempts to identify specific genes that may be involved in IQ.

In the new analysis, the researchers calculated that indeed perhaps 40 percent to 50 percent of individual differences in intelligence--both how much someone knows and how good the are at problem-solving--are due to genetic variations. But it appears that the differences in intelligence are due to many genes, each playing a small role. That means it will be difficult, if not impossible, to ever identify any genes that play a major role in IQ, the researchers say.

Nevertheless, the study “is the first that finds that a substantial proportion of the genetic contribution suggested by” earlier “studies can be found in signals in people’s DNA,” Deary wrote in an e-mail.

“If we have got it right, it looks like some contribution to intelligence differences is made by huge numbers of genes, and that each of these has a very small effect,” he wrote.

Eric Turkheimer, a psychologist at the University of Virginia, called the findings “unsurprising because I was never prone to doubt what we already knew from twin and adoption studies, to wit: intelligence is moderately inheritable.”

“The study does not take any steps in the direction of identifying the genes that are responsible, and in fact it confirms the most pessimistic concerns about our ability to do so,” he said.