Gene variants linked to childhood IQ.

Scientists have for the first time linked a specific gene to high intelligence, at least as assessed on a standard IQ test.

One variant of the gene, located on chromosome 6, appears to exert a small effect on a measure of an individual's general intellectual ability, reports a team headed by Robert Plomin of the Institute of Psychiatry in London. The gene is one of many that have specific variants which, when occurring together, can result in a much higher than average IQ score, Plomin's group proposes.

"The hope is that [this work] will provide discrete windows through which to view neurophysiological pathways between genes and behavior," the team observes in the May Psychological Science.

A DNA marker inserted into the gene for insulinlike growth factor-2 receptor (IGF2R) on chromosome 6 revealed that one form of the gene occurs more frequently in a group of 51 high-intelligence children, with an average IQ of 136, than in 51 youngsters with more modest scores, averaging 103. The same genetic difference appeared more often in a second test: 52 kids with an average IQ of 160 and 50 with an average IQ of 101.

The participants, all of whom are white, ranged in age from 6 to 15.

Plomin and his coworkers suggest that numerous variable genes make small positive or negative contributions to individual differences in IQ. Some folks possess most or all of the positive genetic versions, which add up to a high IQ, while a mix of positive and negative genes underlies average IQs.

The researchers excluded low-IQ youngsters from their analysis because depressed scores on intelligence tests can result from a variety of unusual events, such as oxygen deprivation at birth or prenatal exposure to cocaine.

Plomin emphasizes that the IGF2R variant associated with high IQs is not a "genius gene." It appeared in nearly half of the high-IQ children but also in nearly one-quarter of those with average IQs. The majority of high-IQ youngsters missing this specific gene presumably have other genetic variants that boost general intelligence scores, the researchers theorize.

The IGF2R gene variant accounts for less than 2 percent of individual IQ disparities, or a difference of about 3 IQ points, according to Plomin. The exact function of the gene, as it relates to cognitive ability, remains unclear, he notes.

"This is really important work," comments psychologist Robert J. Sternberg of Yale University, "but we don't know yet whether this approach to the genetics of intelligence will work." For example, Sternberg remarks, the amount of IQ variation attributed to the IGF2R gene is quite small, and it's not known whether enough genes with similar cognitive effects exist to add up to a substantial influence.

Also, a correlation between the presence of a gene variant and high IQ does not mean that the gene directly raises cognitive ability. The IGF2R gene may influence intelligence through interactions with other genes or with certain environments, making the type of analysis reported by Plomin's group difficult to interpret, Sternberg explains.