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Heritability is one of the foundational concepts of behavioral genetics, but its meaning is not always clear. Does a study showing that IQ is highly heritable among affluent children in Denmark have any implications for poor children in the United States? Or is it largely irrelevant?

New research is making it increasingly obvious that the answer is: "It depends." Heritability, as the term is used by behavioral geneticists, is a statistical measure defined in relation to a particular environment and a particular population. The only way to find out whether the heritability of a trait is the same for other environments and populations is to go out and study them.

In a recent study, University of Virginia psychologist Eric Turkheimer, PhD, and his colleagues did just that. Their study explored the heritability of children's IQ in different populations within the United States--those with high socioeconomic status (SES) and those with low SES.

Previous studies of children's IQ have produced conflicting results. On the one hand, some studies of twins and adoptees have found large genetic effects. On the other hand, studies of impoverished children adopted by well-to-do families suggest that the environment plays an important role.

For their study, published last year in Psychological Science (Vol. 14, No. 6), Turkheimer and his colleagues analyzed data from several hundred monozygotic and dizygotic twins included in the National Collaborative Perinatal Project, which followed more than 48,000 mothers and their children from birth to age 7.

Turkheimer and his colleagues found that, among poor families, children who grew up in the same household tended to have similar IQ scores, regardless of how genetically similar they were. Around 60 percent of the variance was accounted for by environment, while genes contributed almost nothing. Among affluent families, the reverse was true. Monozygotic twins with identical genes tended to have much more similar IQ scores than dizygotic twins, regardless of family environment.

The findings suggest that it makes little sense to speak in general about the heritability of a trait such as IQ. For large populations of people who live in diverse environments, such as children in the United States, such broad statements may be meaningless. The environment can make genes extremely important in some subpopulations, but insignificant in others, notes Turkheimer.

Such findings do not challenge the traditional definition of heritability--the proportion of variance on a particular trait that is accounted for by genetic factors within the population as a whole, says Terrie Moffitt, PhD, of the University of Wisconsin and King's College London. But they are important reminders that heritability can vary dramatically depending on the population and the environment that is being studied.