Traditional twin and adoption studies have provided evidence for the influence of genes on many behavioral traits, from intelligence to phobias to TV watching. But the findings are still controversial among social scientists. An ingenious new study design may help overcome the objections, claims Nancy Segal, a psychologist at the University of California, Fullerton. Segal studies what she calls "virtual twins"--pairs of unrelated siblings of the same age, one or both adopted, who have been raised together from infancy. Virtual twins offer the negative image of the identical twins raised apart scenario, sharing a common family environment but no common genes.

In a study of 90 pairs of virtual twins to be published in the September issue of the Journal of Educational Psychology, Segal says she has confirmed one of the most surprising findings from behavioral genetics: the modest influence of shared environment (and hence the major influence of genes) on IQ scores. Segal gave standard IQ tests to the subjects, most aged 4 to 7 and no pair separated by more than 9 months. On average, each child's IQ correlated only loosely with that of its "twin": The correlation coefficient was 0.26, compared with 0.50 for siblings and 0.86 for identical twins.

Segal's results will "allow a 'cleaner' picture of shared environmental effects," says psychologist Tony Vernon of the University of Western Ontario. Those effects are typically greatest at very young ages, adds psychologist Robert Plomin of the Institute of Psychiatry in London. He says that, based on the results of adoption studies, IQ correlations between virtual twins are likely to drop to near zero as they reach adolescence.

Segal says her approach addresses two objections to behavioral genetics studies: that differences in ages and in times of placement obscure the results of adoption studies, and that "twinness"--being the same age and being treated the same--makes twins more alike. This study suggests that's not the case. Indeed, Segal quotes one father as saying that despite efforts to treat them equally, his pair of virtual twins are "like night and day."