Twins: And What They Tell Us About Who We Are by Lawrence Wright

measurable variation to genetic causes. (These studies measure statistical differences within populations. They do not imply that fifty percent of any one individual's personality is genetically acquired.)
Bouchard's team compared the personality scores for separated MZ twins and separated DZ twins against the scores for twins of both types who had been reared together. They concluded that identicals reared apart were about as much alike asin some cases more alike thanidenticals reared together. Moreover, there was not a single one of those personality traits in which fraternal

 

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twins reared together were more alike than identicals reared apart. How could this be? Wouldn't twins who had grown up in the same family, gone to the same schools and churches, and been exposed to the same values and traditions have been similarly shaped by those influences? If, as the Minnesota team was claiming, half of the variance in personality in a population was genetic in origin and the other half was environmental, why wouldn't fraternal twins reared together be far more alike in their personalities than identicals reared apart?
The answer to this paradox had been suggested before, but not with the force of so much data. ''None of the environmental variance is due to sharing a common family environment," the Minnesota team asserted. None? Bouchard and his colleagues repeated the charge in an unsettling 1990 article in Science . "The effect of being reared in the same home is negligible for many psychological traits," the Minnesota team wrote. Even in social attitudes, such as religiosity and traditionalism, adult identical twins were about as much alike regardless of whether they had been reared together or apart. "We infer," the Minnesotans wrote, "that the diverse cultural agents of our society, in particular most parents, are less effective in imprinting their distinctive stamp on the children developing within their spheres of influenceor are less inclined to do sothan has been supposed."
Bouchard's findings confirmed a startling, much-debated adoption study that had been done at the University of Minnesota a few years before his twin studies began. Sandra Scarr and Richard A. Weinberg compared the IQs of adopted children with those of their adopted parents and their biological parents. As is typical in adoption studies, the IQs of the children were higher than those of their biological parents, but still strongly correlated. The inference usually drawn from this information

 

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is that the adopted parents tend to be of a higher intelligence and a higher social class than the parents who surrender their children for adoption, and that the improved scores reflect this enriched environment. Scarr and Weinberg pointed out, however, that the children's IQ did not correlate at all with the IQ or the social class of their adoptive parents. They concluded that most families provide "functionally equivalent" environments; in other words, a person could grow up in one family as well as another and still have, at the end of adolescence, the same IQ in either case. Scarr and Weinberg qualified their observations to point out that they were not talking about extremes of environment; children raised in truly deprived circumstances would no doubt show the effects. But the elevation in IQ that was supposed to follow the rise in economic status simply wasn't present. "Why are the relatively poor families rearing adopted children whose IQ scores are nearly as high as those in professional families?" they asked. "It must be that all of these seeming environmental differences that predict so well the outcome differences among biological children are not primarily environmental differences, but indices of genetic differences among the parents and their biological offspring." In other words, the lower intelligence of the lower working class was a result of their genes, not their situation. It was the same observation made by Francis