BEHAVIORAL GENETICISTS JOKE that people with one child are environmentalists; those with more than one are geneticists. The story implies a self-evident aspect to the inheritance of personality: that children demonstrate temperament and behavioral differences at too young an age to allow environmental explanations. But if all this has always been obvious to parents, it was not just ignored by science for the past fifty years, it was vigorously denied—and still is in some Foucault-besotted corners of academia.

Kenneth Kendler theorizes why many people, including scientists, resist accepting a genetic influence over their actions—for other than the usual political reasons. “They fear,” he said, “that would mean they are not ...

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