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Another example of the parsimony of natural selection is our human xenophobia. We tend to distrust, fear, and dislike other humans who seem different than ourselves. This was adaptive in ancestral times when a stranger stepped out from behind a tree because that stranger might kill you, if you were a male, or to rape you or carry you off, if you were a female. A more selective mechanism would require both strangeness and threatening action to trigger our fear/dislike response. But just "stranger" turned out to be enough. Can this be why modern humans, both New Yorkers and the natives of Papua New Guinea, tend to paint and dress themselves in ways that immediately identify their group membership? And can this be one of the reasons why our contemporary Lockians want to believe in the tabula rasa mythology? "Let us not suppose that xenophobia is natural because then how could we hope to accomplish racial, religious, and social tolerance?" My three sons, white, non-theistic, Aryan types, are happily married to a Catholic, a Jew, and an African American, and have produced my ten beloved grandchildren. Reporting some early results of his celebrated study of twins who were separated in infancy and reared apart, my colleague, Tom Bouchard, pointed out that: "The genes sing a prehistoric song that must sometimes be resisted but which should never be ignored." Our xenophobia can be resisted, as my sons' example attests, but it should not be ignored, or we shall never be able to figure out what to do about Bosnia.

Another ancestral trait that we should not ignore is male sexual jealousy. One human sex difference that even Gloria Steinem cannot deny is that a woman knows that the baby she delivered is hers while her spouse cannot be certain it is his. (DNA data indicate, in fact, that about ten percent of human children could not have been produced by the mother's husband; for bluebirds, by comparison, the figure is about twenty percent.) The CEO of Natural Selection is aware of these facts and has endowed the males of our species with a suite of compensatory tendencies. David Buss has shown that, over many cultures, women are more disturbed by evidence that their mate has an affectionate relationship with another woman (an attachment that might lead him to invest his resources in her and her children) while men are much more concerned to learn that their mate has had sex with someone else. I once did some marriage counseling with a young "hippy" couple who were having problems. Their deeply held principles included opposition to the Vietnam War, support of environmental protections, legalization of psychoactive drugs, and free love. Their problem was that the young man was always grouchy and resentful. The solution to their problem was to accept the fact that most men cannot help feeling grouchy and resentful when their mate persists in having sex with other men. "Oh, Baby, I'm so sorry! I didn't think you cared!"

Another curious fact is that even some evolutionary psychologists, including Steve Pinker's mentors, John Tooby and Leda Cosmides, believe that the genetic differences between people, the very differences on which, during ancestral times, natural selection worked to make us what we are today, no longer exist. "Yes, we all come equipped with species-specific behavioral proclivities. Our infant brains are not just general-purpose computers waiting to be programmed by experience but, rather, they have modules that are preprogrammed to give us a head start at being human. But they are all alike at birth, except perhaps for bits of noisy artifact." Are these folks just being politic, just claiming only the minimum they need to pursue their own agenda while leaving the behavior geneticists to contend with the main armies of political correctness?

The denial of genetically based psychological differences is the kind of sophisticated error normally accessible only to persons having Ph.D. degrees. Even the be-doctored tend to give up radical environmentalism once they have a second child. In our twenty-five years of twin research at Minnesota, monozygotic twins, who share all their genes, have been found to be twice (or more than twice) as similar as dizygotic twins, who share on average half their polymorphic genes, on nearly every trait that we can measure reliably. The few exceptions include birth weight, years of education, romantic choice, and a few interests such as blood sports, gambling, and religious orientation. (Variation in general religiosity, on the other hand, is strongly genetic.) Moreover, monozygotic twins separated in infancy and reared apart, are as similar on most psychological traits as are MZ twins reared together. Middle-aged MZ twins, whether reared together or apart, correlate in IQ more than .70, and this is so whether IQ is estimated from the nonverbal Raven Matrices test administered and scored by computer, or from a standard IQ test individually administered by different examiners in separate rooms. IQ is not all there is to "intelligence" but it is very important. If your child's IQ is less than about 115, she is almost certain never to get through medical or law school.

One of the personality inventories that we use has a Well Being scale that measures current happiness. Like most psychological traits (even IQ), happiness varies from time to time due to the slings and arrows. When we measure Well Being in adult twins twice, ten years apart, the within-twin cross-time or retest correlation is only .55 (.02). But for MZ twins, the between-twin cross-time correlation (Twin A now vs Twin B then, etc.) is virtually the same, .54 (.03), suggesting that most of the happiness "set-point" or stable component is genetically determined. In contrast, the between-twin cross-time correlation for dizygotic (DZ) twins is only .05 (.07).

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