Consider, for instance, this story: A missionary visited a Maori village in 19th century New Zealand and there was a feast in his honor. After the feast, the Maori chief called out, "A woman for the bishop." Noting the scowl on the prelate's face, the obliging chief roared again, even louder: "Two women for the bishop!"

Given the choice, most men would rather have two women than one (although not necessarily at the same time). And they'd rather have three than two. As Margaret Mead once pointed out, monogamy is the most difficult of all marital arrangements. For some time, biologists have had a good idea why. Men are sperm makers. Sperm are cheap and easily replaced. Unlike eggs, they do not require that the guy doing the fertilizing become pregnant, give birth and then nurse his young.

For women -- and females of most other species as well -- the situation is quite different, and, not surprisingly, females tend to be comparison shoppers, and sexually reticent compared to their male counterparts. So the interesting thing about the Jackson affair isn't Jackson's behavior. After all, ever since "The Scarlet Letter" we've been told that even men of the cloth are still men, under the cloth. Rather, it's why Karin Stanford, his paramour, went along.

And here, biologists have finally begun to catch up with common sense. Women, too, have extramarital affairs, and these need not be limited to an unmarried woman having sex with a married man, as with Stanford and Jackson (though it has been reported that she had a boyfriend at the time of her affair with Jackson), or Monica Lewinsky and President Clinton, or Donna Rice and Gary Hart, or ... There are plenty of Madame Bovarys and Anna Kareninas. Adultery is a favorite human topic, in more ways than one.

Enter DNA fingerprinting, and not just for Monica's fabled blue dress. This laboratory technique isn't only useful for identifying unknown soldiers or freeing the falsely convicted. In recent years, it has surprised biologists with a whole new world of screwing around among animals, with likely implications for that troubled animal, Homo sapiens, the one that tries so hard to be monogamous and finds it so terribly difficult.

When we examine the genes of baby birds, even those species long thought to be absolute paragons of monogamous fidelity, we find that 10, 20, sometimes 30 percent of the offspring are not genetically connected to the socially identified father. Social monogamy (what biologists still call, somewhat quaintly, a pair bond) is not the same as sexual monogamy. Several decades ago experimenters vasectomized redwinged blackbirds in the hope of controlling their numbers. But many females, ostensibly mated to only those vasectomized males, laid eggs that hatched! Something funny was going on. But only now, with the accumulation of literally dozens of research studies using DNA data, do we know for sure: Females, even females in species long thought to be sexually faithful, often are not.

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