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Who's your daddy?

Could 10 percent of men really be deceived about the paternity of the children they're raising? Evolutionary psychologists want to know.

(Greg Klee/Globe Staff Illustration)

MY 2-YEAR-OLD son sometimes points to the mailman and gleefully shouts, ``Grandpa!" I am reasonably sure that this is because the mailman, like my own father, has white hair. Yet reading the work of evolutionary psychologists can sow doubts in a man's mind.

For the past two decades, these scientists have been suggesting that roughly one in 10 children born within monogamous relationships was fathered by someone besides the man who thinks he's the father. A 2002 article in Nature Reviews Genetics was even more alarming, suggesting that the figure could be as high as 20 percent.

Could so many men really be deceived about the origins of their children? Kermyt G. Anderson, an anthropologist at the University of Oklahoma, has been skeptical about the figure since he was in graduate school in the 1990s. In an article in the June issue of Current Anthropology he suggests that most of us married males can rest easy: The existing data aren't good enough to settle the issue definitively, but he makes a pretty good case that 10 percent is too high.

``The upshot is that you shouldn't be worried if you are pretty sure the kid is yours," Anderson says.

Anderson's article (``How Well Does Paternity Confidence Match Actual Paternity?") is really a study of other studies: To come up with his own estimate, Anderson examined existing data sets.

The first group of studies he looked at, a set of 22 dating back to 1949, had originally been designed to examine genetic similarities between men and their children, and thus involved fathers who were quite certain their kids were theirs. (Your wife probably wouldn't let you and junior sign up for such a study if there were something she hadn't told you.) Looking at these studies, Anderson found that the average figure for nonpaternity, as determined by DNA or blood tests, was only 1.9 percent.

Of course, in the real world, not all men are confident their kids are their own, so Anderson scrutinized other data sets as well. He located, for instance, 31 data sets from paternity-testing labs, dating back to 1950. Unlike in the genetic studies, you can safely assume that the paternity of children tested in such places is in doubt. Predictably enough, the average nonpaternity figure for these studies was considerably higher: around 30 percent.

Given what he observed about the relationship between doubt and actual nonpaternity in these studies, Anderson then estimated what mix of paternity-confident and paternity-dubious men the general population would have to include in order for the widely cited 10 percent figure to be correct. According to his calculations, 25 percent of fathers would have to have doubts about who the fathers of their kids are.

That seems way too high-and indeed it probably is. Anderson, together with two scholars from the University of New Mexico, had already made a rough estimate of how many men look warily at the mailman. In their study, graduate students randomly approached men waiting in line at an Albuquerque Department of Motor Vehicles office, getting some 1,300 to agree to be interviewed about the paternity of their children. (As if the line at the DMV wasn't demoralizing enough already.) These men expressed doubt about their role in only 1.5 percent of the 3,360 pregnancies that occurred during their monogamous relationships.

. . .

Scholarly interest in human paternity uncertainty has increased with the revelations of just how unfaithful other supposedly monogamous animal species are. For example, a generation ago, ornithologists thought that 90-plus percent of birds in the passerine group-perching birds like crows, finches, warblers, and thrushes-were completely monogamous. But DNA testing has shown just how much cheating was going on behind the backs of male birds (and ornithologists). In fact, only 14 percent of female birds in this subfamily remained faithful to their mates, and 11 percent of their offspring were of what scientists call the extra-pair variety.

According to evolutionary psychologists, the logic behind this behavior is straightforward. In adaptive terms, extra-pair relationships are a way for females to seek out genetically rich males-or just genetic variety-while retaining the security of a life partner.

If Anderson is right, such behavior doesn't happen nearly so often among humans, but that doesn't mean it didn't happen more frequently in our evolutionary past.

Several studies have shown that a mother's relatives pay much more attention to her children than the father's relatives do. One proposed explanation is subconscious suspicions of paternity uncertainty. In 1997, Steven Gaulin, Donald McBurney, and Stephanie Brakeman-Wartell, at the time all affiliated with the psychology department at the University of Pittsburgh, asked 278 college students to rate on a scale from 1 to 7 how much their aunts and uncles seemed to care about their welfare. The students reported that the relatives on the mother's side of the family paid ``significantly more" attention to them.

The degree of reduced interest from the father's relatives was what you would expect, if those relatives were assuming a 13 to 20 percent chance the child wasn't related to them. Similar studies-including one published as recently as this May in Evolution and Human Behavior-have found that maternal grandparents pay significantly more attention to children than paternal grandparents, and have offered similar explanations.

``The issue is, `What were the conditions under which human adaptations evolved?"' says Gaulin, now at the University of California at Santa Barbara. We still fear snakes, he observes, even though snake venom kills almost no one these days.

Christopher Shea's column appears biweekly in Ideas. E-mail

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