The truth about misassigned paternity

Father’s Day inspires sentimental cards, tacky neckties, cheap aftershave and, at least in some circles, suspicion and doubt. What if the child you think is the fruit of your loins actually sprang from someone else’s seed?

With DNA tests now widely available, so-called paternity fraud has become a staple of talk shows and TV crime series. Aggrieved men accuse tearful wives who profess their fidelity, only to have their extramarital affairs brought to light. Billboards in Chicago and other cities provocatively ask, “Paternity questions?” and advise that the answers are for sale at your local pharmacy in the form of at-home DNA paternity tests. Some fathers’-rights groups in Australia have called for mandatory paternity testing of all children at birth, with or without the mother’s consent or even her knowledge.

And people are pretty well convinced there is a need for all this vigilance. When asked to estimate the frequency of misassigned paternity in the general population, most people hazard a guess of 10%, 20% or even 30%, with the last number coming from a class of biology undergraduates in a South Carolina university that I polled last year. I pointed out that this would mean that nearly 20 people in the class of 60-some students had lived their lives calling the wrong man Dad, at least biologically. They just nodded cynically, undaunted. Even scientists will quickly respond with the 10% figure, as a geneticist colleague of mine who studies the male sex chromosome found when he queried fellow biologists at conferences.

But the truth, insofar as we can tell, is much less sensational. According to the most unbiased research, the real incidence of misassigned paternity in Western countries hovers around 1%, with a few studies pushing that number to 3% or nearly 4%. That’s one-tenth as common as the conventional wisdom has it. It appears that we humans are more honest, monogamous and faithful than we think. So what’s behind all that lack of faith in the family?

Not surprisingly, when you press people for the source of their conviction, they rarely have anything much except a notion to offer. They “heard it somewhere,” or (in the case of the scientists) are “sure it was in a paper published awhile back.”

Unbiased studies are hard to find, because most people undergo paternity testing only if they have a reason to suspect a discrepancy between the purported father and the genetic one. Using data from the companies that sell the at-home tests, for example, is certain to yield an overestimate of misassigned paternity.

A handful of medical studies that get around this problem do exist. Most of them gathered information on the parents of children with genetic disorders like Tay-Sachs disease or cystic fibrosis, in which the child has to inherit a copy of the defective gene from both parents to show the disease. When large numbers of families are surveyed for such research, a certain proportion of fathers turn out not to have the gene that their purported child inherited, thus yielding the figures of 1% to 3.7%. Higher numbers, particularly the often-cited 10%, seem to come from more biased samples, or, more likely, simply turn out to be an urban legend, akin to cellphones being able to pop popcorn. (Ironically, one of the paternity testing services advertises on snopes.com, an Internet myth-busting site that will cheerfully disabuse you of the notion that the “Taps” melody came from a scrap of paper in the pocket of a dying Civil War soldier.)

Australian sociologist Michael Gilding thinks we believe in all that duplicity because we’ve been influenced by pop evolutionary psychology, which emphasizes the gains our (male) ancestors would have made by sowing their seed far and wide. I’m not so convinced, simply because that class in South Carolina, for instance, was pretty shaky on the basics of evolution, let alone its application to modern psychology. Maybe we’re all too aware of infidelity and assume that accidents must often happen. And yet, even if people are fooling around in large numbers, the data show they are also taking the paternity of their children seriously — men are still getting matched with their own offspring, even if women change partners.

So take heart, fathers. When 96.3% of you open that tie with the soccer ball print, or sniff that acrid cologne, you can rest assured that at least it came from your very own flesh, blood and DNA.

Marlene Zuk is professor of biology at UC Riverside and author of the forthcoming “Sex on Six Legs.”