Another widespread myth has been busted, this time the belief that a large proportion of fathers are tricked into thinking their children are biologically their own. The myth has sustained a thousand talk shows and made companies offering paternity tests rich, but none of that makes it true. "Media and popular scientific literature often claim that many alleged fathers are being cuckolded into raising children that biologically are not their own," said Dr. Maarten Larmuseau of KU Leuven, Belgium, in a statement. "Surprisingly, the estimated rates within human populations are quite low – around 1 or 2 percent."

The claim that at least 10 percent of children are raised by men who wrongly believe they are the biological father is a favorite on male-dominated Internet threads. The idea feeds into ancient fears, and was bolstered by a growing body of genetic evidence that something similar is true for many species of apparently monogamous animals.

Besides fueling an industry, the claim that extra-pair paternity (EPP) is rife is beloved by evolutionary psychologists and opponents of child support laws. Not surprisingly, tests conducted on behalf of men who suspect they have been lied to about their children's parentage don't form a representative sample of the population as a whole. Not only that, but the claim has been debunked before by individual studies. In Trends in Ecology and Evolution, Larmuseau provides an overview of the data, showing that current EPP is even lower than previously suggested. He follows this up with research that indicates this is not a recent phenomenon brought on by modern technologies or social changes.

One method to test the frequency of EPP before the arrival of the contraceptive pill and no fault divorce is to compare the Y chromosomes of men who are thought to be related through grandfathers and great grandfathers.

"In addition, two further indirect approaches were developed that provide estimates of past EPP rates by analyzing the association between Y chromosomal variation and patrilineally inherited surnames," the paper reported. Four studies, one previously published by Larmuseau using these techniques, produced estimates of EPP rates of around 1 percent in one South African and three European populations. Other studies that gave higher results had methodological problems that likely led to over-estimates, or were done among unrepresentative groups.

These findings, the paper noted, "[pose] an immediate puzzle for behavioral scientists, who estimated that without the availability of modern contraceptives that historical EPP rates should have been much higher, in the range of 10-20 percent."

The authors raise several possible explanations, including the possibility that traditional contraceptives were more effective than previously recognized, or that the "sexual revolution" represented a sharper change than anthropologists have thought, with religion and social mores previously keeping infidelity low among married women.

Further study "is not just of interest to evolutionary biologists," the paper argued, but could have applications in population genetics, medicine, forensics, and the epidemiology of sexually transmitted diseases.