Reading the internet, or even perusing the scientific literature, you'd get the idea that people are constantly cheating on their spouses. Indeed, scientists have estimated that anywhere from 10-30 percent of men are unknowingly raising children who are not their own. This situation is referred to as cuckoldry, or scientifically as "extra-pair paternity." Now, however, it appears that our estimates of cuckoldry rates were way off.
A new survey published in Trends in Ecology and Evolution sums up a number of recent studies that show the actual rate of cuckolds in the general population, based on genetic testing and ancestor research, is 1-2 percent. This challenges evolutionary psychologists who have suggested that human women "routinely ‘shop around’ for good genes by engaging in extra-pair copulation to obtain genetic benefits." This idea came in part from studying socially monogamous songbirds, which mate for life but have roughly 1 in 10 babies as a result of "extra pair" matings.
Scientists were so unwilling to believe that human women were different from songbirds that some suggested the discrepancy between expected and actual rates of cuckoldry was a recent development caused by birth control. One study asserted that women who cheat may be getting pregnant less often than they would have historically. But that assumption turned out to be wrong as well. As the study authors write, human extra-pair paternity rates "have stayed near constant at around 1% across several human societies over the past several hundred years."
The researchers call this a "puzzle," because contraceptives would have been far less available centuries ago. Why are there so few cuckolds? The authors speculate that women may be remaining faithful due to "anti-cuckoldry tactics, such as male sexual jealousy, religious practices that regulate female sexuality, and strongly negative reactions towards female adultery." It's also possible that they fear sexually transmitted diseases, spousal abuse, divorce, or "reduced paternal investment by the social partner or his close relatives if the infidelity was discovered."
Ultimately, conclude the researchers, "the (potential) genetic benefits of extra pair children are unlikely to be offset by the (potential) costs of being caught, particularly in such a long-lived species as humans with heavy offspring dependence and massive parental investment." Basically, women aren't cheating because the costs are too great. Or, you know, it could just be that researchers have completely misunderstood the role of extra-marital sex in women's lives.
Humans seem to have evolved to be primarily monogamous, with occasional cheating, said University of Michigan psychology professor William McKibbin, PhD, at APA's 2011 Annual Convention. As a result, about 4 percent of children worldwide are fathered by someone other than the man who believes he is the father, according to a meta-analysis published in the Journal of Epidemiological Community Health (Vol. 59, No. 9). That tendency allows females to have more genetic variety among their offspring, but for the cuckolded male's genes, it's bad news.
"It's a double whammy," McKibbin said. "Not only are you not having your own offspring, you're devoting your time, energy and resources to another male's offspring."
To defend against cuckoldry, men have developed a variety of behavioral and biological defenses, McKibbin said. In one study, in press in Comparative Psychology, McKibbin and his colleagues found that men at greater risk for cuckoldry (as measured by the proportion of time they'd spent away from their partners) became more interested in having sex with their partners. They also found their partners more attractive and engaged in more "mate guarding" behavior—for example, monopolizing their partners' time at a party. This effect was independent of the amount of time since the couple last had sex, so it wasn't just the result of built-up desire—and it was moderated by how much a man trusted his mate not to cheat, McKibbin found.
This line of research is controversial but important because it may help us better understand—and prevent—sexual coercion and rape, McKibbin said. One such finding, in McKibbin's Comparative Psychology study: Men at risk for cuckoldry were later more likely to pressure their partners into having sex.
These findings, in combination with past research showing that men at risk for cuckoldry produce more sperm, thrust more vigorously and are more interested in their partners' orgasms than males whose partners' haven't had a chance to cheat, suggest that sperm competition has been common throughout human history, said McKibbin.
"Cheating has been around for a very long time," he said.
An act of adultery committed by a married woman against her husband.