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.