Men programmed to avoid sex with best friends' wives: study

A new US study finds that men may have a natural aversion to the idea of having sex with their best friends' wives.

A University of Missouri study has found that adult males' testosterone levels dropped when they were interacting with the marital partner of a close friend. Prior studies have already shown suggested that men's testosterone levels influence competition among males trying to attract females.

"Although men have many chances to pursue a friend's mate, propositions for adultery are relatively rare on a per opportunity basis," says lead researcher and anthropology professor Mark Flinn.

The team posits that humans evolved this way to keep the peace within tribes. "Men's testosterone levels generally increase when they are interacting with a potential sexual partner or an enemy's mate," he adds. "However, our findings suggest that men's minds have evolved to foster a situation where the stable pair bonds of friends are respected."

"Ultimately, our findings about testosterone levels illuminate how people have evolved to form alliances," says Flinn. He adds that the findings could have implications for larger issues, from resolving conflicts to solving mutual threats, such as climate change, that is, if people could "view the Earth as a single community of people," he says.

The study, announced Thursday, appears in the journal Human Nature.

Another study last year on male testosterone levels found that human males are biologically wired to care for their offspring, and that fatherhood lowers a man's testerone levels.

Research has also found that men are much more attracted to their female friends than vice versa -- and that men are also more likely to believe that their female friends are more attracted to them than they are in reality.