Researchers from the University of Missouri and Indiana determined the testosterone levels of the men they studied actually dropped when they spoke to the lover of their close friend.

"Although men have many chances to pursue a friend's mate, propositions for adultery are relatively rare on a per-opportunity basis," said study author Mark Flinn, professor of anthropology at UM.

Testosterone levels typically increase when men communicate with a potential sex partner or an enemy's spouse.

The findings suggest "men's minds have evolved to foster a situation where the stable pair bonds of friends are respected," Flinn said.

"Ultimately, our findings about testosterone levels illuminate how people have evolved to form alliances."

The study is published in the journal Human Nature.