A University of Missouri study that found adult males’ testosterone levels dropped when they were interacting with the spouse of a close friend…

Mark Flinn, professor of anthropology at Missouri and lead author on the study, said: ‘Although men have many chances to pursue a friend’s mate, propositions for adultery are relatively rare on a per opportunity basis.

‘Men’s testosterone levels generally increase when they are interacting with a potential sexual partner or an enemy’s mate.

‘However, our findings suggest that men’s minds have evolved to foster a situation where the stable pair bonds of friends are respected.’