Are men biologically designed to avoid having affairs with their friends' wives?
Adult males' testosterone levels drop when with spouses of close friends
Usually testosterone levels spike in the company of potential sex partners
Researchers suggest this could have been a result of evolution
Untrustworthy men would have harmed social bonds of communities
Well, Duh! And I didn't have to go into hock for a PhD to realize that, early on.
The exception proves the rule... especially as to trustworthiness. When you screw your best friend's girl, what are you saying about yourself?
Same goes for incest of course which is probably why it's so popular a fantasy on here. But that brings up another question...