Hey brother, feel sexually drawn to your best friend's wife? Testosterone, the bad boy among male sex hormones, is supposed to make it easier for you to ignore your friendship and make your move.
However, scientists at the University of Missouri have found that men are biologically inclined toward avoiding a close encounter with the mate of a buddy, and it works the other way around if she is not committed to a friend.
Testosterone seems to be depressed if a friend is involved, but elevated if there is no close relationship, a condition the scientists describe as a "striking reversal" in the role of this powerful hormone. The study was published in the journal Human Nature.
"Men's testosterone levels generally increase when they are interacting with a potential sexual partner or an enemy's mate," anthropologist Mark Flinn, lead author of the study, said in releasing the report. "However, our finding suggests that men's minds have evolved to foster a situation where the stable pair bonds of friends are respected."
The findings should be regarded as tentative, because the number of participants was limited and some data may be compromised by the difficult circumstances under which it was collected, as the researchers note in their own study. The conclusions depend partly on data collected a few years ago in the Dominican Republic.
In some cases, for example, testosterone levels were not determined before the "interaction" with the female, so it's not known how much the level changed during the event, and it was not known if there were prior interactions with the same female.
"Even with these important limitations, the apparent dampening of androgen (sex hormones) levels when interacting with friends' mates is remarkable nonetheless, and consistent with mutual respect of mating relationships and enhanced cooperation among group males," the study notes.
The scientists see their study as much broader than just the sexual temptation involving a friend's mate, because additional research was carried out showing that testosterone is actively involved in a wide range of human activities, probably even international conflict.
They found, for example, that the level of testosterone soared in young men in a Dominican community when they competed in sporting events with a rival from another community, but it remained unchanged if the rival was a close neighbor. And that, they suggest, shows we are biologically determined to form relationships, or coalitions, with those around us -- so we will act less aggressively within our group -- but we are more willing to trample or attack outsiders.
"A victory against friends does not affect testosterone significantly, whereas a victory against outsiders results in elevated testosterone," the study concludes. "Likewise, a defeat by friends has little effect on testosterone, whereas a defeat by outsiders results in decreased testosterone from pre-competition levels."
The researchers take that a step further, suggesting that testosterone remains low to help members of a community work together and it rises to help defeat a threat from outside the community. Thus, they add, it may play a critical role in human interactions, even at the international level.
However, there is still some debate among scientists over testosterone. University of Zurich scientists found that testosterone does not cause aggression if that would threaten one's own status, as in don't pick a fight with a bully if he's really big.