Research has found that when heterosexual men are around an attractive woman, they experience a natural increase in testosterone and, sometimes, become more prone to engaging in physically risky behaviors. For instance, one study found that when male skateboarders performed in front of a female observer, they experienced elevated testosterone levels and attempted more dangerous stunts that increased their likelihood of crashing . Scientists theorize that this spike in testosterone leads men to engage in sexual displays that "demonstrate their value" or manliness. So does this happen every time heterosexual men are in the presence of the other sex? According to a new study published in Human Nature, this increase in testosterone does not occur when men interact with a woman they know is already committed to one of their friends .
In this study, researchers collected saliva samples from men who interacted with one of three different types of women: (1) a female relative (a sister, half-sister, or first cousin), (2) the wife or girlfriend of a close friend, or (3) an unrelated, unattached woman. All saliva samples were collected no more than 20 minutes after the interaction took place and were then evaluated for testosterone.
Results indicated that testosterone levels were significantly higher among men who had interacted with an unattached woman (i.e., a potential mate) compared to guys who had interacted with a female relative or a friend’s partner. This suggests that heterosexual men’s brains and bodies respond differently to women based upon whether they represent a socially appropriate and available mate. The researchers theorize that this may be a potential neurobiological mechanism that helps to maintain harmonious alliances between men from the same group. In other words, perhaps humans have evolved a tendency to not desire their friends’ mates because it helps us to maintain friendships that are adaptive for our survival (an evolutionary “bro code,” if you will).
However, these data should be viewed as preliminary, given that testosterone levels were not assessed before and after the interaction, which prevents us from drawing too many conclusions about change. In addition, the researchers did not assess frequency of previous interactions between the men and women, nor did they study the behavior of the women during these interactions. Thus, we do not necessarily know what was driving the effect (e.g., was it the knowledge that the woman was available/unavailable, or were available women more likely to flirt?). However, these findings suggest the intriguing possibility that human males may naturally moderate their hormonal responses to women in a way that helps to maintain their relationships with close male friends.
 Ronay, R., & von Hippel, W. (2010). The presence of an attractive woman elevates testosterone and physical risk taking in young men. Social Psychological and Personality Science, 1, 57–64.
 Flinn, M. V., Ponzi, D., & Muehlenbein, M. P. (in press). Hormonal mechanisms for regulation of aggression in human coalitions. Human Nature. doi:10.1007/s12110-012-9135-y
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