"From what we can tell now, testosterone is generated to prepare the body to respond to competition and/or challenges to one's status," McAndrew observes. "Any stimulus or event which signals either of these things can trigger an increase in testosterone levels."

It makes sense—in the short-term, testosterone helps make both males and females bigger, stronger and more energetic, all of which would be useful for winning a physical or even mental contest. Testosterone is also responsible for libido in both sexes, and if researchers like Josephs are correct, it powers our drive for social dominance, which is one way that humans decide who gets to mate with whom.

Arguably, the weak correlation between testosterone and violence gives us reason to be optimistic about the human race: Whereas other animals battle over mates as a direct result of their seasonal fluctuations in testosterone and other hormones, humans have discovered other ways to establish pecking orders. Which isn't to say that we can't rapidly adapt to the modern-day manifestations of our violent past: McAndrews's work demonstrated that one surefire way to raise a man's testosterone level is to allow him to handle a gun.