What Dabbs does not address is whether this correlation was the cause or an effect of the environment in which these men found themselves. Which is to say, are high-testosterone males more likely to become violent criminals, or does being a violent criminal raise a man's level of testosterone?
No one really knows the answer, but a growing body of evidence suggests that testosterone is as much the result of violence as its cause. Indeed, both winning a sporting match and beating an opponent at chess can boost testosterone levels. (Losing a sporting match, growing old and becoming obese all reduce levels of testosterone.)
The causal arrow goes both ways, says anthropologist Peter B. Gray of the University of Nevada, Las Vegas, whose work shows that marriage and fatherhood lower testosterone levels. There's evidence in humans that, just as in animals, testosterone is responsive to male-male competition.
Changes in testosterone levels in response to challenges can be further shaped by our expectations. In experiments from the 1990s that put a biological spin on the Civil War divide, psychologists Richard E. Nisbett of the University of Michigan at Ann Arbor and Dov Cohen of the University of Illinois had a volunteer accidentally bump into and then insult men who were raised either in the North or the South. The researchers hypothesized that Southerners come from a culture of honor in which aggressive responses to insults are culturally appropriate, and the results of their experiment bolstered that notion. Not only were Southerners more likely than their Northern counterparts to respond with aggression, but their levels of testosterone also rose as a result.
From what we can tell now, testosterone is generated to prepare the body to respond to competition and challenges to one's status, McAndrew observes. Any stimulus or event that signals either of these things can trigger an increase in testosterone levels.
That breadth of response 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 Josephs and other researchers 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 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. That doesn't mean we humans can't rapidly readjust our hormonal inheritance to the modern manifestations of our violent past: McAndrew's work demonstrated that one surefire way to raise a man's testosterone level is to allow him to handle a gun.