Professional wrestler Chris Benoit's powerful build and muscular grappling maneuvers helped to make him a crowd favorite and propelled him to a world heavyweight championship in 2004. No one was prepared for the shocking turn this past June when he killed his wife and son, then hanged himself in their home near Atlanta. The subsequent announcement by the state medical examiner's office that Benoit's body showed he had been taking injections of testosterone (along with an antianxiety drug and a painkiller) seemed all too predictable, given how often anabolic steroids such as testosterone have been linked to violent behavior.

And yet the official findings might still have offered one surprise: according to medical examiner Kris Sperry, there was no clear evidence that the steroids played a part in the murders. Benoit's levels of testosterone were 10 times normal, but as Sperry was quoted as pointing out, An elevation of that ratio does not translate into something abnormal in a person's thought process or behavior.

It's commonly assumed that testosterone, that stereotypically male hormone, is intimately tied to violence. The evidence is all around us: weight lifters who overdose on anabolic steroids experience roid rage, and castration--the removal of the main source of testosterone--has been a staple of animal husbandry for centuries.

But what is the nature of that relationship? If you give a normal man a shot of testosterone, will he turn into the Incredible Hulk? And do violent men have higher levels of testosterone than their more docile peers?

Historically, scientists had assumed the answer was yes, but the truth has proved more complex. Researchers expected an increase in testosterone levels to inevitably lead to more aggression, and this didn't reliably occur, says Frank T. McAndrew, a professor of psychology at Knox College in Galesburg, Ill. Indeed, recent research about testosterone and aggression finds only a weak connection between the two. And when aggression is more narrowly defined as simple physical violence, the connection all but disappears.

What psychologists and psychiatrists say is that testosterone has a facilitative effect on aggression, comments Melvin J. Konner, an anthropologist at Emory University and author of The Tangled Wing: Biological Constraints on the Human Spirit (Owl, 2003). You don't have a push-pull, click-click relationship where you inject testosterone and get aggressiveness.

Instead what emerges from experiments with surgical and medical castration is a more complex pattern of cause and effect. Testosterone may be necessary for enabling violent behavior, but it is not, on its own, sufficient. In that sense, testosterone is less a perpetrator and more an accomplice--one that is sometimes not too far from the scene of the crime.

In both men's and women's prisons, for example, the most violent inmates have higher levels of testosterone than their less violent peers. Yet scientists hypothesize that this violence is just one manifestation of the much more biologically and reproductively salient goal of dominance.

It has been suggested that the antisocial behaviors related to high testosterone are a function of the manner by which dominance is maintained in these groups, says psychologist Robert Josephs of the University of Texas at Austin. In other words, if researchers were to study other groups of folks--say, the rich and famous--they might discover that testosterone is connected not to violence but to the person who drives the biggest SUV or has the nicest lawn. As Josephs puts it: Slipping a shiv into your neighbor's back might play in the penitentiary, but it probably won't earn you any status points in Grosse Pointe.

The late psychologist James M. Dabbs made a career out of conducting studies connecting testosterone to every kind of lifestyle imaginable. In his book Heroes, Rogues and Lovers (McGraw-Hill, 2001), co-authored with Mary Godwin Dabbs, he notes that athletes, actors, blue-collar workers and con artists tend to have higher levels of testosterone than clerks, intellectuals and administrators.

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.