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The popular belief that testosterone contributes to aggressive behavior in humans may be just that — a belief — according to a new study in the journal Nature. The paper suggests that the hormone may in fact lead to fair, and more altruistic, behavior in some situations, causing aggression only when people believe they are under its influence.

Authors of the new study theorize that the actual effects of testosterone, a hormone produced by the male testes and female ovaries that is linked to brain development and sexual behavior, may be somewhat neutral in nature, leading to what researchers call "status-seeking behavior." Under certain conditions, status-seeking could lead to increased aggression — in prison populations, for instance, where studies have shown that inmates in high-security prisons have elevated levels of the hormone — when fighting seems the only way to the top.

But in other situations, a surge of testosterone may prompt people to engage in more cooperative behavior. For the new study, researchers enrolled 121 women to play what economists call the "ultimate bargaining game": one participant is given a certain amount of money and instructed to offer a portion to another participant. The recipient of the offer gets to accept or reject. If the offer is rejected, neither participant gets any money. Before allowing the women to propose their offer, researchers gave them either a dose of testosterone or a placebo.

The study authors hypothesized that participants taking testosterone would engage in riskier, more aggressive behavior — that is, offer their fellow participant a lesser amount of money. What happened instead was that the women who received testosterone made significantly more equitable offers than those who received a placebo, offering their partners an average of 3.9 money units out of 10, vs. 3.4 money units.

"If you give a lizard testosterone, it becomes more aggressive. But we are not lizards. Our social interactions are nuanced and complex," explains lead author Michael Naef of the Experimental Economics Lab at Royal Holloway College at the University of London. "In many human interactions, it is social rather than antisocial behavior that secures status."

The study's authors also found that women who believed they had received a testosterone supplement — whether they had or not — made much greedier and more self-serving offers, suggesting that the assumption of testosterone's influence became an enabler of antisocial behavior. "It's not the hormone but the myth surrounding the hormone that induced aggressiveness," Naef suggests.

The new research may even undermine the concept of "'roid rage" and therefore have legal consequences, says Naef. In the U.S., some convicted criminals in violent-crime cases have managed to finagle less severe sentences by arguing that their violent behavior was due in part to their use of testosterone as an anabolic steroid — whence the term 'roid rage originates. "There is no direct link between testosterone and aggression," Naef says.

Adam Goodie, a psychologist at the University of Georgia in Athens who studies decision-making, told Naturenews.com that the research has profound implications for neuroeconomics, the study of how biology influences markets, by showing that "not only does biology affect economic behavior — so does belief." But John Coates, a former Wall Street trader and researcher at Cambridge University, warns against extrapolating too much from the study. Coates' own measurements of testosterone levels in the saliva of male traders found a link between higher levels of the hormone and risky behavior. He says there is a "dose-response curve" for testosterone, which means that a small dose of the hormone might result in an opposite behavioral change from a very large dose. "It's entirely possible that at low levels of testosterone you could have higher cooperation but at higher levels you could witness the opposite effect," Coates says.

And while the authors of the current study chose only women to participate (because they tend to have more stable "baseline" levels of testosterone than men), Coates says the hormone influences the sexes differently. "The locations and sensitivity of testosterone receptors is different for men and for women. For that reason, it's very difficult to extend these findings to men."

But Naef counters that Coates' results from the saliva of male traders need not be inconsistent with his own findings. "In the highly competitive arena of trading, high profits lead to social recognition, fueling risky behavior," he explains, while in his experiment, cooperative behavior led to social recognition. What's more, it's impossible to know whether traders engaged in risky behavior because of high levels of testosterone, or whether their testosterone levels became elevated because of their risk-taking. "I think the bottom line is that the picture surrounding testosterone is very complex," Naef says, "but we certainly have to move past the myth that it simply leads to aggression."

• Eben Harrell