You know this guy: bellowing at the bar, cutting off cars in packed traffic or mocking a crestfallen sports star. He’s the testosterone ape, the swaggering embodiment of male aggression.

For years scientists have pointed fingers at him as the living example of testosterone’s brutish, self-centered, antisocial expression.

But Swiss neuroscientist Christoph Eisen­egger of the University of Zurich wondered about this stereotype. To explore it further, he and his team designed a study involving women, not men, along with testos­terone and the root of all evil, money. And he showed that when success depends on fair play, higher testosterone levels encourage cooperation instead of aggression.

The study consisted of a simple, oft-analyzed game involving two people and one pile of money. One player offers a one-time deal on how to divide the money. If the second party accepts the split, both receive their shares. If she rejects it, nobody gets a cent.

The women who offered the deal were given either testosterone or a placebo. To ensure the testosterone would have an influence, the researchers gave the women enough testosterone to pump up their baseline levels by 400 percent. And after administering either the hormone or the placebo, the researchers asked the women to guess which one they had received.

Women who received a placebo but believed they had received testosterone offered fair money splits only 10 percent of the time, probably because they harbored a neg­ative stereotype of testosterone’s effects. Women who were given testosterone but thought it was a placebo, on the other hand, offered fair-share splits 60 percent of the time—significantly more often than those who correctly guessed they got testosterone (30 percent) or a placebo (50 percent).

Ultimately, Eisenegger says, the hormone’s effects vary with the setting. It seems that testosterone feeds the drive to be victorious, no matter what the means are to that end. If being king of the hill is the goal, high testosterone levels can lead to verbal and physical aggression. But in situations where mutual benefit wins the prize, the same hormone engenders cooperation.