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Testosterone gets a pretty bad reputation. It’s been long known as the hormone of aggression. In his 1998 book, The Trouble With Testosterone: And Other Essays on the Biology of the Human Predicament, the neuroscientist Robert Sapolsky writes, “What evidence links testosterone with aggression? Some pretty obvious stuff”: Males tend to have more testosterone than women, and tend to be more aggressive. “Times of life when males are swimming in testosterone (for example, after reaching puberty) correspond to when aggression peaks.”

High testosterone correlates with thrill-seeking, mate-seeking, and dominance in males. If you inject testosterone into a mammal, it will act much more aggressively than normal. Conversely, if you remove the gland that secretes testosterone—in males, the testicles—the mammal will become more docile. Surgery, starvation, illness, and being demoted in a social hierarchy can create lower testosterone levels as well.

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Despite these strong associations, the causal link between testosterone levels and behavior is far from clear. “Study after study has shown that when you examine testosterone levels when males are first placed together in a social group, testosterone levels predict nothing about who is going to be aggressive,” Sapolsky writes. “The subsequent behavioral differences drive the hormonal changes, rather than the other way around.” There appears to be a feedback loop at work—being brave, aggressive, or taking risks increases testosterone, and increasing testosterone increases the likelihood of those behaviors.

“Our findings flatly contradict a simple link between testosterone and male aggression.”

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But a recent study suggests that raising testosterone levels may indeed cause a behavioral change—and in ways unrelated to aggression. Jean-Claude Dreher, the director of the Cognitive Neuroscience Center in Lyon, France, and colleagues, found that administering testosterone in men amplifies their desire to both reward and punish. “Although empirical research and popular opinion center on its role in driving aggressive and antisocial behaviors,” the researchers write, “direct causal evidence for this link is weak in men.” To see if they could strengthen the evidence for that link, they used an experimental setup called “the ultimatum game”—people are put into pairs, and one gets to choose how to split a certain amount of money, say $100, between them.

Shep might choose to give himself $70 and give Alex only $30. All Alex can do, per the rules of the game, is decide whether to take the offer or reject it; if he rejects it, nobody gets any money. You might think that people in Alex’s position would take whatever amount is offered, no matter how little, because a little is better than nothing. But that’s not what happens. In experimental studies, the recipient tends to reject offers of less than 30 percent of the starting sum: They hurt themselves, the hypothesis goes, to punish what they perceive to be unfair behavior. 

In this testosterone experiment, Dreher changed the game a bit. In his version, after Alex accepts or rejects Shep’s offer, he can choose to punish or reward Shep—for the cheap or generous offer, respectively—at a proportionate cost to himself. (So if Alex accepts Shep’s offer and wants to reward him with 10 percent of Shep’s portion, Alex would lose 10 percent of his). Some of the people in Alex’s position, the responders, were injected with a 1-milliliter dose of testosterone enanthate; others were given a placebo. If testosterone drives aggressive and antisocial behaviors, the researchers reasoned, then those who were injected with it should deliver bigger punishments and smaller rewards. But that’s not what happened.

The responders given testosterone were not only more likely to give greater punishments to those who gave unfair offers—they were also more likely to give greater rewards to those who gave generous offers (compared to a placebo control group). In this experiment, testosterone seems to amplify men’s tendencies to punish and reward others. “Our findings flatly contradict a simple link between testosterone and male aggression,” the researchers conclude. “Instead, we find that testosterone’s effect on male behavior depended on the social context, and we show in a single experiment that testosterone can enhance both reactive aggression and generosity.”

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Why might this be? Dreher says this study provides causal evidence for the theory that, in men, testosterone promotes behaviors that increase social status. A leader, for example, has got to dole out goods and punishment to maintain favor.

It’s what alpha male monkeys do. They sustain the social hierarchy, the researchers write, “by not only aggressive behavior but also sharing resources, such as access to food and females.” Dreher thinks it’s no coincidence that, as a male monkey’s social rank increases, so do his testosterone levels.

Jim Davies is an associate professor at the Institute of Cognitive Science at Carleton University in Ottawa, and author of Riveted: The Science of Why Jokes Make Us Laugh, Movies Make Us Cry, and Religion Makes Us Feel One with the Universe. His sister is novelist JD Spero.

Watch: The neurobiologist Robert Sapolsky worries that his field can seem too reductive.

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