The eminent Stanford neurobiologist and science writer Robert Sapolsky has a 1998 collection of his highly entertaining essays on science and society called The Trouble with Testosterone. In the title essay, Sapolsky explains why the myth that testosterone causes aggression is wrong. Using testosterone as an example, he spells out the reasons that a simple one-to-one relationship like testosterone causing aggression rarely holds in biology--there are too many mitigating factors that get in the way. Sapolsky does concede, though, that gargantuan doses of testosterone, amounts that a number of well-known baseball players have used, do produce unprovoked aggression. This has been called "roid rage". Testosterone is one of the anabolic steroids, substances that cause the body grow and develop.

Some years ago, our lab began exploring the effects of testosterone on cooperation. In 2005, we reported that when men were distrusted in a monetary exchange task, their levels of "high octane" testosterone, known at DHT, spiked. Distrust in this experiment meant that a stranger had not sent them any, or much money. This mattered because money sent to another person would triple in value, earning the recipient a healthy payday. The spike in DHT predicted that those who were distrusted would return like for like--when given a chance to share money in return, the distrusted, high-DHT men sent nothing. Women, on the other hand, had no increase in DHT and nearly always returned something to a stranger who had sent them money, even if they were distrusted by being sent very little. So, we just proved that women are nicer than men. And testosterone was the culprit.

In December, 2009, we went several steps further. We created "alpha males" by administering testosterone to men to demonstrate that testosterone caused men to behave badly. Testosterone acts as an oxytocin antagonist and we had previously shown that oxytocin makes men more generous. Like Sapolsky, we found there was trouble with testosterone, but that it was more complicated than we had at first thought.

In this experiment, we drew four tubes of blood from our participants and then had them rub a gel on their shoulders. The gel was either a prescription testosterone replacement called Androgel® or an identically appearing placebo. Testosterone levels peak 16 hours after Androgel® application so we had the men return to the lab the next morning and took four more vials of blood to document the increase in testosterone. Our treatment doubled DHT. Each man then made decisions in which he could be generous towards another guy with money he controlled or stingy, and others could do the same towards him. The crucial part of this experiment was that stingy offers could be rejected by the other person. Rejected offers caused both men to lose all the money on the virtual table (all decisions were made anonymously by computer and all men received identical and complete instructions about the experiment). We gave the men the option of engaging in internecine warfare.

The men then returned four weeks later and repeated the procedure but this time got the other substance to rub on their shoulders. This allowed us to compare the behavior of pharmacologically-created "alpha males" to themselves on placebo.

As we had suspected, alpha males were 27% stingier when it came to sharing money than themselves on placebo. For alpha males it was all about them. Alpha males also engaged in mutually destructive punishment of others for being stingy towards them. This aggression erupted to fight a perceived injustice.

But this is where is gets interesting. Each man in our study knew he would make a decision with another guy only once. By bearing the cost of punishing a stingy person one would never interact with again, these men were helping to enforce an implicit sharing norm for the entire group. Many studies have shown that punishment is an effective way to sustain cooperation in groups. Think of the "three dinner rule." If you have had a new acquaintance to your house for dinner three times and he or she has not reciprocated, they are off the invite list. They are not reciprocators. The men in our study who received testosterone pulled all the food off the table, even their own.

Alpha males probably punish others without much thought. Testosterone receptors in the human brain are largely found in evolutionary-old regions associated with emotions. These impulses are not open to conscious reflection. While men drive too fast and engage in violence of all types much more than women do, we also make up 85% of the active military. Damn it, we like to enforce the rules!

Many societies have found ways to channel the self-centered and aggressive impulses of high testosterone young men into socially useful niches like the military or sports. Those that don't face a plethora of selfish and destructive behaviors.

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