Testosterone and Trust

By Richard Morin
Sunday, December 4, 2005

Finally, an explanation for why bar bets sometimes escalate into bar fights: Levels of a "high-octane" form of testosterone soar when men think others don't trust them.

Economist Paul J. Zak of the Claremont Graduate University in Southern California said scientists have known for years that aggressive behavior in animals is sparked by elevated testosterone (which is present in men and women, though women have significantly smaller amounts). For example, testosterone levels have been shown to spike upward in both sexes before an athletic match as contestants psych themselves up.

Less well understood is whether even mildly negative social interactions -- such as feeling mistrusted by a stranger -- can trigger surges in testosterone, which is what Zak and his colleagues decided to explore.

For this study, Zak's research team gave $10 each to 106 men and 106 women (average age, 22) to play a computerized "trust game" that allowed the subjects to keep any additional money (or absorb any losses) from the decisions they make during the experiment.

The subjects were told that they could choose to send any or all of their $10 (including none) to a randomly selected and anonymous partner. The amount of money they sent was then tripled in the receiving partner's account; the receiver then could send back any amount (including nothing) to the sender. Thus the senders are giving away some of their stash, but they are hoping that the receivers will react generously by sharing the tripled amount.

Zak's team measured mistrust by how much money the receiver could have gotten from the sender, but didn't. Senders who sent $1 were labeled as less trusting than those who sent, say, $5. (In a perfectly trustworthy and pragmatic world, the original sender would pass along the entire $10, which would be tripled to $30 and added to the partner's original $10. The receiver, mindful of the sender's generosity and trust, would split this $40. Thus each player would leave the game with $20 -- the maximum joint payoff -- or twice what they would have collected if the original sender, fearing he or she would get stiffed, kept the initial $10. At least that's the theory behind these trust experiments.)

After the players made their decisions, the researchers drew a blood sample from each and analyzed it. They found that men who received a high "distrust signal" from their partner (in the form of a smaller amount of money) exhibited increased levels of DHT, the biologically active metabolite of testosterone that circulates in both sexes. Women who received a strong distrust signal, however, showed no increase in DHT.

"This suggests that men in our experiment had an aggressive reaction when they received a signal of distrust, while women did not," the researchers wrote in a recent issue of the American Economic Review.

That surprised researchers because, in post-game interviews, women reported that they disliked being distrusted about as much as men did. As a check, Zak's team measured nine other hormones to see if they varied before and after the subjects played the trust game. "None of them was related to the distrust signals received by women or men," Zak and his colleagues wrote.

While neither men nor women like being distrusted, "women don't have the same physiological reaction as men do," Zak says. "Women are just physiologically cooler. Women are probably better negotiators because they don't have these emotionally charged responses. Men get pissed off."

But another researcher found that being pumped up isn't always a bad thing: Men with elevated levels of testosterone were less likely to accept unfair offers in a different game where people were offered ultimatums.

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