Date Published: December 16, 2009
Publisher: Public Library of Science
Author(s): Paul J. Zak, Robert Kurzban, Sheila Ahmadi, Ronald S. Swerdloff, Jang Park, Levan Efremidze, Karen Redwine, Karla Morgan, William Matzner, André Aleman. http://doi.org/10.1371/journal.pone.0008330
Abstract: How do human beings decide when to be selfish or selfless? In this study, we gave testosterone to 25 men to establish its impact on prosocial behaviors in a double-blind within-subjects design. We also confirmed participants’ testosterone levels before and after treatment through blood draws. Using the Ultimatum Game from behavioral economics, we find that men with artificially raised T, compared to themselves on placebo, were 27% less generous towards strangers with money they controlled (95% CI placebo: (1.70, 2.72); 95% CI T: (.98, 2.30)). This effect scales with a man’s level of total-, free-, and dihydro-testosterone (DHT). Men in the lowest decile of DHT were 560% more generous than men in the highest decile of DHT. We also found that men with elevated testosterone were more likely to use their own money punish those who were ungenerous toward them. Our results continue to hold after controlling for altruism. We conclude that elevated testosterone causes men to behave antisocially.
Partial Text: Human beings are both prosocial and self-serving, often exhibiting both behaviors in a short period of time. The neurologic foundations for prosociality are just beginning to be examined –, but the mechanisms that cause a shift from selfless to selfish have not been characterized.
Forty-eight male students were recruited for this double-blind cross-over experiment. The mean age of participants was 20.8 years old (SD = 2.2), and the sample was ethnically diverse (Asian 44%, Caucasian 36%, Hispanic 8%, Other/no data 12%). Only male participants were recruited because the US Food and Drug Administration has only approved testosterone treatment for men, and men were likely to be more reactive behaviorally to its effects . Twenty-five participants completed the entire experiment and are included in our analyses. All participants gave written informed consent for the study, with study phases (testosterone or placebo) separated by six to 12 weeks depending on which sessions participants were in. In every session, approximately one-half of the participants received testosterone and the other half were given the placebo. Session sizes varied from four to eight participants. The experiment was approved by the Institutional Review Boards of UCLA and Claremont Graduate University.
Our primary finding is that manipulating T in men causes them to be 27% less generous in the UG then themselves at baseline. Interestingly, the threshold to initiate costly punishment for those who are less generous towards them increases with T levels. Indeed, participants on Androgel® were more than twice as likely to have exhibited negative generosity (rejection threshold exceeding proposed split) compared to themselves on placebo. This increase in negative generosity between conditions suggests that T infusion interfered with participants’ ability to understand others’ behaviors since rejections of DM1 proposals do not earn participants any money. These results are credible because T was directly manipulated, and the change in T was documented through blood draws. Further, the effects of T on generosity and punishment scale with a man’s T levels, and the comparisons are within-subjects.