Human studies on the relation between testosterone levels and risk-taking behaviour are scarce. Related functions, like aggression, have been related to higher testosterone levels more consistently, especially in the animal literature. Estradiol affects several neurotransmitter systems that play a role in behaviour regulation. Existing human studies on neurocognitive functions and testosterone levels have largely ignored the interrelatedness of testosterone levels and estradiol levels. Therefore, in this study, the effects of a 1-week combined testosterone and estradiol intervention on risk-taking behaviours were investigated. Twenty-one healthy men, with a normal body mass index, were treated for 7 days with an aromatase inhibitor (letrozole 2.5 mg), resulting in high-normal levels of testosterone and low-normal levels of estradiol, or with a combination of an aromatase inhibitor and estradiol (75 μg/24 h), resulting in low-normal levels of testosterone and high-normal levels of estradiol. A randomized experimenter and participant-blind controlled design was applied. Neurocognitive measures of risk-taking and reward and punishment sensitivity were assessed before starting with the medication and after 7 days of drug administration: Balloon Analogue Risk Task (BART), Game of Dice Task (GDT), and Iowa Gambling Task (IGT). A group by time effect was present for the BART, indicating that the high-normal testosterone group showed an increase in risk-taking on the BART, from the first drug-naive BART performance, to the second BART performance (aromatase inhibitor), whereas such an increase was not present in the low-normal testosterone/high estradiol group. No group by time interactions were present in GDT or IGT performance. These results implicate that testosterone levels in healthy men are associated with increased risk-taking under conditions of unknown probabilities, but not in conditions of known probabilities (GDT) or of strategic decision making (IGT).
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