Just a single dose of testosterone, the male hormone, can make women behave more fairly to their peers, the BBC reports. But according to European research, this effect only occurs if women are unaware they’re getting the supplement. If the female test subject realized she’d received the hormone, and not a placebo, she displayed greed and selfishness instead. The study, published in the journal Nature, suggests the mind can trump hormones: testosterone induces anti-social behaviour in humans, but only due to our preconceived notions of its effect, the authors conclude. In the study, over 120 women paired up and played a bargaining game, betting real money. The so-called “proposer” was tasked with suggesting to another player, the “responder,” how to split the money between them, an offer the responder could then accept or reject. If rejected, neither would get any money. The proposers were given either testosterone or a dummy, but were unaware of which they received; those who got testosterone behaved more fairly and were better at interacting with their partners. But women who thought they’d received testosterone, whether or not they really had, acted more unfairly.