Giving women more of the male hormone testosterone can turn them into fairer and more amiable game players, according to tests.
A single dose of testosterone was enough to have this effect, European scientists found, but only if the woman was oblivious to the treatment.
If she realised she had received the hormone and not a dummy drug, she turned to greed and selfishness.
The work in Nature magazine suggests the mind can win over hormones.
Testosterone induces anti-social behaviour in humans, but only because of our own prejudices about its effect rather than its biological activity, suggest the authors.
They believe the same is true in men, although they only studied women.
Power of suggestion
For the study, they asked more than 120 women to pair up and play an "ultimatum" bargaining game with real money at stake.
In the game, one of the pair is the "proposer" and is tasked with suggesting to the other player - the responder - how to split the money between them.
The responder can then only accept or reject the offer.
If they reject it, neither of the pair gets any of the cash.
The researchers gave the proposers either a dummy pill or one containing testosterone, but did not tell the women which pill they had been given.
Once they had played the game, the proposers were asked to say which pill they thought they had taken.
Those who received testosterone behaved more fairly, had fewer bargaining conflicts and were better at social interactions.
However, women who thought that they had received testosterone, whether or not they actually did, behaved more unfairly than those who thought that they had received placebo, again whether or not they actually did.
The researchers, led by Ernst Fehr of the University of Zurich, Switzerland, said the results suggested a case of "mind over matter" with the brain overriding body chemistry.
"Whereas other animals may be predominantly under the influence of biological factors such as hormones, biology seems to exert less control over human behaviour," they said.
UK endocrinologist Professor Ashley Grossman said: "This puts hormones in their place.
"Hormones provide a basic backdrop, but changes in levels will do little to behaviour compared to personality, culture and society."