Increased levels of testosterone can cause a man to engage in both social and anti-social behaviours, finds a new study. In a paper published in Proceedings of the National Academy of Sciences, the researchers concocted a game with 40 volunteers — half were given an injection of testosterone while the other half were given a placebo. They involved money that was split and shared with the aim of maximising the amount participants would get in the end. The men were then asked to play the game in two ways — one group played by responding to predetermined proposals, while the other group played in pairs against one another. In both versions, volunteers were presented with a proposal of accepting money from a split pot of cash. If they accepted the cash, they got to keep it; if not, the other person did not get to keep their share, either. Afterwards, the volunteers were allowed to reward or punish their opponents for being fair or not by using their own winnings to reduce or increase the amount their opponent got. By watching and comparing player behaviour of those that had received the testosterone shots versus the placebo, the researchers found that those that received the shots tended to be more likely to reject proposals and to punish opponents they found unfair — which was expected behaviour. But surprisingly, those operating on heightened levels of testosterone were also found to be more generous with opponents they deemed fair.