Motivational and situational factors and the relationship between testosterone dynamics and human aggression during competition.

Men engage in aggression at a cost to extrinsic reward, and this behaviour is associated with a rise in testosterone. To characterize the factors underlying aggression, men were assigned to one of the four experimental conditions of a computer game in which they were provoked (points were stolen from them or not) and/or received reward for aggression (received points for aggression or not). Men who were provoked but did not receive reward for aggression enjoyed the task the most, demonstrated an increase in salivary testosterone, and were more likely to choose a competitive versus non-competitive task than men in the other experimental conditions. Moreover, individual differences in aggressive behaviour among these men were positively correlated with the extent to which they enjoyed the task and with testosterone fluctuations. These results indicate that costly aggressive behaviour is intrinsically rewarding, perhaps to regulate future interactions, and that testosterone may be a physiological marker of such reward value.