From BBC News:
Scans reveal that being paid more than a co-worker stimulates the “reward centre” in the male brain. Traditional economic theory assumes the only important factor is the absolute size of the reward. But researchers in the journal Science have shown the relative size of one’s earnings play a major role. In the study, 38 pairs of male volunteers were asked to perform the same simple task simultaneously, and promised payment for success. Both “players” were asked to estimate the number of dots appearing on a screen. Providing the right answer earned a real financial reward between 30 (£22) and 120 (£86) euros. Each of the participants was told how their partners had performed and how much they were paid.
Using magnetic resonance tomographs, the researchers examined the volunteers’ blood circulation throughout the activities. High blood flow indicated that the nerve cells in the respective part of the brain were particularly active. A wrong answer, and no payment, resulted in a reduction in blood flow to the “reward region”. But the area “lit up” when volunteers earned money, and interestingly showed far more activity if a player received more than his partner. This indicated that stimulation of the reward centre was not merely linked to individual success, but to the success of others.