I Enjoy Your Pain - Men, women, and the pleasure of punishment. By William Saletan:

"More evidence that men are vindictive: Brain scans show that when somebody they dislike suffers pain, men "but not women" show a) zero activity in empathy-related parts of the brain and b) a surge of activity in pleasure-related parts of the brain. In the study, disliked people were those who had behaved selfishly in a game. Interpretations: 1) Men are born mean. 2) Men are made mean. 3) Men are deeply ethical and reserve empathy for those who deserve it. 4) Men do women a service by instinctively punishing cheaters. "