THE PRICKLY SIDE OF OXYTOCIN
Oxytocin has a touchy-feely reputation, thanks to research showing that it promotes social bonding in a wide range of animals, including humans. But a study published on page 1408 of this week's issue of Science hints that the hormone has a prickly side as well. In experiments with groups of people playing an economic game, those who received a dose of oxytocin behaved more altruistically toward members of their own group. Yet they also displayed more "defensive aggression" toward outsiders, preemptively punishing members of a competing group when their own group was in danger of suffering a heavy financial loss.