Civil wars between ethnic or religious groups have cost millions of lives in recent history (1). In Bosnia and Herzegovina alone, tens of thousands were killed and millions displaced in the war between 1992 and 1995. However, although intense hatred against other groups can lead to tragedy, it often seems to come in tandem with stronger altruism and cooperation within one's own group (2, 3). This combination may have played a key role in early human development (4). Are there ways to mitigate group conflict while at the same time harvesting the potential benefits from stronger cooperation within groups? A report by Alexander and Christia on page 1392 of this issue (5) suggests that the answer is yes, under the right circumstances.