Why are warriors capable of working together, although they like fighting with each other?

Men have a long history of fighting with one another for dominance, but why such duels did not leave tribal unity in tatters and warriors less capable of working together to fend off attacks from predators and hostile clans remains a mystery. One common theory is that men more readily make up after fierce physical conflicts than do women. And an experiment run recently at Harvard University suggests this may be true.

The two researchers set up a second experiment. In this they asked 82 volunteers to place their heads on chin rests and have their eyes monitored as they were shown a variety of street scenes from New York, San Francisco and London. This time, working-class people spent a tenth of a second longer looking at faces than did upper-middle-class people — a difference that did not apply when the same people looked at inanimate objects.