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.

"Nothing is more useful than water," observed Adam Smith, but "scarcely anything can be had in exchange for it. " The father of free-market economics noted this paradox in 18th-century Scotland, as rain-sodden and damp then as it is today. Where water is in ample supply his words still hold true.