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.

Humans are a gloomy species. Some 71% of Britons think the world is getting worse; only 5% think it is improving. Asked whether global poverty had fallen by half, doubled or remained the same in the past 20 years, only 5% of Americans answered correctly that it had fallen by half. This is not simple ignorance, observes Johan Norberg, a Swedish economic historian and the author of a new book called "Progress".