Co-operation within groups leads to antagonism within them -- if you leave out trade

See Why are we nice to strangers? by Matt Ridley. Excerpts:

"The more evolution encourages niceness within groups, the more it produces nastiness between them. Dr. Wilson thinks "the future is bleak if we don't turn our groups into organisms," by which he means entities that emulate the team behavior of cells in a body. But surely the future is bleak if we do turn groups into such organisms. Consider gangs, armies, sports fans and companies, which have taken this advice—and, as a result, fight each other.

As Adam Smith pointed out, kindness works among friends and relatives, but for cooperation among strangers, human beings use a wholly different mechanism: a division of labor that encourages people to engage in mutual service. Plenty of other animals (from chimpanzees to ants) show cooperation within groups and proportionate antagonism between them, whereas none has exchange and specialization between strangers. History shows that it is trade that dissolves hostility between groups.

A few years ago, Joe Henrich of the University of British Columbia and his colleagues did a series of experiments in small-scale societies in the Amazon, New Guinea and Africa. They asked people to play the "ultimatum game," in which a player must decide how much of a windfall he needs to share with another player to prevent the other player from exercising his right to veto the whole deal. The more the small-scale society is enmeshed in modern commerce, the more generous the offers people make. This may shock those who believe in Rousseau's idea of the "noble savage," but not those who believe in the virtues of what Montesquieu called "sweet commerce."