Farmers don’t benefit from competition between their domesticated animals or plants. In fact, reduced competition between individual members of domesticated species is the secret of some big gains in farm productivity, such as the dwarf strains of wheat and rice that made up the “Green Revolution.” Since the elites were in a very real sense raising peasants, just as peasants raised cows, there must have been a tendency for them to cull individuals who were more aggressive than average, which over time would have changed the frequencies of those alleles that induced such aggression. This would have been particularly likely in strong, long-lived states, because situation in which rebels often won might well have favored aggressive personalities. This meant some people were taming others, but with reasonable amounts of gene flow between classes, populations as a whole should have become tamer.
Gregory Cochran and Henry Harpending, The 10,000 Year Explosion: How Civilization Accelerated Human Evolution, New York, 2009, pp. 111-112