In today’s Nature, Peter Richerson and Robert Boyd argue that migration is an engine of social change because the movement of people into societies that offer a better way of life is a more powerful driver of cultural evolution than conflict and conquest (pay-access only, sorry!):
As cultural evolutionists interested in how societies change over the long term, we have thought a lot about migration, but only recently tumbled to an obvious idea: migration has a profound effect on how societies evolve culturally because it is selective. People move to societies that provide a more attractive way of life and, all other things being equal, this process spreads ideas and institutions that promote economic efficiency, social order and equality.