The contagious behavior of schools of fish, insect swarms, and crowds of people is generated by close physical proximity. For most of human history, cultural identities have also been formed through direct contact with the people who are physically nearest to us: small villages, the local community. Large movements of people once were limited to invasions, military conquests, and trade, and these were the main ways in which cultural ideas were disseminated and different languages and ways of life imposed on other communities. All of this has changed irreversibly in the last two hundred years
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