Those final two things you sagely point out, friend, are not mutually exclusive. Social contagion, so-called, facilitates the one and vice-versa.

Social contagion is the hip, updated version of a more ancient word, and an ancient phenomenon: indoctrination. Both facilitate permission, peer pressure, cliques, cults, casques, et cetera ad infinitum, and these in turn reciprocate, so that the process becomes cyclical, the true believers perfervid in defending the territory they've decided to stake.