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It seems that the majority of cultural changes are responses to new or altered circumstances. For example, as Genesis pointed out, crises can spurn cultural change as society adjusts to an altered state. Similarly, when families from particular cultures are placed in new cultures, changes are often made to either fiercely protect and preserve the original culture, or assimilate to the new one (or both). The circumstances may be random and unpredictable, such as with natural disasters, but how society as a whole responds to them seems to be a choice, or at least, not entirely random. That is not to say that every cultural change is ascertained by a vote, but rather that the culture, as a collective of individuals seems to respond similarly, and when they don't, cultures are split, as in Civil War, or subcultures are formed, as with religious sects, for example.

I am uncertain how predictable this cultural change is.