everything a person does is influenced not only by her genes but also by her culture, and cultures evolve, too. Because elements of culture show variation (people invent new things) and selection (other people do or don’t adopt those variations), cultural traits can be analyzed in a Darwinian framework46 just as well as physical traits (birds’ beaks, giraffes’ necks). Cultural elements, however, don’t spread by the slow process of having children; they spread rapidly whenever people adopt a new behavior, technology, or belief. Cultural traits can even spread from tribe to tribe or nation to
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