I decided to cut this from an email, so I’ll put it here instead.

1) A practice hurts individuals, so they stop doing it and warn others against it; or a practice helps individuals, so they keep doing it and tell others to do it. (Inuits and red berries on SSC.)

2) A practice helps cultures, so the cultures that practice it gain an advantage over the cultures that don’t. This can have multiple consequences:
2a) The cultures that don’t practice it notice that the cultures that do have an advantage, so they adopt it. (Russia and Japan bringing in German modernization advisors.)
2b) A culture with the advantage gains enough power to influence other cultures. (America spreading democracy and bad pop music across the world.)
2c) A culture with the advantage uses it to conquer and destroy nearby cultures that don’t. (Europeans and advanced technology (guns, etc.), or Mongol war tactics and the eradication of the Tangut.)

3) The widespread adoption of a practice has non-immediate harm. Once the harm is noticed, the practice is removed. (Lead.)

4a) A practice is tried and fails to work. (That Tom Wolfe article about how hippies discovered the utility of hygiene by being incredibly unhygienic and coming down with obscure diseases.)
4b) A belief is put into practice. It may be a new belief, or it may be an old one that was previously in conflict with some higher-ranked constraint or that was unprincipled-exceptioned out of practice. (Peggy Dennis’s ultrafeminist mother’s opposition to marriage and reproduction.)

5a) A practice dies out by suppressing the fertility of its adherents and failing to win enough converts to keep up its numbers. (Shaker celibacy.)
5b) A practice spreads by increasing the fertility of its adherents and being passed down from parents to children sufficiently reliably. (Certain well-known Anabaptist sects.)