As Steven Pinker observes, the logic of loyalty is particularly clear in the domain of romantic relationships: You’re a great catch, but there is bound to be someone out there who’s got everything you’ve got plus a little more. Knowing that your partner might someday meet such a person, you’d be reassured by the knowledge that your partner isn’t going to leave you as soon as something better comes along. This would make you much more willing to settle down with your partner and start a family—a high-stakes cooperative endeavor if ever there was one. It’s wonderful that your partner fully
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