First, the light came on when I read this: "In its sick way, playing favorites does give her a boost -- she gets an easy bond with the favored one, and the needy, unflagging attention of the unfavored one. Case in point: You're 27 with no feelings for her except pain and resentment, and you're still bucking for her approval." Wow. I'd never realized that it does work just like that for the person who's playing favorites.
Second, about evolution: "In Africa, those who carried the mutation [for lactose tolerance] were able to leave 10 times more progeny, creating a strong selective advantage." Ten times! That's amazing!