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People repeat behaviour that leads to flooding their brains with pleasurable chemicals. The short-term reward loop acts over hours to years, and the long-term reproductive success loop over generations.

Attention is the way social primates measure status. It is highly rewarding because it causes the release of brain chemicals such as dopamine and endorphins.

Evolution acts slowly. Our psychological characteristics today are those that promoted reproductive success in the ancestral environment.

Real competition can drive up testosterone, which boosts libido.

Birth-control is effecting, and promising to effect, many functions in our social life.

The management of fertility is one of the most important functions of adulthood.

When trust improves, the mood improves.

I need to be thinking about a few things at once. I think it actually helps because you're cross-fertilizing yourself.

When a girl marries, she exchanges the attentions of many men for the inattention of one.

When we make progress quickly, it feeds our emotions. Then, when there's a period of waiting or we hit a plateau, we find out how committed we really are and whether we're going to see things through to the finish or quit.