Love demands an explanation. Less than 5% of mammal species live monogamously, with males and females staying together beyond mating, and fathers helping mothers care for babies. We humans aren’t the most monogamous species of the bunch, but we’re closer to that end of the spectrum than the other end, where mating is little more than ships bumping into each other in the night.

A biological explanation for love–as with any biological explanation–has two levels. On one level are the molecular circuits that produce love, and on another level are the evolutionary forces that favor the construction of those circuits in the first place. It turns out that in this case one of the best guides to both levels of explanation