You’ve heard that 150 is an approximate upper limit on the number of our family-and-friend relationships because that’s how many connections we can track? That’s Dunbar. You’ve read the theory that language evolved as a sort of replacement for hands-on grooming among our primate relatives when group size got big? That’s Dunbar too. Now, in The Science of Love and Betrayal, Dunbar, who is Professor of Evolutionary Anthropology at Oxford, asks seductive questions about love and friendship. Why do men and women pair-bond when so many other animals don’t? How do biology and sociality intersect in explaining human attraction to others?