This column is often concerned with the evolution of sexual behavior and sexual anatomy, but instead of attributing everything to sex, for once let's accept a view like that of Bertrand Russell.

"Love," he said, "is something far more than desire for sexual intercourse; it is the principal means of escape from the loneliness which afflicts most men and women throughout the greater part of their lives."

So if love is about something other than getting more sex, what is that thing? And -- as this is Natural Selections -- how did love evolve?