Since Darwin's day, we've been told that sexual monogamy comes naturally to our species. Mainstream scienceas well as religious and cultural institutionshas maintained that men and women evolved in families in which a man's possessions and protection were exchanged for a woman's fertility and fidelity. But this narrative is collapsing. Fewer and fewer couples are getting married, and divorce rates keep climbing as adultery and flagging libido drag down even seemingly solid marriages.How can rea ...