Since Darwin's day, we've been told that sexual monogamy comes naturally to our species. Mainstream science--as well as religious and cultural institutions--has 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 reality be reconciled with the accepted narrative? It can't be, according to renegade thinkers Christopher Ryan and Cacilda JethÅ. While debunking almost everything we "know" about sex, they offer a bold alternative explanation in this provocative and brilliant book.

Ryan and JethÅ's central contention is that human beings evolved in egalitarian groups that shared food, child care, and, often, sexual partners. Weaving together convergent, frequently...