So, the Playboy ethic was right?

Psychologist Christopher Ryan is out to defeat an archetypal figure in the mythology of monogamy. No, not prince charming; he’s after the widespread belief in a prehistoric hunter who would slay an antelope on the plains and heroically haul it back to his nuclear family.

You might wonder what this has to do with monogamy. Well, Ryan argues that in actuality the meat would have been shared with the entire tribe, because pre-agricultural societies shared everything — including sex. This is a key point he and co-author/wife Cacilda Jethá make in “Sex at Dawn,” which was released last year in hardcover and this month in paperback. Our hunting and gathering ancestors were nonmonogamous, they argue — the implication being that, biologically speaking, sexual exclusivity is unnatural.