Sex columnist Dan Savage calls Sex at Dawn "the single most important book on human sexuality" in the last 60 years. Its authors, Christopher Ryan, PhD, and Cacilda Jetha, MD, attack the "standard narrative" of sexual evolution—that humans mate monogamously and for life—like nutritionists slamming supermodel imagery. They see it as a contrived ideal that "invite[s] punishment upon ourselves, upon each other, and upon our children."

Instead, Ryan and Jetha argue that until agriculture and private property got big about 10,000 years ago, humans mated with multiple partners and communally raised children. They pore through piles of psychological, anthropological, and archeological research, couching their conclusions in accessible language and bad professor jokes. Though often diverging from their thesis in both interesting and superfluous ways—you can skip Part III on the state of nature—the overall case is convincing.

Comparisons to other primates are especially persuasive. Like humans, our closest cousins, chimps and bonobos, have testes on the outside (though theirs are bigger), and male animals are typically 10 to 15 percent larger than females. Gorillas, though, like more distant cousins on steroids, have small penises (balls on the inside), and males that are twice as big as females. Because gorillas mate polygynously (multiple ladies for each dude), males must be imposing to win access to females (specifically, their vaginas). But for chimps and bonobos, "multimale-multifemale" maters, the battle is on the inside—large volumes of sperm fight to the egg, even leaving traps for competitors. Our genital similarities to chimps and bonobos thus reveal our promiscuous past.

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To explain why this multi-mate model of human sexuality has never been embraced, Ryan and Jetha dissect opposing theorists along with their theories. Darwin, for example, was an uptight, 30-year-old virgin who projected his priggish Victorian mores onto his understanding of human sexuality. The idea of a horny female would not only have made his proper British head explode, it would have discredited his entire theory.

There are a few too many arguments in Sex at Dawn, some stretched too thin, and some that contradict, but the overall product is a rich buffet of sexual history. Open to a random chapter and you're likely to find something worth quoting. The book might not make you have better sex, but it will definitely improve how you argue about it.