When we think of the first swinger parties most of us imagine 1970s counter-culture, we don’t picture Top Gun fighter pilots in World War II. Yet, according to researchers Joan and Dwight Dixon, it was on military bases that “partner swapping” first originated in the United States. As the group with the highest casualty rate during the war, these elite pilots and their wives “shared each other as a kind of tribal bonding ritual” and had an unspoken agreement to care for one another if a woman’s husband didn’t make it back home. Like the sexy apes known as bonobos, this kind of open sexuality served a social function that provided a way to relieve stress and form long-lasting bonds.

For the husband and wife team Christopher Ryan and Cacilda Jethá in their new book Sex At Dawn, this example is one of many that suggests the human species did not evolve in monogamous, nuclear families but rather in small, intimate groups where “most mature individuals would have had several ongoing sexual relationships at any given time.” We are the descendants of these multimale-multifemale mating groups and, even though we’ve constructed a radically different society from our hunter-gatherer forebears, the behavioral and psychological traits our species evolved in the distant past still manifest themselves today. Ryan, a psychologist, and Jethá, a psychiatrist, argue that understanding human sexual evolution this way helps to explain our species’ unique creativity inside (as well as outside) the marriage bed. It may also shed light on why fidelity has been such a persistent problem for both men and women throughout recorded history.

For Ryan and Jethá there is little doubt that human beings are an exceedingly sexual species. As an example they detail how in 1902 the first home-use vibrator was patented and approved for domestic use in the United States. Fifteen years later there were more vibrators than toasters in American homes (today this number could be as high as fifty million nationwide). In 2006, according to U.S. Pornography Industry Revenue Statistics, people around the world—the majority of whom were probably men—spent an estimated $97 billion on pornographic material ($13.3 billion in the U.S. alone), a figure that exceeded the annual revenue of Microsoft, Google, Amazon, eBay, Yahoo!, Apple, and Netflix combined. To judge human sexuality based on consumption patterns, as Stephen Colbert would say, “the market has spoken.” When this is combined with estimates that people engage in hundreds, and sometimes thousands, of copulations per child born (more than any primate, including chimpanzees and bonobos) there’s little denying that the human animal is one sexy beast.

But why should a species often described as monogamous be so hypersexual? Monogamous animals by definition don’t have to compete for reproduction and, as a result, are generally characterized by a low level of sexual activity. But according to Ryan and Jethá humans top a very short list of species that engage in sex for pleasure. “No animal spends more of its allotted time on Earth fussing over sex than Homo sapiens,” they write. In fact, the animal world is filled with species who confine their sexual behavior to just a few periods each year, the only times when conception is possible. Among apes the only monogamous species are the gibbons whose infrequent, reproduction-only copulations make them much better adherents of the Vatican’s guidelines than we are. In this way, Ryan and Jethá argue, repressing our sexuality should not be confused with reining in an “animal” nature; rather, it is denying one of the most unique aspects of what it means to be human.

The suggestion that humans did not evolve as a monogamous species is not as radical an idea as it may sound. In The Descent of Man Charles Darwin wrote, “Those who have most closely studied the subject [particularly the anthropologist Lewis Henry Morgan] believe that communal marriage was the original and universal form throughout the world.” Yet ever since the nineteenth century anthropologists have struggled over how to identify the mating system of human beings. In 1967 George P. Murdock’s Ethnographic Atlas reported that only 14.5% of modern preindustrial societies could be classified as monogamous. Yet, in the West, researchers commonly refer to humans as “serially monogamous,” based on the pattern of repeated monogamous marriages throughout men and women’s lifetimes. But with over half of divorces occurring because of infidelity and one in 25 dads unknowingly raising children that they didn’t father, this is not a picture that fits comfortably with monogamy of any sort, serial or otherwise.

However, by looking at modern indigenous societies and comparing the findings of anthropologists with the latest results in behavioral psychology and biology, Ryan and Jethá piece together a remarkably coherent pattern from an otherwise fractured understanding of human sexuality. From societies that believe that multiple men are necessary for a successful pregnancy (what researchers refer to as “partible paternity”) to those where not having an extra-marital tryst will cause a man to be labeled “stingy of one’s genitals” by his female suitors, the authors conclude that marriage may be an established social arrangement among many hunter-gatherers but it’s one in which sexuality is decidedly fluid. A range of physiological evidence from Western populations is further offered to support this position, from the year-round libido in both sexes, to the unusually large size of men’s genitalia compared to other apes, to the shifting sexual strategy during various stages in women’s reproductive cycle (and lest we forget multiple female orgasms?). All suggest that our species is adapted for several concurrent sexual partners.

This is, of course, not a new idea in human evolutionary research. Primatologist Sarah Hrdy advocated a promiscuous mating system for humans in The Woman That Never Evolved (1999) while psychologist David Barash and psychiatrist Judith Lipton detailed their own argument in The Myth of Monogamy (2001). In Sex At Dawn Ryan and Jethá cover some similar ground as these previous authors but provide a great deal of additional material that was unavailable a decade ago. They also emphasize the ways in which monogamy has been used as a means of controlling women in patriarchal societies and make a number of insightful connections between the invention of agriculture 12,000 years ago and how sedentary societies influence the structure of human mating. However, with a relaxed writing style and numerous examples from modern popular culture, their discussion of these topics remains readily accessible even to those who may be encountering such ideas for the first time.

Sex At Dawn is a provocative and engaging synthesis of the latest research on human sexual evolution that has the added benefit of being a joy to read. While the authors’ conclusion that healthy relationships can be both committed and open may come as a shock to some readers, others will likely find it refreshingly honest. As their example of WWII fighter pilots emphasizes, human sexuality has numerous social as well as emotional functions and there has never been only a single path chosen by the human species. In offering a fresh look at a fascinating and controversial topic Sex At Dawn is a book sure to generate discussion, and one likely to produce more than a few difficult conversations with family marriage counselors.