Everyone with a stake in the culture wars needs to read Sex at Dawn. It presents an idea that is often absent from public discourse: that monogamous pair bonding (our traditional idea of marriage) is not part of our evolutionary heritage and that, in fact, it is a social imposition contrary to deeply ingrained human inclinations. The alternative and, according to the authors, more natural behavior is a sort of promiscuity that they take pains to point out is not what we think of in the vernacular (sex with strangers and almost-strangers) but rather closer in spirit to polyamory: sex as a social bonding act between various members of a tribal group.

The authors advance three lines of evidence in favor of this thesis. The first is based on observation of the mating behaviors of our close animal cousins. Most animals are not monogamous but most species have a limited and advertised period of female fertility during which the males are fiercely protective of their (generally temporary) mates and aggressive toward other males. Bonobos and humans, by contrast, hide the period of female fertility and use non-procreative sex as a way to bond individuals in the tribe. Moreover, this shared paternity makes males invested in the outcome of all children in the tribe and is hence more rather than less adaptive for the group.

The second line of evidence relies on observations (from the days of European colonization to the present) of peoples that still have a forager lifestyle similar to how it is supposed our ancestors lived. Many (most?) have a very fluid sexuality where sex with multiple partners is often not only permitted but expected. Children are considered the community’s children. The authors claim that it was the shift from foraging to agriculture (which they describe as the most significant event in human history) that led to a preoccupation with individual property and its inheritance (as opposed to the shared resources heretofore used). That, in turn, led to shrinking the sphere of sharing (sexual relations, child rearing, resources) from the community to the nuclear family. This is where the “standard narrative” of sexuality arose (men want to spread their seed as widely as possible, women want to make sure their man will provide for them and their children) and led to the inferior status of women that has historically plagued Western societies. As an aside, the authors suggest that, after agriculture led to the notion of property, property in turn led to the notion of poverty.

The final type of argument is the observation that if the “standard narrative of sexuality” were really as natural as its adherents claim, we would not need so many strictures so often enforced to guard against pre- and extra-martial sex: those behaviors would be rare. Moreover, the fluid sexuality model provides a better evolutionary framework in which to understand homosexuality: it survives in the group because it is a form of pair-bonding.

One of the interesting themes running through the book is how easy it is for scientists, both social and natural, to be biased by their own culture. Darwin himself did not venture to challenge the Victorian notion of marriage (though there are some suggestions he may have suppressed thoughts heading in that direction), while modern anthropologists and primatologists appear to contort their interpretations to make our notion of marriage inevitable.

I expect many people will dismiss or attack this book because it is threatening—threatening to the way we’ve constructed our lives and society, threatening to “traditional marriage,” threatening to our human exceptionalism. This perceived threat does not mean the claims are false, of course. Indeed, if they better describe who we are and how we got here, then perhaps we can better understand how hard it can be to live up to our cultural ideals, and be sympathetic to each other when we stumble.

Should we change our society in light of these findings? That is a more complicated question. Whether we like it or not, the fact is that we today are members of a society shaped by a history of agriculture, property, differing sexual roles, and a monogamous definition of marriage. These legacies will not disappear overnight; it is not clear that they all should (do we really want to give up agriculture? can we?). That said, some parts of our cultural legacy we have been succeeding in improving (slavery, women’s rights), and maybe marriage, the concept that tries to capture the essence of our emotional and sexual bonding, will get its turn.

For Seattle readers: One of the authors will give a reading next week.