Book SummarySex at Dawn, by Christopher Ryan and Cacilda Jethá

1-Page Summary1-Page Book Summary of Sex at Dawn

In Sex at Dawn, Christopher Ryan and Cacilda Jethá argue that everything we think we know about prehistoric human sexuality is wrong. Contrary to popular belief, humans haven’t always formed monogamous pair bonds—instead, the authors say, prehistoric humans lived in foraging societies that encouraged casual sex with multiple mates. In their view, humans only reluctantly embraced monogamy about 10,000 years ago when we stopped foraging for food and started farming.

In this guide, we’ll begin by covering the “standard narrative,” or widely accepted set of beliefs around human sexuality. Then,...

Sex at Dawn Summary Part 1: The Standard Narrative

Before we dive into the authors’ arguments, we need to understand the traditional description of evolutionary human sexuality—the “standard narrative”—that Ryan and Jethá argue against. The standard narrative describes the way men’s and women’s approaches to reproduction evolved over time. (Shortform note: In general, when Ryan and Jethá refer to “men” and “women,” we can infer that they’re specifically referring to cisgender people. Sex at Dawn was published in 2010 and does not explicitly mention transgender or intersex people.)

According to the standard narrative, the human mating system works like this: If a man and a woman find each other desirable, they’ll form a long-term, monogamous bond (from which they’ll periodically escape for flings with other partners). This pairing offers women the security of access to resources and offers men the all-important certainty that they are their children’s biological father.

(Shortform note: In Mating in Captivity, couples therapist Esther Perel argues that the nature of monogamy has continued to evolve beyond this narrative in that [modern monogamy is less of a practical...

Sex at Dawn Summary Part 2: Evidence From Great Apes

To refute the standard narrative, Ryan and Jethá use evidence from three sources: the sociosexual habits of great apes with close genetic links to humans, observations of remote hunter-gatherer societies, and human biology. In this section, we’ll begin by examining the evidence from the great apes: specifically, chimpanzees and bonobos, which are species with non-monogamous (or multimale-multifemale) mating systems.

Let’s look at those mating systems in detail.

Chimpanzees

According to the authors, twentieth-century scientists thought chimpanzees were a nearly perfect model of ancient, unrestrained, “primal” humans because while they exhibit very human behaviors, they’re far less inhibited and more openly brutal than modern humans. These scientists also speculated that chimps’ approach to sex must represent ancient humans’ primal reproductive instincts: instincts that we still have, but repress.

(Shortform note: This belief that chimp behavior—sexual or otherwise—is equivalent to primal human behavior is why, in The Chimp Paradox, psychiatrist Steve Peters refers to humans’ most instinctive urges and emotional...

Sex at Dawn Summary Part 3: Evidence From Foraging Societies

In addition to great apes, the authors also use evidence from modern foraging societies to support their arguments against the standard narrative. These societies are geographically isolated from other people and still practice the type of hunter-gatherer lifestyle that our prehistoric ancestors did. The authors present two aspects of modern foraging societies that cast doubt on the standard narrative: partible paternity and non-nuclear families.

(Shortform note: There are about 30 modern foraging societies that we know of. However, as of 2018, there were also roughly 100 uncontacted tribes in the world. Because these tribes have avoided or rejected interactions with outsiders, we don’t know for sure whether they embrace the same hunter-gatherer lifestyle as early humans did.)

Partible Paternity

According to the authors, one way that modern foraging societies refute the standard narrative is through partible paternity, or the idea that more than one man can be the biological father to a child. This argument arises...

Sex at Dawn Summary Part 4: Evidence From Modern Human Biology

Lastly, the authors use several aspects of modern human biology as evidence for their argument that prehistoric humans lived in promiscuous societies. These include body-size dimorphism, the size and shape of the human testicles and penis, and female copulatory vocalization. Let’s explore each in more detail.

Body-Size Dimorphism

Body-size dimorphism is the average difference in body size between males and females of the same species. According to the authors, the more males in a species have to compete over females, the bigger the body-size dimorphism. That’s because bigger males tend to win competitions for females and pass on their genes, so each generation of males gets a little bit bigger. On the other hand, when there’s no need to compete, the genes for body size remain equally distributed in the population, so each generation of males stays about the same size.

In humans, adult men tend to be, on average, 10% to 20% bigger than adult women. The authors believe that’s a relatively small difference compared to other species like gorillas, in which males can be up to 100% larger than females. They argue that the relatively small body-size dimorphism in humans...

Sex at Dawn Summary Part 5: The New Narrative

Ultimately, Ryan and Jethá argue for a new narrative of human sexual evolution to replace the standard narrative. Their new narrative is based on the assumption that, before the advent of agriculture, humans lived in multimale-multifemale mating systems, not polygynous harems or monogamous pair bonds.

(Shortform note: It’s important to note that this is just one possible theory of many. As we’ve explored, there isn’t a universally agreed-upon narrative—other researchers have come to different conclusions despite using the same evidence as Ryan and Jethá. For example, recall that the authors use average human testicle size as evidence that humans evolved in a multimale-multifemale mating system, but William Buckner used the same data as evidence that humans evolved in a polygynous mating system.)

This underlying assumption gives rise to a set of new conclusions about the nature of human sexuality:

Conclusion 1: Monogamy Isn’t “Natural”

According to the authors, monogamy is extremely rare in the natural world, occurring in just 3% of all mammals. Furthermore, among humans, adultery is a common occurrence in...

Shortform Exercise: Reflect on Monogamy

Ryan and Jethá believe that monogamy isn’t “natural” and may not be the best approach for every relationship. Take a moment to reflect on your feelings about monogamy.

Growing up, what kinds of relationships were modeled to you? Did you see any examples of consensual non-monogamy?