EDIT: This review, from the journal Evolutionary Psychology, is the one I would’ve written if I’d been better read in the field and had more time to read extensively in it. Read the linked review if you really want to understand the problems with Sex at Dawn.
Furthermore, “The Myth of Promiscuity: A review of Lynn Saxon, Sex at Dusk: Lifting the Shiny Wrapping from Sex at Dawn” discusses the (many) problems with Sex at Dawn in a more complete fashion than I did. So if you’re looking for a deeper discussion than the one I can offer, consider Sex at Dusk.
My bottom-line assessment of Sex at Dawn: The Prehistoric Origins of Modern Sexuality is that the book would never get past peer review because so many of its descriptions of existing research and ideas are wrong or skewed. The book argues that humans are not “naturally” monogamous. That might be true. But Sex at Dawn doesn’t prove it. The data are ambiguous.
The biggest problem with the book starts on page 46, with the chapter “A Closer Look at the Standard Narrative of Human Sexual Revolution.” But there is no standard narrative of human sexual revolution: there are a wide array of people who have made inferences about the evolutionary basis of sexuality, but their narratives aren’t consistent and new papers and ideas constantly jostle or replace old ones. Ryan and Jethá don’t cite anyone else who claims a “standard” narrative, because to my knowledge no one has, and the standard narrative they cobble together is just that: cobbled together from a variety of sources with a variety of views.
I mentioned the lack of citations as a problem that occurs in their chapter on the standard narrative. It continues throughout the book. On page 293, Ryan and Jethá say that “To avoid the genetic stagnation that would have dragged our ancestors into extinction long ago, males evolved a strong appetite for sexual novelty and a robust aversion to the overly familiar.” But they don’t have any evidence for that. Similarly, they accuse scientists and others of claiming that monogamy is “natural” or inborn and cite, the anthropologist Owen Lovejoy as saying, “The nuclear family and human behavior may have their ultimate origin long before the dawn of the Pleistocene” (34). And he’s right: such behaviors may have their origins there. Or they may not have. Good scientists tend to be more tentative than polemicists because scientists recognize the fragility of so much human knowledge.
In Melvin Konner’s The Evolution of Childhood, he writes:
A double standard of sexual restriction is common across cultures; still, most human marriages have been mainly monogamous, owing either to environmental constraint or cultural principle. Modern cultures are monogamous in principle, but both adultery and serial monogamy are common. In at least thirty-seven countries, men express preference for women several years younger than themselves and place more emphasis on appearance, while women prefer men several years older and emphasize status and wealth (41).
The “environmental constraint” is important because it takes a lot of resources to support multiple spouses; this means that most men in most places and most conditions cannot afford to support multiple women. One woman might be able to support or be supported by multiple men, but polyandry is far less common than polygyny, as Konner points out. This is probably as close to accurate as one is likely to get regarding the historical or anthropological record on the subject of polygamy. It also has the advantage of coming from someone who spent his entire career on the subject of childhood development and who is deeply familiar with the vast literature surrounding evolution, anthropology, and childhood.
Ryan and Jethá also have many sections where they ask rhetorical questions or pit themselves against imaginary foes of great power; the page after the Lovejoy quote, they say, “This is what we’re up against. It’s a song that is powerful, concise, self-reinforcing, and playing on the radio all day and all night . . . but still wrong, baby, oh so wrong” (35). Enough with the polemics: if you’re right, show us that you’re right and leave the judgment up tot he reader.
Dan Savage called Sex at Dawn “the single most important book about human sexuality since Alfred Kinsey unleashed Sexual Behavior in the Human Male on the American public in 1948.” The statement is hyperbolic and unlikely but nonetheless demonstrates the power of the book, especially when America’s most famous sex columnist is pimping it, so to speak.
In addition, Kinsey was at least doing original research by taking and compiling sexual histories. Ryan and Jethá aren’t: they’re rehashing a variety of other people’s research, and in doing so regularly misrepresenting that research. Furthermore, Kinsey was reacting to a much, much different culture than ours today; Sexual Behavior in the Human Male had essentially no real forerunners, while Sex at Dawn is a weak entry to a crowded field of evolutionary biologists and psychologists like Geoffrey Miller (The Mating Mind), Sarah Blaffer Hrdy (The Woman Who Never Evolved, and David Buss). All three get cited, but out of context, and their deeper arguments are never really engaged. I don’t think it a coincidence that all three are academics.
For another example of imprecision in Sex at Dawn, Ryan and Jethá point out that men are only 10% – 20% larger than women (in polygynous species, the larger the size difference between sexes, the greater the number of sex partners). But that raw size or height difference way underestimates how that size translates to muscle. Consider David Potts’ work:
When fat-free mass is considered, men are 40% heavier (Lassek & Gaulin, 2009; Mayhew & Salm, 1990) and have 60% more total lean muscle mass than women. Men have 80% greater arm muscle mass and 50% more lower body muscle mass (Abe, Kearns, & Fukunaga, 2003). Lassek and Gaulin (2009) note that the sex difference in upper-body muscle mass in humans is similar to the sex difference in fat-free mass in gorillas (Zihlman & MacFarland, 2000), the most sexually dimorphic of all living primates.
These differences in muscularity translate into large differences in strength and speed. Men have about 90% greater upper-body strength, a difference of approximately three standard deviations (Abe et al., 2003; Lassek & Gaulin, 2009). The average man is stronger than 99.9% of women (Lassek & Gaulin, 2009). Men also have about 65% greater lower body strength (Lassek & Gaulin, 2009; Mayhew & Salm, 1990), over 45% higher vertical leap, and over 22% faster sprint times (Mayhew & Salm, 1990).
(That’s from Puts, David, A. “Beauty and the Beast: Mechanisms of Sexual Selection in Humans.” Evolution & Human Behavior 31.3 (2010): 157-75.)
The weird thing is that this information supports their assertion that humans are polygynous but hurts their assertion that early societies were mostly kind and peaceful, which they probably weren’t, per Lawrence Keeley’s War Before Civilization. Both the Potts paper and the Keeley book are the kinds of things that peer reviewers should be apt to point out.
Even when they aren’t simplifying the research others have done or selectively quoting writers without fully engaging in their arguments, Ryan and Jethá are merely poor writers. Take this: “For better or worse, the human female’s naughty bits don’t swell up to five times their normal size and turn bright red just to signal her sexual availability,” which is true in many species of apes. But note how bad this writing is: the sentence starts with a cliche, moves on to a childish description of women more appropriate to 14-year-olds than a real book and that also reinforces the very cultural forces the authors are trying to counteract, and then proceeds to something that has already been stated earlier in the chapter. The writing in much of the book is equally bad, the reasoning sloppy, and the thought underdeveloped. Which isn’t to say the book doesn’t have interesting or useful elements—it does—but those tend to get subsumed by its flaws.
The more I read about humanity, history, and the rhetoric of authenticity, naturalness, human instinct, and the like, the more I think there aren’t such things and the claims about what is “natural” reflect more about the person making the claim than anything about humanity itself. I would say that it’s natural for people to make claims about what is natural, but relatively little else is; circumstances affect so much that it’s hard to perceive many higher order behaviors as anything other than reflecting the bizarre combinations of self and environment.
People simply vary widely in their preferences, and most appear to view whatever society and subculture they grew up in as normal and natural. I posit that it’s not normal or abnormal to be polygamous or monogamous: in some circumstances one might make more sense, and in others the other strategy would. And people are too variable to say one mode is completely correct for all people under all circumstances.
I had actually begun this post before I read Paul Graham’s latest essay, “The Top Idea in Your Mind.” This part especially resonated:
I’ve found there are two types of thoughts especially worth avoiding—thoughts like the Nile Perch in the way they push out more interesting ideas. One I’ve already mentioned: thoughts about money. Getting money is almost by definition an attention sink. The other is disputes. These too are engaging in the wrong way: they have the same velcro-like shape as genuinely interesting ideas, but without the substance. So avoid disputes if you want to get real work done. 
To really catalog everything that’s wrong with Sex at Dawn, I’d have to go back through at least five or six books (and probably more) and at least a dozen papers. It would take me all day. Why spend that much time on a book that’s not very good? A while ago I promised myself that I wasn’t going to write many more posts on books that are bad in a generic way that doesn’t do anything special because I’m usually not spending my time in an optimal way. And reading Sex at Dawn is unlikely to be an optimal use of your time.