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Is Sexual Promiscuity More Natural than Commitment?

What "Sex at Dawn" gets wrong about fatherhood.

Sex at Dawn

is a popular book about sex and human nature, written by PT blogger Chris Ryan and Cacilda Jethá. Some other PT bloggers seem to be huge fans of the book (for instance, see this recent post by Nadja Geipert, or Rachel Clark's response to that post). I've just finished reading Sex at Dawn, and while I found it provocative and often fun to read, there is unfortunately quite a lot to criticize about it. The book's problems are in fact way too numerous to confront in any one post, so I'll just say a few words about them here. I may say more about them in future posts, if this topic proves popular with readers.

Sex at Dawn

The book's basic argument is that human sexual nature is essentially promiscuous. Ryan and Jethá imagine that our hunter-gatherer evolutionary past was characterized by an absence of exclusive, committed relationships; males mated promiscuously with many females, and females mated promiscuously with many males. Because everyone was having sex with everyone else, there was no way for males to know which children were their own, or if they even had any children of their own. But that was OK, because nobody cared; "certainty of paternity" was a non-issue for ancestral hunter-gatherers. Instead of worrying about caring for and protecting (investing in) one's own children, everybody in the group simply invested equally in all children.

Based on their assumptions about our evolutionary past, Ryan and Jethá suggest that human nature is fundamentally promiscuous. In their view, people have been experiencing paternity concerns, and engaging in committed, sexually exclusive, long-term relationships (such as monogamous or polygamous marriages), only over the past 10,000 years or so; these unnatural and noxious behaviors emerged as by-products of the agricultural revolution. (Exactly why agriculture is to blame for the emergence of committed relationships is never coherently explained, but they suggest that it has something to do with private property).

So what's wrong with this picture? Well, first off, let me clarify how Ryan and Jethá's vision of human sexual nature conflicts with the standard view from evolutionary psychology (EP). Most evolutionary psychologists see people as flexible mating strategists, who are adapted to employ promiscuous strategies under some conditions, and more committed strategies under other conditions. The adaptiveness of any one strategy versus another can depend on a variety of environmental variables (e.g., the importance of male parental investment in that environment) and on the characteristics of the people involved (e.g., the potential mate's genetic qualities). Another standard EP view is that a man will be more inclined to engage in a committed relationship with a woman, and to invest in any offspring that this relationship may (apparently) produce, when he has higher paternity certainty (e.g., greater confidence in her sexual fidelity). Evolution favors those who pass on their own genes, and ancestral men who didn't mind being cuckolded tended to do badly in that department.

These standard EP perspectives are based on hundreds of studies conducted in psychology, anthropology and other fields; for a review see an introductory EP textbook such as Buss (2008). Ryan and Jethá aim to overturn this massive body of scientific research in one bold stroke of library research. They don't agree that people are flexible mating strategists; they instead see human sexual nature as inflexibly committed to promiscuity, even if civilization may force people to suppress and deny their true sexual nature. Similarly, they see any male concern with paternity—including subsidiary symptoms of such concern, such as sexual jealousy—as a disease of civilization.

To support their strong claims for an exclusively promiscuous human nature, it would be nice if Ryan and Jethá could produce some strong evidence. But they don't. Their case rests heavily on their notion that the highly promiscuous bonobo is useful as a model of human nature. The bonobo is one of our two closest primate cousins; it and the chimpanzee are equally related to humans. It is also an entirely different species from humans, separated from us by about six million years of evolution. While comparative studies among species are often highly revealing, using the bonobo as a model for human nature makes roughly as much sense as using the bonobo as a model for chimpanzee nature, or using the human as a model for bonobo nature. At some point we have to accept that the only valid "model" of human nature is humanity itself.

That's not to say that Ryan and Jethá don't focus on humans too; indeed, they often refer to evidence from the anthropological record. Unfortunately their presentation of this evidence is not at all systematic or even-handed. They cherry-pick ethnographic anecdotes and case studies that seem to support their arguments, while ignoring oceans of contradictory evidence. They leave in all accounts that seem consistent with cost-free and conflict-free promiscuity, and leave out abundant examples of long-term committed relationships, paternity concerns, and sexual jealousy in preliterate societies. For a more in-depth look at how they misrepresent the ethnographic record, see this illuminating review of their book (PDF).

The book's biggest flaw is probably its misunderstanding of how EP views paternity certainty. The main prediction that follows from this EP view is not (as Ryan and Jethá suppose) that men will be equally preoccupied with paternity concerns in all cultures; rather, it's that across cultures (and across species), male willingness to invest in offspring will tend to correlate positively with paternity certainty. Consider, for example, fatherly investment in the bonobo and chimpanzee. Despite all their attention to these apes, Ryan and Jethá somehow overlook the fact that bonobos and chimps are both characterized not just by high promiscuity and low paternity certainty, but also by an absence of male parental care (Geary, 2000).

Instead of acknowledging that low paternity certainty leads to low male parental care, Ryan and Jethá instead emphasize the "inconsequentiality" of paternity certainty. For instance, they state that among the Mosuo (the society referenced in the posts that I link to above), many women mate promiscuously, and "paternity certainty is so low and inconsequential that men... raise their sisters' children as their own" (Ryan & Jethá 2010, p. 126). But their description undermines their own conclusion that low paternity certainty is "inconsequential". There is a consequence: low fatherly investment. That's why this investment has to be provided instead by the mother's brothers. (Note also: the fact that this replacement investment is provided by the mother's genetic kin is exactly what standard EP theory would predict. This fact is inconsistent, however, with Ryan and Jethá's expectation that everyone in "the group", regardless of kinship, will invest equally in these children).

Whether or not we all agree about the value of male parental investment in modern environments, it's important to be realistic about the trade-offs that promiscuity entails: when promiscuity is more prevalent, males will be less committed to long-term relationships, and less devoted fathers.


Buss, D. M. (2008). Evolutionary Psychology: The New Science of the Mind, Third Edition. Pearson.

Ryan, C. & Jethá, C. (2010). Sex at Dawn: How We Mate, Why We Stray, and What It Means for Modern Relationships. Harper.

Geary, D. C. (2000). Evolution and proximate expression of human paternal investment. Psychological Bulletin 126: 55-77.

Copyright Michael E. Price 2011. All rights reserved.