In Sex at Dawn: The Prehistoric Origins of Modern Sexuality (2010), Christopher Ryan and Cacilda Jethá look at the evolution of monogamy in humans and within human mating systems, challenging conventional wisdom about many aspects of human sexuality. According to the authors, prior to the establishment of agriculture, sex was fairly promiscuous, and paternity of little concern, in a similar manner to the mating system of Bonobos. Far from creating jealousy, sexual interactions within groups strengthened bonds of trust, social equilibrium, and reciprocal obligation. Turning the “standard narrative” of human sexual evolution on its head, the authors contend that monogamy is not quite as natural as once thought. Ryan and Jethá open by asserting that humans evolved within egalitarian hunter-gatherer groups in which sexual interaction was viewed as a shared resource, much like childcare, food, and protection. Much of our evolutionary psychology today has a bias regarding human sexuality, they say. Researchers and the public are both guilty of the “Flintstonization” of hunter-gatherer societies, meaning modern assumptions and beliefs are projected onto earlier societies. Pair bonding in marriage, monogamy, and even the nuclear family are all cultural constructs that date from after the rise of agriculture and civilization, and the benefits of these constructs were as much economic as they were societal. As a result, Ryan and Jethá claim this bias has led people to assume that our species is primarily monogamous, despite what they see as evidence suggesting otherwise. They argue, for example, that certain factors suggest a non-monogamous history, including human sexual dimorphism, female copulatory vocalization, testicle size, and appetite for sexual novelty. Male selection, they say, was not the subject of intragroup competition among humans of the pre-agricultural era because sex was neither commodified nor scarce. Instead, sperm competition was more important in terms of paternity factors than sexual selection. This behavior persists among certain existing hunter-forager groups that believe in partible paternity. Ryan and Jethá state that monogamy is not found in any social group primate, except—if the traditional narrative is to be believed—humans. They claim that conventional wisdom regarding human nature, in addition to what they call the “standard narrative” of evolutionary psychology, is incorrect. According to the standard narrative, males and females assess each other’s mating value based on their differing reproductive capabilities or agendas. Males search for signs of youth and fertility as well as the absence of prior sexual experience and the likelihood of sexual fidelity. Thus, he searches for a healthy and fertile young mate who has childbearing years ahead of her and no current children who would drain his resources. On the other hand, a female searches for signs of wealth, or prospects of future wealth, physical health, social status, and the likelihood her mate will stay to protect and provide for the children. Her mate must be capable of materially providing for her, particularly during pregnancy and breastfeeding, as well as their children. Assuming each meets the others’ criteria, they mate and establish a monogamous bond. Following this period, she will watch for signs he is considering leaving or being unfaithful, which would threaten the female’s access to the male’s resources and protection, while also looking for an opportunity (particularly around ovulation) for a quick fling with a man who is genetically superior to her husband. Meanwhile, the male will be sensitive to indications of her sexual infidelity, while taking advantage of sexual opportunities with other women as his sperm are plentiful and easily produced. The authors go on to note that they do not see these narrative elements as part of human nature as much as adaptations to social conditions, many of which came with the introduction of agriculture roughly 10,000 years ago. Ryan and Jethá go beyond commenting on sexual behavior to assert that humans are actually more selfless and egalitarian than is commonly thought. Sharing was so widespread, they say, because it is simply the most effective means of distributing risk among a group of individuals. However, the advent of agriculture changed people’s lifestyles entirely, leading to the accumulation of private property and power. This entirely altered the way people behave, leaving modern human beings in a state in which their instincts are at odds with the society in which they live. Like primates, they say, our prehistoric ancestors utilized sex as a currency, a buffering mechanism, and a way of keeping the group on an even keel. Thus, there is a reason we all possess a sex drive that does not stay fixed on one mate; deciding to keep it fixed there is possible, just unnatural. While Ryan and Jethá do not take a specific position in terms of morality or the desirability of monogamy in modern society, they do argue that people should become aware of our behavioral history in order to make more informed decisions.

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Cacilda Jethá

Christopher Ryan

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