Humans Evolved to Be Sexually Promiscuous, Not Monogamous

Christopher Ryan is an American psychologist who is best known for his book, Sex at Dawn (2010) which looks at the prehistoric origins of human sexuality. The book has attracted quite a bit of controversy due to its main thesis, which says that our ancestors evolved to be sexually promiscuous, not monogamous. Dr Ryan is careful to point out that we should not interpret promiscuity in this context to mean loose morals or the willingness to sleep with anyone, but as having multiple sexual partners at any given time; most likely sexual partners who you know quite well. Since our ancestors lived in small (100-150 people) close-knit groups, their sexual partners would not be strangers or “one-night stands”. 

Monogamy has two meanings in modern usage; the first refers to sexual exclusivity between partners (partners having sexual relations only between themselves) and the second refers to the institution of marriage (a social or legal contract between two people). In both senses, monogamy is artificial, in that it is not an evolved sexual behaviour, but a cultural invention. Dr Ryan argues that there is an impressive variety of evidence to support this thesis.

According to Dr Ryan, for 95% of our history, humans were sexually promiscuous, and it was only after the advent of agriculture, around 10,000 years ago, that humans became monogamous. Before agriculture, our hunter-gatherer ancestors shared partners in a “free-love” egalitarian-type society. Sex was shared in much the same way that food was shared among the group. However, with agriculture came the concept of private property, the idea that an individual could have ownership of something and not be expected to share it. Women, unfortunately, became a possession that men could own. From then on, sexual behaviour was either monogamous or polygynous (where men have more than one sexual partner/wife – or a “harem” of females).

Ryan challenges mainstream theories of human sexuality which say that this form of human sexuality existed amongst our ancestors. Most evolutionary psychologists assert that males and females have been in a constant battle, with women naturally inclined to seek food and protection from men, whilst men seek to impregnate as many women as possible. However, in light of the evidence, this is not really an accurate picture of how human sexuality evolved.

Couple therapists and marriage counsellors may try and convince partners that their loss of sexual interest is a sign of a failed relationship, but in actual fact, it is a sign of human behaviour. Humans are more inclined to be sexually curious, and seek sexual novelty (in the form of fetishes, different partners, pornography etc.) than find actual fulfilment from an enduring relationship with one partner. Evidence for this lies in the statistics – nearly half of all marriages end in divorce, not to mention the sexual frustration, betrayal, instability and dysfunction that characterises many monogamous relationships. 

If humans have evolved to be monogamous, then why is it so difficult and unsatisfying for us to remain with one partner for the rest of our lives? Further evidence for the evolved promiscuity of humans comes from observations of our closest cousins, the chimpanzee and the bonobo. Ovulating female chimps have intercourse with all males who are willing and bonobos, who we share more DNA with than chimps, enjoy group sex as a way to appease conflict and strengthen social bonding.

The human body itself also points towards a highly promiscuous species. Body-size dimorphism (the difference in body size between males and females) is about 10-20%. Body-size dimorphism reflects male competition for females, so that if a male is significantly larger than a female, this indicates that the males of that species are involved in fierce competition for females. They need to big in order to compete. The 10-20% body-size dimorphism of humans is the same figure as that of chimps and bonobos, who are promiscuous. Moderate body-size dimorphism is therefore a sign of promiscuity.

Small testes, which gorillas, orang-utans and gibbons have, are either a sign of polygamy or monogamy. Larger testes are associated with more promiscuous behaviour, since species that copulate more will need larger testes in order to house more sperm for ejaculations. Chimps and bonobos have the largest testes, which is unsurprising considering how promiscuous they are, while humans have moderately sized testes, although not anywhere near as small as a gorilla’s. This volume is still far beyond what is needed for monogamous or polygynous mating.

Even the shape of the human penis has evolved in response to the fact that females will have multiple sexual partners. The glans of the penis are shaped as they are to remove any previously deposited sperm. During ejaculation the man’s glans will then shrink to ensure that his own sperm are not removed by the same process. Sperm also contains chemicals that defend against and attack sperm from other males and the large ejaculate that men have (the largest of all the Great Apes) is a sign of sperm competition. And there is sperm competition because women are having sexual relations with more than one partner, so males need to increase their chances of paternity certainty (being the father of the child which is eventually born).

Another piece of evidence to add is the fact that women are capable of having multiple orgasms (each orgasm taking longer to fulfil) whereas men lose interest in sex after orgasm (which does not take as long to fulfil). This suggests that women are actually more inclined to seek multiple sexual partners. What also runs up against the standard narrative of human sexuality is the fact of female copulatory vocalisation (FCV), which basically means that females vocalise (make loud noises) during sex. If humans were meant to be monogamous, then why would females draw attention to themselves by making these vocalisations? The answer is that the groans and moans are invitations for other males to come along. FCV is therefore associated with promiscuous mating, not monogamy. 

The last and very important kind of evidence that supports Dr Ryan’s thesis is the anthropological studies of other societies – those whose social organisation most resembles those of our prehistoric ancestors. Amongst the Siriono of Bolivia and the Mosuo of China, sexual promiscuity is the norm and it is not a source of shame, guilty, jealousy or social dysfunction in these societies. Every adult is responsible for every child and every child is respectful of every adult. Shame, for the Mosuo women, would actually come from the promise of fidelity (faithfulness).

It appears then that there is convincing evidence - from the disciplines of psychology, evolution, anatomy, primatology and anthropology – which supports the conclusions that humans evolved to be promiscuous, not monogamous. Despite this, Dr Ryan is careful to point out that the facts do not necessarily have to inform the choices that we make. Just because we are driven to have multiple sexual partners, this does not mean that we should. What is does not imply what ought to be done (as David Hume’s famous maxim goes). If people want to be monogamous, that is up to them, and there is nothing preventing them from creating a stable, satisfying relationship. Dr Ryan is just drawing out attention to how this choice might be a struggle, given the facts about human psychosexuality. 

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