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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