After listening to the podcast, “Polyamory: When two just won’t do”, from Stuff You Should Know, I wanted to delve deeper into this topic. As psychology was a compulsory part of my course last semester, I decided to create an extra individual assignment that focused on monogamy and polyamory where I designed the questions I wanted to answer. Here is the original piece I submitted.

Are humans naturally monogamous?

1) Define monogamy, polygamy, polygyny, polyandry, and polyamory as is intended to be used to answer the following questions.
2) What biological factors in humans suggests that as a species we are naturally monogamous or polyamorous?
3) What social factors in modern era has encouraged humans as a species to be monogamous in the Western world?
4) Are people fighting their instincts by forced monogamy? Is it natural to cheat?
5) To this end, what do you think is the primary reason for humans to be monogamous in modern, Western civilisation?



Of the 1,231 cultures globally, 84.6% are polygynous, 15.1% are monogamous, and 0.3% are polyandrous (Gray, 1998, p. 86-136). Polygyny however, is a rather taboo subject in the Western world. But why is this so? Upon beginning to research the topic of polyamory, it would seem that the reasons for humans (in Western culture) being so opposed to polygamy can be categorized as social or biological — although these two complement one another and are often difficult to truly distinguish.

For the sake of keeping this essay straightforward and easy to follow, the gender spectrum is not being taken into account (i.e. scenarios will only consider those identifying as either female or male), and partnerships involving two people will be addressing heterosexual couples, unless stated otherwise.


1) Define monogamy, polygamy, polygyny, polyandry, and polyamory as is intended to be used to answer the following questions.

Upon looking further into the topic, a few more terms were deemed necessary to define (in addition to those stated in Question 1.), in order to ensure minimal ambiguity.

Monogamy = The state of having a sexual relationship with only one other person (during a period of time or for a lifetime).

We could dig even deeper and differentiate between social monogamy and sexual monogamy, but for the case fluidity in this piece, monogamy will refer to a couple who are sexually exclusive and cohabit, as for humans in Western society, social and sexual monogamy tend to go hand in hand.
Social monogamy = The living arrangement where a male and female share territory and,     according to Reichard (2003), engage in “behavior indicative of a social pair […] without     inferring any sexual interactions or reproductive patterns” (p. 4).
Sexual monogamy = Reichard (2003) described sexual monogamy as “two partners remaining sexually exclusive with each other and having no outside sex partners” (p. 3).

Serial Monogamy = In which a person has more than one sexual partner during their lifetime, although they are only with one person at a given time. Known as sequential monogamous pairings by Wright (1994).
[When referring to monogamy in this piece, it will most likely be serial monogamy.]

Polygamy = The state of being in a sexual relationship with more than one person at a time, where all parties involved are aware of this. A blanket term for all sexual relationships involving more than two people.

Polygyny = Polygamy where a man has multiple female sexual partners. [By far the most common form of polygamy.]

Polyandry = Polygamy where a woman has multiple male sexual partners.

Polyamory = Similar to polygamy, but will be used in the following questions in regard to sexual relationships with more than two people, where all involved can be sexually active with all others in the group. There is more of an egalitarian structure rather than a patriarchal or matriarchal structure and there is not a set sex ratio i.e. there needn’t necessarily be one male; there could be two males and two females. All parties involved are fully aware and informed of the situation and everything is consensual.

Bigamy = The offence of marrying somebody while being already married to another. It is only bigamy if it is illegal to marry more than one person at a time in that judicial district.

Group/Plural Marriage = When multiple people are married to one another in a legal sense and form a family unit — marriages between the involved individuals are not considered separately. Not legal in many Western countries.


2) What biological factors in humans suggests that as a species we are naturally monogamous or polyamorous?

Upon analyzing the various biological factors of the human species to determine whether or not we are monogamous, there are no definite answers pointing in one direction or another. For example, polygynous species tend to display quite prominent sexual dimorphism (differences between females and males of the same species, in body characteristics beyond solely the sexual organs), with polygynous, mammalian males being significantly larger than females. Monogamous species tend to have a less of a size difference between the sexes, such as the gibbon. Humans have a definitive male to female size difference, with males being on average 9% taller and 16.5% heavier in the United States of America, (McDowell, 2008; Ogden, Fryar, Carroll, & Flegal, 2004), but this is significantly less than most polygynous primates, where the difference is more pronounced and males are often one and a half to two times as large as females. Overall, the sexual dimorphism in humans would point towards polygyny. However this ratio is not as pronounced as that of the polygynous chimpanzee and orangutan, the latter of which has a male to female size ratio of nearly two to one (Plavcan, 2001, pp. 25-53).

According to Barash and Lipton (2002), most animals in the animal kingdom are polygynous, with only 3% of mammals and 15% of primates estimated to be monogamous. Many bird species are known to “mate for life”, and over 90% of bird species are (socially) monogamous. However, lesser known is that only 10% of birds are sexually monogamous (Morell, 1998), as upon studying offspring in nests, Angier (1990) found that around 30% of baby birds were not the offspring of the couple raising them. This infers that nature tends to encourage polygamy, and so the probability of humans being polygamous is more likely. But simply because more animals are polygamous, that of course does not mean that humans cannot be a part of the 3%.

Animals that fit into the monogamous category include wolves and otters, and notably our closer related primates like the Night Monkey and Tarsier. But this must be taken with caution as we have a tendency to compare ourselves to closely with our primate relatives, despite our genetic branches having split around 6 million of years ago with the chimpanzee-human last common ancestor (“When humans and chimps split”, 2005). In this time there is plenty of scope for change and significant evolution to curve away from one sexual norm to another.

Males in polygynous species tend to have larger testes than monogamous ones, presumably because there is more of a time pressure to impregnate a female during intercourse in polygynous societies. Monogamous males have no immediate pressure to impregnate their partner because their relationship is (hopefully) exclusive and they will be having sexual intercourse on a regular basis. This also ensures that a male can be reasonably certain that any offspring is theirs, a topic that will be touched upon later in the social factors for monogamy. On one end of the spectrum, chimpanzees are polygynous and have very large testes, and on the other end are gibbons who are (socially and at times sexually) monogamous with small testes (Reichard, 1995, pp. 99-112). Unfortunately for anthropologists, human penis size lies fairly in the middle of the spectrum, making it impossible to use this biological feature for or against the monogamy argument.

Another aspect of the human penis that fascinates anthropologists is the lack of “horny papillae”, a feature found on many other mammals (Engber, 2012). These are hardened ridges along the length of the penis that tend to shorten the length of time to ejaculation during sexual intercourse as Engber (2012) has said they “enhance sensation […] and shorten a mating male’s delay to climax”. This would be ideal in polygynous societies, where males must often be quick when impregnating females lest an opponent arrive to challenge. The lack of horny papillae in humans means that males needn’t rush sexual intercourse and it is therefore likely an evolutionary feature to promote monogamy; it allows for deeper pair bonding. With increased time until ejaculation, human couples spend more time focused on one another while copulating and develop deeper emotional connections, important for building trust and raising a family. This all would point towards humans being monogamous. Notably, Morris (1967, p. 56) mentioned that male baboons spend no more than eight seconds between mounting a female and ejaculating and females do not visibly show any sexual pleasure. (Though as with many evolutionary theories, there is not as clear a correlation between horny papillae and faster intercourse as some would want to believe. Hawks (2011) showed galagos to be prime examples, having pronounced ridges on the testes and yet spending a significant amount of time copulating). Human bodies also release the well-known hormone, oxytocin, during intercourse — much more so than most other animals. Known as the “Cuddle hormone” or “molecule of monogamy”, oxytocin promotes trust, empathy, and intimate relationships, again invaluable for a monogamous pair (Bartz et al., 2010). Although found in other situations such as a mother-child bond, during intercourse the release of oxytocin reaches a peak, making it difficult for humans to have sex without becoming emotionally attached to their partner — this is even more so the case for women, who have a larger limbic system.

The females of polygamous species tend to overtly display their fertility, so that when they are ovulating, males will not miss an opportunity to pass on their genes. Monogamous females on the other hand, tend not to display such changes in fertility because (similar to why monogamous males need not have large testes where there is no immediate pressure to impregnate) they will likely be copulating on a frequent and regular basis with their partner — not on a fertility-dependent one. Hiding fertility might also be an evolutionary tactic to promote commitment in a relationship and encourage males to stay and care for offspring. When looking at humans, the lack of obvious changes in the female body throughout the menstrual cycle would indicate that humans are monogamous. However, as this is a popular topic for science, multiple studies have been conducted to see whether females are considered more attractive when ovulating. And the results would suggest, ‘yes.’

Firstly, women are more flirtatious when in the ovulatory phase (Cantú, 2013), and dress more provocatively (Haselton, Mortezaie, Pillsworth, Bleske-Rechek, & Frederick, 2007, pp. 40-45; Durante, norman, & Haselton, 2008, pp. 1451-1460). They tend toward the color red, long known to be associated with passion, lust, and romance. But findings suggest that these choices are not made consciously, as women are not particularly successful at pinpointing when they are fertile — Barber (2009) found that women tend to guess correctly around 60% of the time. Breasts become more symmetrical during fertility and the waist-hip ratio is accentuated, proven to be an important feature of attracting heterosexual males — which self-fulfilling because a 0.7 waist-hip ratio is the most attractive and also one that correlates with optimal oestrogen levels and therefore fertility.

During ovulation women’s odours change and their voices become more high-pitched (Bryant & Braselton, 2009, pp. 12-15), which increases attractiveness, and their dancing and gait is rated as more attractive (Fink, Hugill, & Lange, 2012, pp. 759-763; Guéguen, 2012, pp. 621-624). A popular study (Miller, Tybur, & Jordan, 2007) showed that lap dancers earn significantly more during their “heat” period (around $335 for a five hour shift in comparison to $260 and $180 during their “luteal phase” and when menstruating, respectively). Males being handed female clothing return results that the clothes worn by women when ovulating smelt more attractive (Kuukasjärvi et al., 2003) — but only those not taking oral contraceptives because, like with women who are not fertile, women on contraceptives obviously do not change their scent throughout the different phases of their menstrual cycle. Another study by Miller and Maner (2010, pp. 276-283) showed that smelling clothes from ovulating women resulted in testosterone levels greater than those who smelt unscented clothes or those worn by non-ovulating females. There is a heightened redness in cheeks when ovulating, and males have consistently rated redder female faces as more attractive (presumably because this suggests health and youthfulness). However, because the increase in redness is not visible to the human eye, the researchers (Burriss et al., 2015) into this particular study concluded “that changes in skin color are not responsible for the effects of the ovulatory cycle on women’s attractiveness.” The increased redness might be due to women’s body temperature increasing slightly during the ovulatory phase.

From all this it would have to be concluded that females do in fact display their fertility, but in such a subtle way that it would be plausible that it was intended for these traits to be gradually phased out evolutionarily in order to promote monogamy, but was never quite fully completed.

On an evolutionary level, polygyny makes sense as humans are less likely to die out as a species when females are more likely to be impregnated quickly, and by an optimal mate. Polygyny reserves the privilege of impregnating females nearly exclusively to the superior males, meaning that beta males are outcasts from society, and have little to no chance of passing on their genes. The more successful males are more likely to be stronger, resistant to disease, and physically and mentally more adept — characteristics that a mother would want for her child.

In societies where there is a scarcity of males, polygyny is clearly a sensible route to ensure the  continuation of the species. When humans were concerned about their lineage continuing, polygyny ensured that females were virtually always pregnant and therefore the society would prosper. Another interesting aspect to consider is “culling”, the well known phenomenon that in times of hardship, the female body tends to abort a higher percentage of male foetuses. A female’s liver produces more of the Cortisol hormone, which the male foetuses cannot as easily deal with as female foetuses — male foetuses are weaker than female ones. Essentially, in times of hardship and stress, more females are born, gearing toward a more polygynous society (few males are needed in polygynous groups, rather it is the number of females limiting how many offspring will come into the group). As the human species has progressed and there is less of a struggle for survival, there is a lesser gender imbalance in births. This could lead to the suggestion that humans are only polygynous when it is in the interest of survival, and that otherwise humans should be monogamous. This is quite a stretch though.

Taking all of these biological considerations into account, on balance, humans are probably no longer wired for polygynous societies where a select number of males “hoard” females, but this does not suggest that humans are no longer biologically capable of being in intimate, loving, polyamorous relationships with three or more people. Polyamorous relationships provide even more support for every member — which is a concern with polygyny where multiple females all rely on one male.


3) What social factors in modern era has encouraged humans as a species to be monogamous in the Western world?

In Benin, 55% of women share their husbands (Barber, 2008, pp. 103-117), yet in the United States of America a unanimous Supreme Court decision in 1878, banned polygamy (United States Supreme Court, 1878). An immediate answer to why polygamy is such a taboo in the Western world, is the influence of Christianity on these countries. However, although Christianity played an important role in the institutionalisation and enforcement of monogamy, it had much less of a role to play in introducing monogamy than people might like to think.

Regarding the earlier mentioned point about monogamy allowing males to be (reasonably) certain offspring are theirs, this encourages males to take care of the offspring more than in a polygamous society where the child could be that of any male. It is obviously in the interest of a child’s wellbeing and development to have a father who can share the burden of raising a child, so this would suggest that humans became monogamous to encourage males to help take care of children and thereby further the human species. If a child is given longer to mature, it would give the human brain the opportunity to develop more and increase intelligence. The brain development and time given to child-rearing likely both coaxed one another on. This would especially be the case in Western countries, which are overall more northern in latitude and so have to deal with colder, harsher winters, where food is scarce, and children need more care.

After reading The Naked Ape (Morris, 1967), it is clear that Desmond Morris had a very similar, if more elaborated, explanation for the development of monogamy. As noted in detail by Morris, the trend towards monogamy started back as far as when humans were evolving from their primate roots, into more carnivorous mammals. Being inferior in comparison to the already specialised carnivorous killers on the open plains, the ape from which humans descended only had its brain to compensate for lack of strength, agility, endurance, and speed. This ape needed to develop tools and advanced hunting strategies instead, and in order to do this the brain had to develop significantly. Modern-day monkeys’ brains are already 70% of their final adult size when born and reach maturity within the first 6 months of life. Humans on the other hand are born with their brains at 23% their final size and this full size is only reached at around age 23 (Morris, 1967, p. 30). With increased time taken for the evolving ape (or “hunting ape” as Morris calls it) to mature and develop and learn from its surroundings, it required two parents to dedicate themselves to the lengthy process. These mates had to develop a “pair-bond” in order to work together and stricter gender roles had to be formed with the female becoming home-bound and dedicating herself to the rearing of children. Males on the other hand had to work in groups to hunt for the mothers and children — the ape was of course not physically capable of going on the hunt by itself and used its advanced brain to develop more detailed communication systems and more complex manoeuvres. The parents had to trust one another and be sure of one anothers’ loyalty, especially the males, who needed to work as one, cohesive unit on the hunt. This would not be possible if there were group tensions and rivalry for females. In essence, the demands of ever more intelligent children brought about monogamy. In order to ensure that couples were truly monogamous, more and more social developments were brought about, such as clothing to hide anything that might be perceived as sexual, the introduction of ‘marriage’, and laws to punish infidelity.

The biological change to help encourage pair-bonding and the normalisation of couples is probably the reason for the widespread phobia of polyamory. The Christian Church had such an influence on ordinary people’s lives, that by ruling sexual relations to only be permitted between two people, it was now near impossible for polygamy to be practiced without severe consequences. (Even though in the Old Testament of the Bible the likes of Abraham and Solomon engaged in polygyny). Simply because it is unusual, people cannot accept that there could be something other than a pair of lovers, even though having three parents could be even more beneficial to a child who would have more care and attention focused on them.
Another intriguing social reason for monogamy put forward by Price (2011 (September)), is military advantage. Monogamy meant that inferior males were still nearly guaranteed a wife, and so would not need to leave the society in search for a mate. Keeping these inferior males helped the society as they could take taxes from these men and they would fight in their armies. Essentially, women were rewards for males staying to fight and help the economy. On the whole, monogamy is better for all men as they are all nearly guaranteed wives. To keep the common man on rulers’ sides, giving him a wife was a small sacrifice for dominant males to make.

When considering why polygyny might else have ‘died out’, it is worth remembering that in the small-scale societies of ancient civilisations it would have been difficult to hoard wealth. Men would not have been able to support more than two or three wives. However, this reason does not align with the emergence of larger-scale societies in more recent history, where industrialisation allowed for wealth hoarding and therefore would have made polygyny more possible and even sensible (Price, 2011 (August)). Perhaps by then monogamy was already too interwoven into society. Much of the Western World has become very urbanised in the past few centuries. In more agriculturally focused areas, having more children (possibly through polygamy) is often beneficial, as they can help with tending the land and livestock. But in cities it is much more challenging to start a large family, due to the sheer expense a single child imposes on its parents. Western countries’ urbanisation could also be a supplementary factor for why the Western world specifically is so opposed to polygamy.

Now that monogamy is so institutionalised, it is unlikely that the human race will see a trend towards polyamory any time soon, especially as in this developed stage of monogamy in culture, the ideals of monogamy are omnipresent. From the beginning of a child’s life, a loving, sexual relationship with one other person is seen as the relationship to strive for. In popular culture; love songs, books, films, the “Happily Ever After” faerie-tale ending, parents, friends, religion, tales of “The One”. That finding this other person is what will lead to happiness. Having this so overwhelmingly impressed on someone their entire life, makes it very difficult — if not impossible — for some people to even imagine beginning a polyamorous relationship.


4) Are people fighting their instincts by forced monogamy? Is it natural to cheat?

This question had essentially been answered in the extended sections on the biological and social factors resulting in monogamy, above. But to summarise, humans are likely not fighting their instincts due to forced monogamy by Western culture, because their instincts ought to be that they want to be monogamous. Humans have biologically developed enough since the introduction of monogamy to want to be monogamous — examples of these ‘new’ biological traits include the high releases of oxytocin, and extended time taken for sexual intercourse to tighten pair-bonds. Humans became monogamous in order to survive, which is possibly the most natural reason for a change in behavior.

However, some of the primate genetics are likely still inherent within human DNA due to the sheer speed at which the “hunting ape” developed. This could explain the tendency for people to cheat on partners; we are not so developed that we can always be fully satisfied with a single pair-bond. This of course does not excuse cheating, because most cultures have made it clear that cheating is not a socially acceptable thing to do, and so the fear of social punishment should overwhelm any natural instincts in most instances (Morris (1976) would disagree, believing that any social rules that go against biological impulses will never end successfully). It is no longer natural for humans to cheat on partners due to the changes in genetic make-up that seemed to come about to promote monogamy. But there is no scientific evidence that it is not natural for people to have a sexual relationship in a group of three or more people, and that they cannot equally love two people in a similar way.


5) To this end, what do you think is the primary reason for humans to be monogamous in modern, Western civilisation?

The primary reason for humans to be monogamous in modern, Western civilisation is the initial, primal reason that the “hunting ape” needed to increase its intelligence in order to hunt and survive upon leaving the forest dwellings. Intelligence required more time for a child to grow and develop its brain, which required two dedicated parents because rearing such children is extremely demanding and draining. After this primary biological change that introduced monogamy to the human race, it was the institutionalisation of laws and religions etc. against polygamy, simply because it was uncommon. The reason for humans being nearly exclusively monogamous now, is self-inflicted. There are severe consequences for those practising polygamy — social outcasting and legal action being the main two — and people are so exposed to the ideals of monogamy from birth, that it would require serious reflection and commitment to engage in polygamy. The former reason of children requiring more care however, does not explain why polyamory and group marriages are not practised often in Western culture; surely having more parents would only aid a child’s development? The latter of social conditioning though, makes complete sense to explain why polyamory is so rare.


Conclusion and Reflection

It was difficult to base this piece on the polyamory that can lead to group marriages because the vast majority of research is based on polygyny. Research nearly exclusively considers a dominant male with females who ‘share’ him, rather than relationships where the females might have sexual relationships with one another as well. Because of this, this piece became one predominantly focused on why the Western world is not polygynous, as opposed to polyandrous (this lacks feasibility in most instances) or polyamorous (because of the lack of research). It was more about the traditional polygamy: polygyny. It would have been nice to detail why Western culture is so opposed to more of a group marriage style polyamorous relationship. Either way, it was still fascinating to look into this field, and what can be concluded on the whole, is that here are stated only a few of the many reasons and theories for why modern Western civilisation is nearly exclusively monogamous and not polygynous (as opposed to polyamorous). Because polygyny is not in the interest of children’s development or the majority of the male population, it becomes unusual and is disdained. This then becomes integrated into the law, and after much conditioning, it is so unusual that it seems taboo, unnatural, and alien. At this point, it would be more useful to examine whether isolated humans would tend towards an arrangement like a group marriage or a traditional, “Christian” marriage.

My sections (4) and (5) are extremely short, but I got rather carried away with the second and third sections, which essentially detail the answers to (4) and (5) in a rather extended way. Trying to separate the biological from the social factors regarding monogamy may not have been the best idea for a successful essay because the two are so intertwined, and biological factors often spurred on social ones and visa versa.

Initially when beginning this piece, I had anticipated concluding that humans are naturally polygynous, which could develop into polyamory if it became more socially acceptable. I was surprised to find that after digging into the biological and social changes in humans, I had to change my mind and accept that humans are likely naturally monogamous. Some food for thought on this topic where there are no definitive answers, are the words of Jared Diamond (1997, p. 62), “Perhaps our greatest distinction as a species is our capacity, unique among animals, to make counter-evolutionary choices.” (Although this is a slightly self-contradictory statement in itself, as all choices are evolutionary, they might just not make much sense on a survival-basis level…)



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