Is Monogamy Natural?

Now that we have a working definition of monogamy (a dyadic and sexually exclusive relationship), we can determine if this state is natural for human beings. The word natural means “in accordance with or determined by nature.”[1] So, as this definition suggests, when we ask if monogamy is natural, we are inquiring as to whether the state of having exclusive sexual relations with one other person is consistent with our human biology and inherent impulses. This chapter will seek to resolve whether monogamy is an innate condition for humans or whether it is a constructed social convention.

We Are Designed to Reproduce

Despite our intellectual and technological advancements, we must remember that humans are still animals. And as much as we like to think we are a superior enlightened species, our thoughts and actions are still driven by the basic instincts of reproduction [2]. Sure, we may choose to focus our physical and mental energies on other pursuits such as careers, creativity, commercialism, or contemplation, but this does not negate the fact that reproduction is our biological reason for being [3]. And while today, sex may be chosen as a means of pleasure rather than procreation, it does not take away from the millennia of evolution that have shaped us with one purpose in mind — to reproduce.

“In the human species, it [the reproductive instinct] is one of the strongest of the instincts; so strong is it that the control and regulation of its impulse is one of the most difficult problems for the individual and for society.”

Monogamy then appears to be somewhat counter to the human instinct for reproduction, limiting the human potential for procreation by:

• Confining men to the reproductive cycle of only one female
• Preventing men from breeding with other females during a partner’s pregnancy
• Restricting women from maximising their fertile times by accessing sperm from multiple partners and engaging in sperm competition (more on this later).

Monogamy is Rare in Nature

It is understandable, then, that monogamy would not be a natural choice for animals whose prime priority is to reproduce. Science supports this, with research showing almost all mammals are not monogamous. One study has found that only three per cent of our mammal cousins form pair bonds to rear their young. However, most of these couples also have sex with others outside the dyad, and so while form couples are not technically monogamous. [5] Sexual sovereignty is so widespread and persistent in mammalian species that scientists developed the term social monogamy to describe the situation where the partners display the array of social and reproductive behaviours associated with pair bonding while not necessarily displaying sexual fidelity [6].

The proportion of mammals that actually maintain sexual exclusivity is much closer to 0.6 per cent. Only a few dozen out of 4,000 mammal species maintain restricted sexual relationships. These include a few species of bats, foxes, a few primates (marmosets and tamarinds), a handful of mice and rats, some south American rodents, the northern beaver, a few species of seals, and some small African antelopes[7].

Monogamy is in the Minority

And just in case you would like to counter with claims of human ascendency, here are some statistics from our species:

• Only around 21 per cent of global cultures are exclusively monogamous and explicitly outlaw polygamy (where there are multiple spouses for either sex).
• Well over three-quarters of the world’s cultures, both historically and currently, practice some form of polygyny (the practice of having multiple wives) [8].

These statistics show that most of the world’s population lives more aligned with the natural human instinct for reproduction. Notwithstanding the distressing reality that most of this is done unequally, with women paying the price for the power of patriarchy. While men have the advantage of securing the reproductive assets of multiple women, the same sanctioning of sexual agency is not bestowed upon their female partners. Many western nations have used the potential for polygamous arrangements to exploit and repress women as a reason for not considering legalising these arrangements in their own countries.

The following map shows how these cultures are spread across the globe[9].

In addition, across 160 different societies, it was found that infidelity was by far the most common rationale for ending a marriage [11], with extramarital affairs being the prevalent reason for around 40% of American divorces [12]. Even more interesting, though, is the finding that the vast majority of men (74%) and women (68%) would have an affair if they knew they would never get caught [13]. It appears that without the fear of retribution, most people would quite happily not be monogamous.

Why is this so? Why would so many people in committed relationships desire to have sex with other people? The answer is in our bodies and our brains.

“You can take the person out of the Stone Age, not the Stone Age out of the person.”

Monogamy is Not in Our Biology

The human body, both male and female, is designed to support our biological imperative — reproductive success. Because through reproduction, we not only ensure the continuation of our gene pools but the survival of our species. The way the sexes achieve this aim, though, is significantly different.

The sperm that men supply during sex is relatively easy to produce and, therefore, a cheap input into the fertilisation process. Because it is plentiful, men can spread it around many women and, in doing so, work towards building the dominant gene pool. For men, there is no advantage to maintaining monogamy when it is possible to copulate with many females and leave a larger genetic legacy[15]. In this way, the males’ path to reproductive success is through the quantity of copulation. This desire for genetic dominance is hardwired. However, it may be tempered today by the reality of enforced child support payments for proven offspring.

Compared to men’s relatively fleeting contribution to reproduction, women have a much larger investment. They will bear the physical burden of pregnancy for around nine months and the mortal risks of childbirth. If the child is breastfed, the woman will also be responsible for the infant’s nourishment for potentially the first few years of its life, tying her to the child. Motherhood is an immense physical and psychological venture. Therefore each woman needs to make sure they get a good return for their huge investment of time and energy. They want the best offspring possible, which means getting the best sperm. How is this done? Through sperm competition.

You see, women have one adaptation that allows them to game the reproductive system: hidden ovulation. Unlike other animals where the female is obviously in oestrous (on heat), human females have no visible sign. Only the woman, if she observes her mucous throughout the month, can determine when she is most fertile. This gives women immense power to time copulation with several men and creates a competitive environment where she can ensure the strongest sperm will survive. In contrast to men, women’s strategy for reproductive success is quality — creating the circumstances to get the best possible sperm and, thus, the best possible offspring. The woman’s body, too, is well suited to non-monogamy. But women would never be so downright deceitful these days to engage in sperm competition, would they?

The differences in biological imperatives for males and females are outlined in the following table[16].

Where the interests between males and females converge is the child’s survival. This shared goal makes the creation of pair bonds a logical arrangement. For women, the most obvious bond to be formed is with the child’s father, given that his vested interest is more likely to secure the safety and well-being of her offspring. And for the father, it is understandable that he will want to provide for the mother and child to ensure they survive to pass on his genes to the next generation. There is certainly a mutuality in maintaining a relationship between the couple. However, while forming this pair is practical, especially in the children’s early years, this does not mean there needs to be sexual exclusivity during this time. It can be argued that parental responsibilities can still be fulfilled without relationship restrictions. And once the offspring are independent and childbearing activities have ceased, there are also no parental responsibilities that would demand the continuation of monogamy in the pair.

Instead, the behavioural ecologist Geoffrey Parker and evolutionary theoretician, Robert Trivers, have concluded that the best reproductive strategy for humans is a mixed one[17]. They suggest that males and females should create a pair bond for the rearing of offspring but be available for additional copulation if the opportunity arises. This strategy will benefit humanity by ensuring the transmission of the strongest genes and, therefore, creating the healthiest communities.

Our Brains Are Not Wired for Monogamy

One may think that the advanced logical functions of the human brain provide a higher level of morality than other animals. Surely our greater intellect allows us to comprehend the moral failing of infidelity and curb any animalistic urges as they arise. Again, you would need to be corrected. The way our brains are wired actually supports non-monogamy with ease.

1. Old brain — The Old Brain is the section we share with our reptile and primate ancestors. It is located in the brain’s core and is also known as the Limbic System or Animal Brain. Our Old Brain houses the centres that register pleasure and fear. Its key functions are identifying, remembering, and reacting to enjoyable and threatening experiences.

2. New brain — The New Brain makes up the brain’s outer layers. It includes the two large cerebral hemispheres full of dense grey matter, also known as the neocortex. This recent evolution enabled humans to develop language, abstract thought, imagination, and consciousness. The neocortex is where we see an immense ability for neuroplasticity and, therefore, the gift of almost infinite learning capacity.

The only other instinct as strong as reproduction is to secure our survival. Despite having the intelligent New Brain, every piece of information we receive in the brain still first passes through the Old Brain and is tested for how it may enhance or threaten our lives. Everything we see, hear, taste, touch and smell are initially assessed by the Old Brain to determine if the information suggests:

• The possibility of pleasure. If this is the case, it will initiate actions to move towards this stimulus.
• The likelihood of pain. In this case, it will act to move us away from the potential source of threat.

These two basic drivers, pleasure and pain, dictate most, if not all, of our actions.

“Nature has placed humankind under the governance of two sovereign masters, pain and pleasure. It is for them alone to point out what we ought to do, as well as to determine what we shall do.” ~ Jeremy Benthem

Our neurology then explains why, even though we may be in a loving, committed relationship, we turn our heads to follow someone we find attractive. The pleasure centres are activated well before we have time to think and decide to look away lest we spur the partner’s jealousy and wrath. And it is these ingrained animal instincts that lead the vast majority of people to admit they would undertake affairs if they did not have to deal with the pain of getting caught. Avoiding pain keeps people faithful, not a biological imperative or innate need to be monogamous.

It is natural for people to move towards those things that bring them pleasure. Sex feels good and, as a bonus, through reproduction, contributes to the survival of our genes. As stated by Brunning:

“Human behaviour is informed by evolutionary biology and is understood in terms of what best serves the interests of our genes.”

In addition to our primal pleasure and pain responses, it has been proven that three separate systems in our brains guide our mating behaviours and intimate relationships [20]. These are the systems of:

1. Sex drive. Creates the impetus to search for a sexual partner.
2. Romantic love. Draws a person in to focus on a potential sexual partner and creates the emotional conditions conducive to sex.
3. Deep attachment. Keeps people together long enough to raise a child.

These systems are not wired together, meaning they can act independently and interact in various combinations. One system does not depend on another. For example, a deep attachment can exist for one person, while feelings of lust or romance can be felt for another person simultaneously. The sex drive system can also operate either in combination with or independently from feelings of romantic love or deep attachment, creating the desire for additional sexual partners beyond these emotional connections. You can hold in your mind a great love and care for your partner but still flirt with and potentially follow through with the stranger smiling at you from the other side of the bar.

The independence of these three neural systems indicates that our brains are wired to support non-monogamy. They allow us to compartmentalise social interactions and not require deep-seated emotional preconditions for physical pleasure and procreation. They allow us to function in socially secure partnerships while also undertaking extramarital affairs. As stated by Helen Fisher:

“our brain architecture easily accommodates infidelity.”

This finding is revolutionary, as it dissolves the notion that if someone cheats on you, they don’t love you. They can still very much hold you dear, but their system of sex drive is operating independently of this love.

Monogamy Is A Social Ideal

Our physical makeup and neural networks are not built to support monogamy. In fact, our bodies and brains largely encourage and support sex outside of a pair bond.

So, if monogamy is not ingrained in human biology, it must be purely a social construct. The conclusion can be drawn then that monogamy is a cultural creation and predominantly a social ideal [21], and is not a natural relationship arrangement for humans.

That monogamy is not natural for humans was recognised way back in the 1930s, when Robert Briffault, in his book Sin and Sex [22], stated that:

“For a male and female to live continuously together is biologically speaking, an extremely unnatural condition.”

The consequences of monogamy not being natural are serious for those cultures who choose to make it a moral standard. In these societies, for monogamy to occur, enforcement is required.

“The persistent and explicit prohibitions against adultery in Western traditions confirm the biological argument that strict monogamy is not automatic. It needs to be enforced and reinforced. Otherwise adultery happens.”[23]

As soon as we begin discussing the need for enforcement and reinforcement of behaviour, we also introduce concepts of struggle, conformance, rebellion and prosecution. The less natural our behaviours are, the more difficult they will be to maintain. And the further we move away from a natural state, the more tension will arise, and the more fuel is created for conflict. Monogamy then begins to feel less like the happy-ever-after scenarios from the Disney Princess movies we watched as a child and much more like the Shawshank Redemption.

If we want to build relationships based on love and compassion, then we first need to begin with kindness towards ourselves[24]. Understanding our natural tendencies and how monogamy differs from these can be a starting point. However, just because monogamy may not be in tune with your biological imperatives, if it is in line with your values, then you need to understand and prepare for the challenges that may arise. Knowing that monogamy is not natural will allow us greater insight into difficulties that may occur in an exclusive partnership arrangement and provide the potential for caring responses. Far from condoning harmful behaviour, this wisdom can turn shameful weakness into valid vulnerability and open the doors to critical and constructive conversations. And far from destroying relationships, this information can strengthen understanding and empathy and empower lifelong love.

[1] Natural. (2022). The Merriam-Webster.Com Dictionary. https://www.merriam-webster.com/dictionary/natural

[2] We’ve Been Designed to Reproduce | Psychology Today

[3] Sharpe, R. M. (2018). Programmed for sex: Nutrition–reproduction relationships from an inter-generational perspective. Reproduction, 155(3), S1–S16. https://doi.org/10.1530/rep-17-0537

[4] William McDougall: An Introduction to Social Psychology: Chapter 10: The Reproductive and the Parental Instincts. (n.d.). https://brocku.ca/MeadProject/McDougall/1926/1926_10.html

[5] Mock and Fujioka. 1990. Monogamy and long-term pair bonding in vertebrates. Trends in Ecology and Evolution 5(2): 39–43

[6] Fisher, H. (2017). Anatomy of Love: A Natural History of Mating, Marriage, and Why We Stray (Completely Revised and Updated with a New Introduction ed.). W. W. Norton & Company.

[7] Barash, D. P., & Lipton, J. E. (2002). The Myth of Monogamy: Fidelity and Infidelity in Animals and People. Holt Paperbacks.

[8] The research of Walter Scheidel, as quoted in Brunning, L. (2020). Does Monogamy Work?: A Primer for the 21st Century (The Big Idea Series) (1st ed.). Thames & Hudson

[9]File: Status of polygamy worldwide.png — Wikimedia Commons. (n.d.). https://commons.m.wikimedia.org/wiki/File:Status_of_polygamy_worldwide.png

[10] Toyofa and Spitzberg. 2007. The dark side of infidelity: Its nature, prevalence, and communicative functions.

[11] Betzig 1989. Causes of conjugal dissolution. Current Anthropology 30:654–76

[12] Marín, R. A., Christensen, A., & Atkins, D. C. (2014). Infidelity and behavioral couple therapy: Relationship outcomes over 5 years following therapy. Couple and Family Psychology: Research and Practice, 3(1), 1–12. https://doi.org/10.1037/cfp0000012

[13] S. (2021, March 11). Infidelity: The Cold Hard Truth About Cheating. LA Intelligence. https://laintelligence.com/infidelity-the-cold-hard-truth-about-cheating/

[14] How Hardwired Is Human Behavior? (2022, February 3). Harvard Business Review. https://hbr.org/1998/07/how-hardwired-is-human-behavior

[15] Fisher, H. (2017). Anatomy of Love: A Natural History of Mating, Marriage, and Why We Stray (Completely Revised and Updated with a New Introduction ed.). W. W. Norton & Company.

[16] Based on the discussions found in Social Basis of Human Sexual Behavior. (n.d.). https://public.wsu.edu/%7Etaflinge/socsex.html

[17] Ref — Male mixed reproductive strategies in biparental species: American naturalist 2005

[18] Author Removed At Request Of Original Publisher. (2015, October 26). 3.2 Our Brains Control Our Thoughts, Feelings, and Behavior — Introduction to Psychology. Pressbooks. https://open.lib.umn.edu/intropsyc/chapter/3-2-our-brains-control-our-thoughts-feelings-and-behavior/

[19] Brunning, L. (2020). Does Monogamy Work?: A Primer for the 21st Century (The Big Idea Series) (1st ed.). Thames & Hudson

[20] Fisher, H. (2017). Anatomy of Love: A Natural History of Mating, Marriage, and Why We Stray (Completely Revised and Updated with a New Introduction ed.). W. W. Norton & Company.

[21] Brunning, L. (2020). Does Monogamy Work?: A Primer for the 21st Century (The Big Idea Series) (1st ed.). Thames & Hudson

[22] Robert, B. (2020). Sin and Sex (Routledge Revivals) (1st ed.). Routledge.

[23] Barash, D. P., & Lipton, J. E. (2002). The Myth of Monogamy: Fidelity and Infidelity in Animals and People. Holt Paperbacks.

[24] Based on the teachings of Pema Chodron

Originally published at https://moraldilemmaofmonogamy.substack.com on January 12, 2023.