Why Do We Cheat?: Infidelity From the Evolutionary Perspective


Love, sex, and cheat are some of the strongest driving forces in life. They have served as inspiration for some of the finest artworks ever created, modified the course of history, provided us with endless hours of gossip and entertainment, and, most recently, opened a window to the study of the human mind.

The human psyche and human relationships are extremely daedal. So, it is not a simple task to conclude why people cheat on their partners. Yet, this article will try to shed light on this aspect of human behaviour with the help of evolutionary psychology.

Banana and strawberries pretending to cheat. The banana shows lipstick kisses and a strawberry saying 'I though I was special!'

Are we built to be monogamous?

When we look around, monogamy is a rarity. In the animal kingdom, matting with a single partner for life is not widespread. And when it comes down to our closest relatives —chimps and bonobos— it is virtually unheard of. So, are we humans an exception?

It is difficult to tell as today’s people are the result of millennia of evolution and adaptation. Regardless, anatomy and physiology have something to say regarding our nature.

The truth is we present an array of traits suggesting our species, like our ape relatives, have evolved to be non-monogamous. For instance, in promiscuous species, the males tend to be 15-25% larger than the females. Copulatory vocalization is another enthralling trait common to polygamous species.

Comparison of male and female bonobos suggesting physique is related to cheat behavior.
Sexual dimorphism is a common characteristic of many polygynous mammals. Source: National Geographic. 

Withal, the masculine reproductive system holds the key to this affirmation. Its anatomy and physiology allow it to engage in what is known as ‘sperm competition’, a clear polygamy sign. An excellent example is the shape of the penis, capable of producing a ‘vacuum’ effect inside the female channel, removing possible sperm traces from other men. Likewise, human sperm contains about 40% of kamikaze sperm, which are aggressive swimmers hampering foreign sperm’s way to the egg.

It is in our genes

Research from the University of Queensland avers our demeanour shapes our DNA and descendants. That way, cheating could be sourced to our genes as a genetic behavioural component.

Moreover, scientists believe the desire to cheat could be connected to a dopamine receptor related to alcoholism and other addictions; the DRD4 receptor. In fact, a study from Binghamton University found that those presenting a variation in the abovementioned receptor were more likely to cheat, as they required higher stimuli to process the pleasure hormone: dopamine.

But if monogamy is a social construct and not a natural aspect of humans, why do we pair up? And why do we experience such a gutted visceral feeling when someone betrays us?

Cheating is natural, and monogamy is too

While monogamy does not seem to be part of our biology, it is preached in most cultures across the world. As a result, polygamy is spurned, making the single-partner association the most common and socially desirable model. Such is the influence of this standard that most individuals deem polygamy as an absence of love.

Nonetheless, sex drive, romantic love and attachment are feelings we have developed as part of our reproductive strategy, and they aren’t always connected.

The anthropologist and biologist Helen Fisher asserts that during the last 300,000 years (the period modern humans have existed), we have refined a dual reproductive strategy, explaining why both monogamy and infidelity are natural within us.

Reproduction-wise cheating presents some advantages. Monogamy does so. Yet, due to a clear difference in parental investment, whatever the benefits are, they will vary between sexes.

The advantages of polygamy and cheating

Undoubtedly, diversifying the number of individuals we mate with boosts our reproductive success chances. Having extra partners to reproduce with increases the amount of DNA one person can spread.

Seven fluffy dog puppies.
The higher our number of descendants is, and the more variated they are, the bigger the chances of perpetuating our lineage.

Due to the low parental investment males have to withstand, it is pretty easy to understand what benefits they take. But, what are the benefits for females?

Studies show that polyandry provides adequate sperm provisions, as a single ejaculate is not sufficient considering egg production. Likewise, it provides fitness benefits and, most importantly, several resource sources to guarantee the survival of the progeny.

In the case of chimpanzees, polyandry is a tool to prevent offspring infanticide; when several males do not know whether the progeny is theirs or not, they tend to provide protection. Something similar could have happened with the hunter-gatherer humans.

Why polygamous-anatomic beings choose monogamy

About 12,000 years ago, we humans stopped being hunter-gathers to become farmers. This recent change altogether modified the paradigm; in the brink of an eye, humans had property and began to call dibs on it. New rules arose, and humans did what they do best: adapt.

Some anthropologists reckon that’s the cornerstone, the moment when we might have included monogamy as a reproductive strategy and the point when we integrated jealousy as an adaptation for it.

In this new system, infidelity poses a significant threat to both the reproductive success of men and women. In the case of men, the greater risk is now spending —and even depleting— resources on someone else’s spawn, making sexual affairs the most significant menace. However, when it comes to women, emotional infidelity is the new decisive hazard as it means relocating their children’s resources to a different person.

In short, monogamy allowed humans in a new context to ensure there is an offset to the investment reproducing entails. Its relevance turned out to be primal, to the point it translated into a legal agreement; marriage. A contract bonding two people together to make alliances, peace treaties, business, or increase their labour force.

Two golden engagement rings on top of a wedding set.
Western cultures conceive marriage as a display of monogamic love, yet feelings were not always at its core.

Who cheats more? Men or women?

Traditionally, society has considered men to be the randy sex. For decades, biology studies have supported them in getting away with the ‘spreading the seed’ argument when committing infidelity. But as we have seen, the two sexes are biologically programmed to seek multiple partners.

The stereotyped idea that one sex gets a higher pay-off from unfaithfulness lays its foundations in the Victorian era when evolutionary biology became an area of study. As predictable, the patriarchal society of the time failed to consider women as sexual beings with libido.

Nowadays, taboos still hamper research in this area. Regardless, the latest studies are overturning this idea deeply-rooted in our minds. We are all designed to be titillated by erotic novelty and, surprisingly, women are more sensitive to the absence of it.

The anthropologist, Wednesday Martin, shows that females are more likely to cheat due to how sex is built in their brains. Men have more robust neurological pathways for it and thus respond better to spontaneous desire. That’s not the case for women, who are more sensitive to triggered desire (the arousal a given stimulus provokes), causing an easier loss of interest in their partners and a quicker plunge in their libido than in the case of men.

So, contrary to popular belief, women are more prone to cheat than men. Even so, they fall behind in statistical studies of infidelity, showing that a predisposition to something does not directly translate to an act.

The human mind is mysterious and complex, just like our ways are. Indeed, we can state there is no specific reason why people stray out of their relationships.

Yet evolutionary studies give us some clarity in this area; there is a battle between the type of animal we are and the society we have built.

We constantly face ambivalent impulses and make decisions based on them. Sexual exclusivity provides benefits but also sabotages a part of ourselves. Sex drive is an impulse, love a feeling. However, monogamy is a rule we invented to adapt to a new context; consequently, we have to work for it.

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