Til Death Do Us Part: The Evolution Of Monogamy

At a young age, a familiar playground song teaches us the supposed trajectory of adult romantic relationships: “First comes love, then comes marriage, then comes baby in a baby carriage.” It leads us to believe we’ll live happily ever after in blissful monogamy.

But since half of marriages end in divorce and as many as one-third end because of infidelity, we might ask ourselves, is monogamy really a “one-size-fits-all” approach for relationships? As humans, are we naturally monogamous or do we fall somewhere in between?

The Animal Kingdom

First off, it’s important to distinguish between sexual and social monogamy. To be sexually monogamous is to be mutually exclusive with one sex partner. To be socially monogamous is to depend on a partner for shelter, food, and finances, and to engage in “flings” outside of the sexual relationship.

In the animal kingdom, only 3 to 5 percent of the 5,000 mammal species (including humans) are known to form sexually monogamous bonds with partners. Evolutionarily speaking, it's advantageous for men to copulate with several partners in order to “spread” their genes. Meanwhile, it is evolutionarily advantageous for women to focus on a single partner who will likely stick around to help her raise her children.

R. Robin Baker and Mark A. Bellis, authors of the book Human Sperm Competition, suggest natural selection favors promiscuity, because the more promiscuous someone is, the more likely their genes are to survive and reproduce. Although this goes against social norms of monogamy and fidelity, those who are promiscuous and cheat on their partners do increase their chances of passing on their genes.

Now, before you say you're "spreading your seed" as an excuse to cheat on your spouse, let’s take a look at the biological basis of monogamy. Is there something embedded in us that affects our propensity to cheat?

Cheating: Is It In Your Genes?

Animal studies have long helped us explore how hormones affect humans. One such study published in the journal Nature compared the effects of the hormones vasopressin and oxytocin in two types of rodents: prairie and montane voles. (Scientists have used prairie voles, which are sexually monogamous, as a model for human monogamy.) They found when vasopressin, a hormone that impacts trust and empathy levels, became active in prairie voles, it triggered a tighter bond with their mate.

The opposite was true for montane voles, the promiscuous cousin. Vasporessing receptors are located in distinct areas of the brain in both types of vole; in the prairie voles, it's in the nucleus accumbens, the reward center, while in montane voles it's in the amygdala, a region associated with fear and anxiety. For these reasons, the researchers found vasopressin stimulation had no effect on sexual monogamy in montane voles — they had various partners.

Another 2015 study published in the journal Evolution and Human Behavior actually analyzed the relationships between vasopressin, oxytocin, and infidelity in humans. Vasopressin is believed to play an important role in bonding, social behavior, and sexual motivation, with a brief half-life of just 16 to 24 minutes.

After collecting saliva samples and sequencing genomes, the researchers found there was a link between specific mutations of a receptor gene for the hormone and infidelity in women, but not men. Nothing, on the other hand, connected the oxytocin genes to the sexual behavior of either sex.

The differences in humans who practice monogamy and non-monogamy may be seen in how their brains respond to reward. Previous studies suggest a "cheater’s high" motivates some people to cheat. Researchers found people experience a boost in dopamine after getting away with unethical behavior, such as cheating on a test or faking a disability. This is because when oxytocin and vasopressin are released, dopamine trickles into the brain’s pleasure center, motivating a person to make a move.

“Since the primary motivation for engaging in sex has long been pleasure, it reasons that the same mechanisms that attract some people to pleasures in general, might also attract them to broader sexual practices,” Dr. Nicole Prause, sexual psychophysiologist and neuroscientist, told Medical Daily in an email.

Gender Differences In Cheating

Brain chemistry and genetics provide the reason for why some people are more susceptible to cheating than others, but what role does gender play? Research has shown that almost 60 percent of men and over 45 percent of women will cheat at some point in their marriages. Dr. Fran Walfish, Beverly Hills psychotherapist, author of The Self-Aware Parent, and expert panelist on WE TV’s Sex Box, believes it's a lot less challenging for women to be monogamous than it is for men because of the way they perceive sex.

“Men are more practical by nature and women are more emotional and when it comes to sex. Men experience foreplay through their genitals, and women experience foreplay through their hearts and emotions. So when a woman bonds and attaches with a man, she gives her heart and soul and can remain monogamous easier than a guy who has feelings, but it's all about orgasm and the mechanics of sex,” she told Medical Daily .

Moreover, some men cheat because of an early wound, or from trauma caused by abandonment or severe rejection in their early life. Being the attachment figures they are, parents who die or get divorced and disappear could make the person "under stress and anxiety long to be close to a woman and feel loved by important love objects,” Walfish said. “It's a much deeper thing than a conscious choice. It is a profound fear on a deeper level.”

Meanwhile, the thing that drives women to cheat is more of a feeling of neglect or deprivation in their relationship with their man. In this type of relationship, no one talks about the issue with each other, Walfish says. Or maybe one partner will try to bring it up, but the other typically shuts it down with denial or a false explanation.

Monogamy: Structure vs. Natural State

Studies have viewed social and sexual monogamy in humans as more of a social construct than a natural state. One reason why monogamy has been instilled in our species, according to a 2013 study, is because men stayed with one woman to ensure their young were not killed by other men. This would guarantee their children survived to reproduce and carry on their genetic line. In addition, the men would help raise the children so that the mother could reproduce again sooner.

Contrastingly, a 2013 study suggests monogamy works for the purpose of location and supply. Monogamy tends to occur in areas where there is a low female population and where men cannot battle other men for more than one woman because they’re too spread out — this distance would make it hard for men to determine whether the child a woman is carrying is theirs. Researchers suggests women also tend to be solitary and intolerant of other women because their nutritional needs are greater. Therefore, they ward off competitors for food resources, which also helps them stay monogamous.

The status of our relationship is based on what works best for us. Some of us need labels, while others don’t. Some of us will stay while others will stray. And some of us will maintain healthy, purely sexual relations with others while still maintaining deeper emotional connections with our significant others. On the other hand, as relationship expert April Masini says, “monogamy is still something that is a very popular practice — even when it’s serial monogamy."