Monogamy in humans: Why is it so common?


There are some who believe that monogamy is natural to humans and others who believe that humans are naturally polygynous. For a detailed discussion on monogamy vs. polygamy, see article Monogamy vs. Polygamy: What is natural for humans?

Regardless of which of the two camps is right, it’s hard to deny that monogamy is prevalent in humans. This article explores the reasons why monogamy is so common in humans.

The end goal of sexual reproduction is to successfully pass on one’s genes to the next generation. To motivate us to achieve this goal, we’re wired to find certain members of the opposite sex attractive and form relationships with them.
A typical relationship can be defined as a long-term or an expected long-term bond between individuals each of whom will ultimately contribute half of their genetic code to produce an offspring and raise it.

The long-term mating strategy (monogamy), as opposed to the short-term mating strategy, is the most preferred mating strategy worldwide. Since marriage is found in nearly all the cultures, even in those cut off from the modern world, it can’t be merely a social construct.

The desire to seek a long-term partner and raise offspring with them is wired into the human psyche. But why?

The evolution of monogamy

Our early ancestors probably engaged in unconditional short-term mating. They basically banged whenever they could. They had to. Survival was extremely hard thanks to predators and diseases. Only by reproducing indiscriminately and as often as possible could they increase the chances of successfully passing on their genes.

But about 2 million years ago, something changed. Our early ancestors became bipedal i.e. they stood up and walked. The female pelvis grew narrower but the human brain grew significantly in size as we freed our hands, made tools, started fires, and developed language.

So for a baby’s big head to pass through the narrow birth canal, the baby had to be born well before the brain could develop fully. This evolutionary trade-off resulted in a highly vulnerable and helpless baby with prolonged childhood, wholly reliant on its parents.

It was probably at this stage in our evolutionary history that monogamy became the dominant mating strategy. It facilitated adequate support, protection, and care that a helpless baby requires from both its parents.

monogamy in humans
Our ancestors were always on the move so newborn infants had to grasp their caregiver’s bodies tightly to avoid falling off. A baby’s grasp can be strong enough to support its own weight!

Women pursuing monogamy

Women pursuing monogamy makes all the sense in the world. Since they can produce a limited number of offspring with many time constraints and huge costs, they can best ensure their reproductive success by raising their few offspring well.

Although they can benefit from short-term mating, the costs are pretty huge.

Men pursuing monogamy

Men pursuing monogamy is an interesting phenomenon. They have a lot to gain from short-term mating and very little to lose by pursuing the short-term mating strategy.

The fact that many men pursue long-term mating tells us that the benefits of monogamy can outweigh the benefits of short-term mating.

Men are wired to pursue short-term mating so that they can father as many children as they can. This equation of reproductive success for men changes when you factor in vulnerable babies with big heads and prolonged childhoods.

It’s no longer about quantity but about quality of offspring. By providing adequate care to his offspring, a man can ensure that it survives and successfully reproduces later in life.

There’s little reproductive success in fathering dozens of offspring who all die or fail to reproduce because they didn’t receive adequate care.

Pulling off the anchor

There are psychological programs for short-term mating and long-term mating strategies in both men and women. What strategy they choose will depend on the cost/benefit calculations they carry out in their heads in a specific context.

Men are pulled more strongly toward the short-term mating strategy because they can benefit from it reproductively. The need for significant parental investment and the additional care that a human baby requires anchors them to long-term mating.

If we pulled off that anchor, what do you think would happen?

What if parental investment and support of a baby are taken care of by something else like government funds, childcare policies or even well-to-do mothers?

In such a situation, men are likely to revert to pursuing short-term mating or serial monogamy because each offspring they father will likely be cared for. Now, in order to maximize their reproductive success, quantity again becomes important.

In developed countries with good childcare support, divorce rates are high and men start new families every now and then. This has resulted in the rise of single moms.

In the developing countries that are poor and barely supporting their own economy, let alone supporting children, ‘single mom’ is an alien concept or a highly stigmatized one. Single dads do exist but they’re far less common worldwide because that kind of a strategy is not very reproductively beneficial for men.