How Polygamy Shaped Who We Are

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First, let’s get one thing straight. My wife and I have been married for over 21 years, and I love her deeply. This post is not about me! There, I’ve said it — for the record.

That said, as an evolutionist who is intrigued by the interface of evolution and human behavior, I’m extremely interested in how features of our ancestral worlds have shaped who we are and how we behave today.

A powerful and crisp treatise on the nature of polygamy is found in my fellow Psychology Today blogger David Barash's book Out of Eden: The Surprising Consequences of Polygamy (with polygamy defined as some mating system in which an individual has more than one partner — such as a man having three wives).

A while ago, my graduate student, Olivia Jewell, and I were fortunate to be asked to provide a review of this book for Quarterly Review of Biology. Let’s say that it was pretty eye-opening.

Completing the Jigsaw Puzzle of Human Behavior

One of the things that I love about the evolutionary perspective in psychology is that it’s like putting together a jigsaw puzzle. In fact, I’d say that most of psychology is like putting together a puzzle. What makes evolutionary psychology different is that it’s like being able to look at the completed picture on the box of the puzzle to help guide you. That is, the evolutionary perspective provides a broad framework for helping behavioral scientists ask and answer questions about any feature of human behavior (see Geher, 2014).

The evolutionary perspective achieves this by having researchers think about such big questions as (a) how this behavior might have led to increased survival and/or reproductive success among our pre-agrarian ancestors and (b) how modern environmental conditions are different from the kinds of conditions that would have been typical under ancestral times. By considering these questions as a backdrop to any psychological research study, evolutionists are always able to think about the big picture while they are on the hunt to find answers to specific questions. Again, like putting together a jigsaw puzzle with the completed picture right in front of you so that you know what you are looking for.

Polygamy and Ancestral Humans

In examining pre-Westernized societies that exist in various parts of the globe now, along with evidence regarding the mating patterns of pre-agricultural humans, Barash paints a picture of polygyny (the most common, specific form of polygamy in humans, when a male has more than one female partner) — from mild polygyny on one hand to an all-out harem lifestyle on the other — as being extremely common in human evolutionary history. In fact, institutionalized monogamy, which largely follows from modern monotheistic religions in connection with urbanization, seems to have been very much the exception rather than the rule during the lion’s share of human evolutionary history.

Thus, as an expert evolutionary biologist, Barash asks about the adaptive nature and evolutionary history of polygamy. He then uses this evolution-based framework to help us understand various aspects of the modern human condition, as well as how these aspects have been importantly shaped by our long history of polygamous mating systems.

Three Surprising Consequences of Polygamy

It turns out that dozens of features of modern humans owe something to our history of polygamy. (If you want the full tea on this one, then you’ll have to read Barash’s book.)

For the purposes of this post, below are three specific consequences of being human that follow from our evolved history of polygamy (focusing on polygyny in particular).

1. Sexual dimorphism: Men are larger than women.

Sexual dimorphism in size simply means that in some particular species, members of one sex are consistently larger than are members of the other sex. In evolutionary biology, there is a pretty standard rule when it comes to sexual dimorphism as it relates to polygamy. As mating becomes more competitive, as is found if only a few males in a species do most of the mating (as is the case with elephant seals off the California coast), members of the sex who are doing the competing tend to be physically larger than members of the other sex. This is because large size is advantageous if there are going to be physical altercations in an effort to obtain a mate.

In elephant seals, a very small number of males do the bulk of the mating. And, not surprisingly, males are very aggressive toward one another — and males are much larger than females are. About four times larger, in fact. That is thousands of pounds!

Across sexually reproducing species, we can make inferences about how polygamous the ancestors of the species were during the evolution of that species by examining sexual dimorphism in size. In humans, males are larger than females in both overall size and muscle mass (by ratios of 1.20 and 1.53, respectively). There’s a clue for your jigsaw puzzle!

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2. Sex differences in preference for a variety of sexual partners.

If humans had a long history of polygamy during human evolution, and if males were more likely to have multiple partners than were women (i.e., if polygyny existed), then it would make sense that males, compared with females, would have a sexual psychology that fits this part of our ancestral ecology. For instance, we would expect males to be more interested in having sexual relationships with a variety of sex partners compared with females. And one of the largest cross-cultural studies in the area of sexual psychology found exactly this. Across the globe, with virtually no exceptions, males are more likely than are females to desire a relatively large number of sexual partners across one’s lifetime (see Schmitt et al., 2003).

3. Males die young.

I remember once when I was a kid asking my father why men die at younger ages compared with women. He said that it’s because men have to work harder all their lives. Actually, with all due respect to my dad, that’s a terrible answer! And this is not the answer that experts in the field have arrived at based on mountains of research. Sorry, Dad!

In fact, males die young relative to females. But they also die at higher rates across the entire human lifespan. In a major evolution-based treatise on sex differences in mortality across the lifespan, Kruger and Nesse (2006) found that, across the globe, males are more likely than are females to die at any stage of life. And this trend is exacerbated during the prime courtship years (from about the ages of 15-25).

How does this all relate to polygamy? Think about it this way. Males are more likely to die prematurely than are females at this key life stage, because they are often in intense competition with other males. And intense male-male competition during the courtship stage of life is exactly what would be found in a polygynous species in which only a subset of males successfully find female mates. If humans were purely and fully monogamous, there would be no need for such intense competition, because there would be no chance of a male getting shut out of mating. In fact, during these courtship years of life, males are about four times as likely to die, from a broad array of causes, compared with females. And this dark fact of the human condition owes largely to a deep history of polygamy in our species.

Bottom Line

Why are men, on average, larger than women are? Why do men seek more variety in sexual partners relative to women? Why do men die at higher rates than women do — across the entire lifespan? Thanks to evolution-based thinking, we have a powerful answer: Humans evolved with a deep history of polygamy.