Polygyny: the culture we dislike might not be the culture that is evolutionarily disfavored

There’s a new paper out in the Proceedings of the National Academy of Sciences by David W. Lawson and colleagues that looks at whether the cultural practice of polygyny is disadvantageous. It’s a question that should be fascinating to anyone who is interested in sexual conflict or cultural evolution. At first glance, polygyny appears to be a classic case of male reproductive agendas trumping what’s good for females and their offspring, a phenomenon that is well-documented in other animals. Although it is not inevitable that male and female reproductive interests will fail to align — and therefore conflict — it’s a possibility that needs to be entertained in human populations because sexual conflict is relatively common in nature. And if polygyny was good for the reproductive fitness of males and not for females, that would reinforce a common intellectual narrative applied in Western societies: that patriarchy puts the needs of men ahead of the needs of women (and perhaps their children).

Polygyny is a cultural practice, and for this reason it is interesting to consider how it relates to our cultural evolution. There’s a lot of evidence that polygyny was practiced by at least some of our ancestors, but today most developed countries ban the practice. That monogamy is legislated so prevalently is also culturally interesting. Why does there appear to be a move from polygyny to monogamy in larger-scale societies, and does this change reflect any advantage to the cultural practice of monogamy? Put more bluntly, is monogamy culturally outcompeting polygyny because it creates better overall outcomes for societies?

These are the core questions taken on by this scientific paper. I was not familiar with this literature before reading this paper, but based on their summary of previous work the anthropological consensus seemed to be that polygyny increases male reproductive success at the expense of women and children. These conclusions were based on science… in a sense. To test the hypothesis that polygyny was bad for women and children, researchers compared polygynous households with monogamous households in terms of various health and welfare metrics; given that the prevailing view of polygyny was as a male-imposed social-structuring system, it was not surprising that polygynous households showed higher rates of illness and increased risk of childhood mortality.

And maybe we should be a little more careful when we are not surprised, because this case shows how we can be tricked — or just trick ourselves — with data that appear to scientifically answer a clearly-asked question.

To understand why these initial data were deceiving, it helps to look more closely at the data that was collected and how it was analyzed. It also helps to be an ecologist, because ecological thinking really helps to understand the nature of this data. It is not disputed that polygynous households have lower overall health metrics. What Lawson and colleagues show is that this comparison does not actually answer the question is polygyny good for men but bad for women and children?

To actually answer this question, you need to make sure that you are comparing polygynous and monogamous households experiencing the same environmental background. That is to say you cannot compare people living under different ecological conditions. And that is exactly what previous studies appear to have done: compared monogamous households that experienced better overall ecological conditions than polygynous households. In other words, environment was a confounding factor that was not accounted for by early studies. What Lawson and colleagues show is that when you take ecological background out of the comparison, male-headed polygynous households actually have better health and wealth outcomes than similarly-situated monogamous households as well as multi-female households headed by a woman. Polygyny does not appear to be the product of sexual conflict. In fact, it may represent an adaptive form of cooperation in the face of ecologically-challenging conditions.

There are a couple of things about this paper that I find fascinating. As I suggested in a tweet about the paper:

Fascinating how science can uncover cultural biases as well as illuminate evolution’s bias toward adaptive culture https://t.co/JQ7XQ2XRaz

— Christopher Jensen (@cxjjensen) October 27, 2015

What’s clear in this situation is that polygyny can be — under particular conditions — an adaptive cultural practice. That people practice polygyny is likely to be an indication that they are facing tough environment conditions, and simulation studies have suggested that harsher environments incentivize cooperation (Andras et al. 2007). If multiple marriage involves some sacrifice (sharing the contributions of a single male spouse) but leads to better outcomes for all (because multiple females choose a male who is better-equipped to compete for resources in harsh environments), that sounds like a pretty classic case of cooperation. If so, the right cultural rules about when and where to be polygynous would be advantageous regardless of which sex employed them. We need to recognize that just like biological traits, whether a cultural trait is adaptive really depends on what environment that cultural trait faces.

The other thing that I find fascinating about this paper is how it really unmasks the kinds of intellectual bias that can be involved in anthropological research. Most anthropologists are from monogamous cultures, and so inevitably carry a bias towards the idea that monogamy is a more adaptive cultural practice than polygyny. This bias is reinforced by a couple of extra boosts to the monogamous cause: the assumption that Western industrialized cultures are more successful than those of developing nations, and a feminist critique of patriarchy. Ironically it is often social scientists with a feminist bent who criticize science — rightly so in many cases — for being culturally biased against hypotheses that empower women. In this case, high-quality science reveals what is actually going on, which if we value women’s outcomes might shift how we act in relation to polygynous societies. We all need to be mindful of how our cultural practices can influence the way we practice science.

The post above was based on Proceedings of the National Academy of Sciences “No evidence that polygynous marriage is a harmful cultural practice in northern Tanzania” (Lawson et al. 2015)