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Are humans monogamous or polygamous by nature?

There have been endless debates on the topic of whether humans are monogamous or polygamous by nature. There are sound arguments for both polygamy and monogamy with regards to human mating so the answer probably lies somewhere in between.

As is true for many other phenomena, people are keen on obtaining clear-cut answers even if there may be none. This leads them to create false dichotomies and falling prey to the either-or bias, i.e. ‘either this exists or that, there’s no grey area’. While such clear-cut dichotomies may exist in some phenomena, this way of thinking helps little in the pursuit of understanding human behavior in general and human mating in particular.


Polygamy

When we look at nature, a good way to predict whether or not a species is polygamous is to look at the physical differences between the two sexes. Polygamy mostly appears in nature in the form of polygyny and polyandry is relatively rare. In general, the larger the males are compared to the females, the more likely it is that the species is polygamous. This is because many males of the species, in competition to get females, evolve to become larger so as to fend off other males.

Therefore, if the physical differences between the sexes are large, the species is likely to be polygynous and vice versa. For example, in elephant seals which are polygynous, a dominant male may keep a harem of about 40 females. Similarly, an alpha gorilla gets to mate with most females, which is why gorillas tend to be so huge and formidable. 

In humans, there are obvious general physical differences between males and females in terms of body size, strength, and height. But these differences are not as stark as in elephant seals and gorillas. Hence, humans can be said to be moderately polygynous.

Another evidence for the polygamous nature of humans comes from testis size. The more intense the competition in a species among males to acquire the females, the more likely it is that the species will be polygamous. This is because intense competition produces few victors and a large number of losers. When males of a species cannot compete with other males with formidable strength and size, they may do so with their sperm. 

For instance, chimpanzees may not be as large as gorillas but their testes are large, enabling them to produce large quantities of sperm that may supersede the sperm of a competitor in the female reproductive tract. Needless to say, chimpanzees are polygamous. The lesser the competition among males for females, the smaller will be the testis size because there is little or no sperm competition. Human males have average-sized testis compared to other mammals and hence, are moderately polygamous.

Historical records also point toward polygyny being the dominant form of human mating. Kings, rulers, despots, and monarchs have repeatedly kept large harems of women not unlike what elephant seals and gorillas do.




Monogamy

Monogamy is widespread in modern humans which is rare for not only primates but also mammals. As David Barash points out in his book Out of Eden, only 9% of mammals and 29% of primates are monogamous. The most important concept closely tied to monogamy is parental investment. Polygynous males invest little or nothing in their offspring but males that form monogamous pair-bonds invest a lot of resources in their offspring. 

Also, in polygynous societies males have no incentive to invest in the offspring because they have no way of knowing that the offspring is theirs. When males and females form a monogamous relationship, the male is likely to invest because there’s a greater probability that the offspring is his own. In other words, there is greater paternity certainty.

Another likely reason why monogamy evolved in humans is how human offspring are virtually helpless after being born (see Why is the long-term mating strategy (monogamy) so prevalent). In such a scenario, it isn’t advantageous for a male to invest effort, time and energy into securing a mate, reproducing, and letting any offspring produced die at the hands of other males or because of a lack of resources. Therefore, by raising the offspring with a female- at least until the offspring can grow up and take care of itself- a male benefits reproductively.  

Many male mammals have hardened spikes on their penises that supposedly enhance sensation and reduce their delay to climax. This is consistent with their polygamous and short-term matings. Since human males lost this feature millions of years ago, it is argued that longer lasting sex promoted more monogamous and intimate relationships.

Generally monogamous, moderately polygamous

Modern humans can be described as being generally monogamous and moderately polygamous. Nesting birds whose degree of parental investment matches that of humans also show a similar trend in their mating behaviors.1 So humans aren’t either monogamous or polygamous. They exhibit a whole spectrum of mating behaviors ranging from pure monogamy to polygamy. This strategic pluralism of human mating behavior allows them to choose the optimal strategy in a given set of circumstances.2 

Throughout our evolutionary history, monogamy and polygamy may have switched places as being the dominant human mating strategy number of times. Australopithecine males, for instance, that lived millions of years ago were 50% heavier than females.3 While this may seem to indicate a trend toward monogamy in human evolution, monogamy is not a recent phenomenon imposed after Western Imperialism. Rather monogamy has been a striking feature of human sexuality for 3 million years now.4 Again, what strategy becomes dominant depends on prevailing conditions and this is best exemplified by a shift toward polygamy that occurred after the agricultural revolution.

The agricultural revolution meant that humans clustered near fertile lands and started accumulating resources. This created conditions for polygyny as some men accumulated more resources than others. When we read about kings with multiple wives, this is the era that is described. However, toward the end of this era, a shift occurred toward monogamy again resembling how humans mated in pre-agricultural revolution times. This despite the fact that variability in resources-acquisition has exponentially increased since the Industrial Revolution. There are a couple of plausible explanations for this.

First, the clustering of humans in small areas increased the chances of infidelity and sexually transmitted diseases.5 Social regulation of human mating became important and hence the laws that emerged during this era stressed curbing infidelity and promiscuity. Second, since high-status men paired with a number of females, this left many unpaired men in the population that were predisposed to anger and violence.6 

If a society wants to be peaceful, a large proportion of unpaired males are the last thing it wants. As education levels rose, democracy and striving toward peace and took hold, monogamy became prevalent and this trend continues to present times.



References:

1. Barash, D. P., & Lipton, J. E. (2002). The myth of monogamy: Fidelity and infidelity in animals and people. Macmillan.

2. Buss, D. M. (Ed.). (2005). The handbook of evolutionary psychology. John Wiley & Sons.

3. Barash, D. P. (2016). Out of Eden: the surprising consequences of polygamy. Oxford University Press.

4. Baker, R. (2006). Sperm wars: Infidelity, sexual conflict, and other bedroom battles. Basic Books.

5. Bauch, C. T., & McElreath, R. (2016). Disease dynamics and costly punishment can foster socially imposed monogamy. Nature communications7, 11219.

6. Henrich, J., Boyd, R., & Richerson, P. J. (2012). The puzzle of monogamous marriage. Phil. Trans. R. Soc. B367(1589), 657-669.