From: https://www.psychologytoday.com/us/blog/pura-vida/201603/how-monogamy-helps-men

In subsequent posts, I’ll develop some of the most cogent evidence for our simultaneous polygyny plus polyandry – i.e., polygamy in the broadest sense. But for now, I want to focus on a fascinating and as yet unanswered question, suitable for others to ponder. Given that monogamy is not “natural” to human beings, why is it currently so widespread? (Note: the fact that monogamy isn’t natural doesn’t mean that it isn’t possible or even desirable; I argue that some of the best things about human beings are revealed when we do things that are difficult, and that don’t come easily, such as learning to play the violin ... or, perhaps, being monogamous. But that’s another topic.)

So my question for now is: Why did such a large segment of human society switch from polygamy to monogamy? And my first answer is: at present, we don’t know. My second answer is a guess, which goes as follows. (I propose it simply as a hypothesis, in the hope that readers will not only find it interesting but also useful in generating informed discussion and, if possible, meaningful research.)

Imagine a polygynous society with an average harem size of, say, ten. This means that for every male harem-keeper, there are nine unsuccessful, sexually and reproductively frustrated, resentful bachelors. The simply reality is that polygyny is disadvantageous not only for women – for complex reasons – but even more so for men, since with a 50/50 sex ratio, there are unmated men in proportion as polygyny obtains. This, btw, runs counter to the lascivious imaginings of many men, who, when I describe the evidence for primitive human polygyny, often express regret that they weren’t alive in those days, imagining that they would be a happy harem-holder. This is actually rather comical, analogous to those charlatans who claim to remember their past lives, when they were Napoleon, or Tutankhamen, or perhaps Cleopatra or Catherine the Great ... whereas the overwhelming statistical likelihood is that they would have been some poor boob who froze to death on the Russian steppe, or an equally unknown laborer who struggled to construct a pyramid.

In short, whatever its benefits when it comes to child-rearing, monogamy is a great democratizing institution, enabling men to have a wife and a chance at a family, the great majority of whom would otherwise be left out. The hypothesis thus goes that Western society in particular inculcated monogamy as a trade-off, whereby powerful men essentially agreed to forego the obvious trappings of polygyny in return for a degree of social peace and harmony. Monogamy is part of our egalitarian ethos, and for good reason. Consider the contribution that socio-sexual frustration has apparently made to unrest in the Arab world, where limited economic opportunities has contributed to limited mating opportunities. Similarly, look at the situation of modern China, where an excess of men seems poised to generate its own (currently unpredictable) disorder.

The subsequent posts he references if you're curious:

People are Polygynous

Polyandry (One Woman, Many Men)

Double Mother Suckers

Edit: Some quoted text became unquoted after posting, fix typo

Edit2: Title should be David P. Barash, Ph.D., - this isn't his PhD thesis or anything

Edit3: I just copied his bio from the site which says

David P. Barash, Ph.D., is an evolutionary biologist and professor of psychology emeritus at the University of Washington. He has written 38 books covering evolution, animal and human behavior, and peace studies. He is a Fellow of the American Association for the Advancement of Science. He is married to psychiatrist and frequent co-author Judith Eve Lipton. He was named as one of the "101 most dangerous professors" in the U.S. by conservatives.

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