Evolutionary psychologist David P Barash gives us the comprehensive case for humans being naturally polygamous, with more evidence than you might want. Barash challenges two competing assumptions about human nature those in the west are most comfortable with: that humans are either by nature monogamous or promiscuous. The former monogamy owes to the dominance of custom from the Greco-Roman past as well as from Judeo-Christianity. The latter promiscuity is the great of hope of social radicals and egalitarians since the French Revolution who see marriage of any kind as despotic and unnatural.
Barash's thesis is that humans are naturally inclined toward polygamy, both polygyny and polyandry, due to our primate past, and that monogamy for everybody is a social imposition, though not exactly unnatural. Barash's case has two major lines of evidence: 1) lingering differences in males and females called dimorphism and 2) comparative anatomy with ape relatives.
The basic argument for the first line of evidence is that polygamous species males face greater competition against other males for reproductive partners. The ratio of male to female partners is limited by the fact that a human female can only give birth 9 months at a time from a single male partner. So if one male has exclusive access to several females, the other males are left out of the gene pool. This would incentivize males to develop stronger bodies to compete against one another for mates since the competition isn't "fair." The expectation is for men to be physically larger than women, take longer to mature, and be more violent in a polygamous species
The physiological differences between men and women are more striking than I realized. Men are on average five inches taller than women and 10-20% larger overall, but when fat is discounted, men are 40% heavier, with 60% more muscle mass and 80% more muscle in the arms. The male-female difference in crime is 10 to 1. The difference is slight in petty crime, greater in robbery, even greater with assault, and most dramatic in homicides. Homicide rates are very different in Iceland and Honduras but ratio the same. So this strongly suggests humans are by nature polygamous.
It isn't as if females are inclined to monogamy and males to polygamy though. Though overt polyandry, several male partners for one female, is rare but it is practiced covertly. This is obvious given the existence of adultery and legal divorce. Through divorce in fact one woman can over her lifetime has offspring from several male partners. This points to a predisposition to polygamy being human and not just male nature.
The second line of evidence is comparative anatomy with primates. Gibbons are monogamous and their females are equal in size. Gorillas are polygamous and males are larger than females. The dimorphism among humans suggest our ancestors were closer to the gorilla. Chimps and bonobos are the closest to us genetically and have multiple sex partners. They however openly practice promiscuity among both males and females rather than taking mates. This is because male competition occurs among their sperm. If humans were closer to the chimp model of promiscuity, male sperm count should be pretty large. It is in the middle. The evidence is that we are mildly polygamous, or serially monogamous.
A survey of 849 societies found that prior to western imperialism, 708 (83%) had polygamy. One half were usually polygynous and one half occasionally so, though most men end up with one wife. 16% were officially monogamous and fewer than 1% polyandrous. Most men in a polygamous society however don't have more than one wife. A harem is for men of wealth and power. The case Barash is making isn't that everybody was polygamous, just that it was practiced in some way for those who could afford it.
There are strong forces which induce humans to pair bonding and monogamy. Pair bonding is affective and monogamy is political. The "four horsemen" which promote monogamy are parental attachment, mirror neurons, neuroplasticity, and the hormones oxytocin and vasopressin. Humans are born into the world helpless and develop strong attachments with their caregivers, which in turn shapes our future relationships to greater emotional dependency. This condition elicits greater male provision of resources to children. Mirror neurons are a proposed explanation for empathy by exciting the same brain activity when observing somebody else as if we are doing the same action, which makes interpersonal relationships possible on a deeper level. Our brain's neuron connections grow and brain regions develop in response to continued interaction. The hormones oxytocin for females and vasopressin for males induce positive feelings from partner from physical arousal.
Strict monogamy as opposed to pair bonding however is political. Following Freud's speculations Barash argues that social and legal monogamy arose as a form of social leveling, to ensure equal access among males for females to maintain social peace and harmony. Polygamy means having a large portion of males condemned to bachelorhood with no prospects of accumulating wealth. Societies with mass polygamy were tyrannical and highly stratified. It is possible that democracy cannot work with widespread polygamy, though it seems to me that mass polygamy is more a consequence than a cause of social inequality as providing for a few hundred wives would be quite expensive in a hunter gatherer society. Prior to such accumulation of wealth humans were mostly serially monogamous with maybe a few having more than one or two partners.
"Monogamy may therefore have emerged as a sop to men, reducing the number consigned to frustrated bachelorhood, in a kind of unspoken social bargain whereby powerful men gave up the overt prerequisites of polygyny in return for obtaining a degree of social peace and harmony."
Barash even speculates that the drive to accumulate wealth derives from our polygamous heritage.
"We've seen that polygyny in particular is a direct outgrowth of high male variance in reproductive success. Is it possible that income and socioeconomic inequality- not just the tolerance of such differences, but the establishment of circumstances underlying their very existence- is associated with a polygynist mindset, whereby some individuals accrue more than others? Certainly it is more than their "fair share." It seems at least possible that capitalism's fondness for more...is a consequence of an evolutionary process that has- at least in the past- rewarded those who obtain more."
The Abrahamic god the father meme is also speculated to have some of its appeal from our harem alpha ape past.
"Much in the evolutionary psychology of Homo sapiens renders our species susceptible to God as portrayed in the Abrahamic religions. We are deeply sensitive to dominance hierarchies and especially to the need to respect the silverback make and his prerogatives. We are subject to sexual impulses that in our evolutionary past contributed to the success of our ancestors but that also risked serious trouble is they were not employed cautiously; hence, we are endowed with urges that are powerful but that we also recognize as potentially dangerous to ourselves, especially if they evoke jealous anger from the powerful male."
"In his book The Naked Ape, zoologist Desmond Morris wrote that religion's extreme potency is simply "a measure of the strength of our fundamental biological tendency, inherited directly from our monkey and ape ancestors, to submit ourselves to an all-powerful, dominant member of the group."
Extrapolation to culture though takes us to cultural evolution through memes which isn't biological but influenced by biology. The argument would be that we are predisposed towards certain memes like patriarchal religion or worship/attraction of successful men. That men are socially valued for competition and women for fidelity would be explained by the polygamous urges.
I think the case Barash makes is less surprising than he makes it out to be. Human nature is multifaceted and it seems we have some predispositions toward polygamy, but I think it's really for most of us serial monogamy of having multiple partners over a lifetime but one, overt, partner at a time. Our primate past could very well have been polygamous, I think it's plausible. I think that the transition from ape to human induced us to become more monogamous during the Pleistocene but that the rise of civilized life and wealth reactivated these powerful tendencies toward polygamy, and a moral-social revolution changed it to official monogamy.
"Here, then, is something of a monogamy bottom line: human beings have several deeply evolved predispositions, which don't always coexist comfortably. In particular, we are almost certainly endowed with a string inclination for pair bonding, piled on top of alongside or incorporated within and sometimes without another, contradictory inclination: for multiple sex partners. To paraphrase Walt Whitman, do we contradict ourselves? Very well, we contradict ourselves. We are vast; we contain multitudes, which include simultaneous contradictory impulses toward both polygamy and monogamy."
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