There are plenty of examples of monogamous, polygamous, and promiscuous mating systems found across mammals and primates. Every single mating system is utilized by some primate or other. Our cousins the gorilla have "a unimale social system and a polygynous mating system." Our closest cousins, the chimpanzees, are promiscuous with lots of variations within the promiscuous mating system, both within and between species (common and bonobo chimps). Various human cultures have utilized most of the mating systems as well.
Rape or "[s]exual aggression by males toward females is widespread among social mammals." Non-human females don't seem to be terribly bothered by sexual coercion in that they seem to remain fully functional in its aftermath. Human females may be uniquely fragile in that they sometimes suffer debilitating trauma from unwanted sexual activity.
The difference in human and chimp genomes is 1.23 percent:
the publication of a rough draft of the chimp genome in the journal Nature immediately told scientists several important things. First they learned that overall, the sequences of base pairs that make up both species' [i.e., humans and chimps] genomes differ by 1.23% -- a ringing confirmation of the 1970 estimates -- and that the most striking divergence between them occurs, intriguingly, in the Y chromosome, present only in males. [emphasis added]The emphasized words could be interpreted to mean that human females are mostly somewhat refined, hairless chimpanzees, while the human male is significantly more evolved from our closest cousins:
As far back as 1972, Elaine Morgan, a feminist, writing in The Descent of Woman, noted that in fact the role of females hadn't changed much from chimp to human. Mothers nurse and care for their offspring in basically the same way chimps do. In terms of social role, there really isn't much difference between human females and other animals.
What has changed is the role of males. Among chimps, males hang out in groups, form alliances, forage together, and do a lot of bickering over status. They do not participate at all in child rearing. By the time hunting-and-gathering tribes arrive, however, men have been folded into the family. Monogamy predominates and both parents participate in child rearing. The extraordinary innovation is "fatherhood," a role that doesn't really exist elsewhere in nature.Of course, not all human societies are monogamous. And with the War of the Sexes, we seem to be moving rapidly away from monogamy to some combination of polygamy and promiscuity. Promiscuous as far as having sex goes, though having sex with the intention of avoiding children may not count as "mating." As far as having children goes, my guess is that we're moving mostly towards a polygamous society with the alpha males servicing (but not marrying) the majority of females.
Relative to polygamy, monogamy benefits beta males, allowing them to mate. It also benefits alpha females, allowing them to monopolize an alpha male. In a species where the females are completely responsible for child-rearing (which is the vast majority of species), polygamy hugely benefits beta females. In a species that relies on males to help with raising children, and in particular in a species where males will only willingly help with raising their own children, polygamy is substantially less advantageous for beta females.
In the past, when western civilization was much less wealthy and had almost no safety net, it was a real struggle for a single woman to raise children on her own and survive (and have the children survive). It was far better for her to be stuck with a beta male helping raise the family - survival for her and her brood was much more likely. Indeed, they might even thrive in good times. Sure, the children might be genetically inferior relative to having an alpha male as a father, but it was still better than no surviving children at all.
The west is much richer now. With safety nets in place, survival is virtually guaranteed. As a result, it's far less imperative that a beta female attach herself to a beta male. There may still be advantages to being married to a beta male, but having healthy children and being able to raise them through adulthood is not one of them. Thus, it seems predictable that beta females would become much more demanding regarding which males they would be willing to mate with and they would be willing to leave that male much more quickly if even the possibility of a better opportunity presented itself. The downside is pretty small, especially with the reasonable, or even favorable divorce settlements that women often get.
My guess is that this female "hypergamy" is the basis for women and men "going their own way." Beta women are logically choosing to mate above their "station" (where "mating" means having children as opposed to just having sex) and beta men realize they have nothing to offer relative to alpha males and look for alternative ways to fill their lives. My guess is that the impact on civilization as a whole will be significant, but not devastating, but those are topics for future posts in this series.