Which brings be back around to Christopher Ryan's belief that humans aren't inherently monogamous (or at least would be happier if they weren't). Now, I realize I am apparently the only person in the entire blogosphere that disagrees, but hear me out.
Ryan has continued his argument that modern humans sexual nature was/is polygamous, and uses things like testosterone surges (leading to lower mortality) in the presence of an unknown woman as evidence that we (men) are hardwired for polygamy. He suggests that a woman's rapid ovulation cycle, and 12 pregnancy opportunities per year suggest women are hardwired to mother children that aren't necessarily those of their cohabitant male, rather the children of the male she happens to be mating with at that point in the year.
Yet, he's not addressing the behavioral compulsions of both pre- and post-agricultural human societies that cause monogamy to be the rule, not the exception. Is it any coincidence that serial monogamist societies are (by and large) the Industrialized nations of this planet? Is it any coincidence that HIV is far less prevalent in heterosexuals that practice monogamy?
When faced with these issues, Ryan retreats to pre-agriculture for defense:
While there were no doubt occasional outbreaks of infectious disease in prehistory, it's unlikely they spread far, even with high levels of sexual promiscuity. It would have been nearly impossible for pathogens to take hold in widely dispersed groups of foragers with infrequent contact between groups. The conditions necessary for devastating epidemics or pandemics didn't exist until the agricultural revolution.
But if we are to assume that humans were built into "widely dispersed, small groups of foragers," wouldn't inbreeding be minimized if monogamy were adopted? It seems that Ryan's model has as few people as possible to breeding outside of their little group, but as many as possible breeding within that group. Small polygamist (and to a lesser extent monogamist) groups cannot escape inbreeding without the regular input of external genetic data; the harmful mutations just continue to escalate. If you have a society with four men and four women, and each woman fathers four children, one from each father, within one generation all the children in the group are half brothers and sisters. If you have four monogamous pairs, it takes three times as long for all the progeny to become at least half-siblings.
And what about the thoroughly modern concept that a man simply might find it less worth his time to spend enormous physical, mental, and fiscal capital raising children that are clearly not his own. In almost every post-agricultural society I can think of, humans have placed a lot of weight, either in financial inheritance or in social standing, on the ability to parent your own progeny. For women this is an inescapable simplicity, but for men, the cultural pressure to father as many children as possible must be counterbalanced with the desire to minimize chances of mistakenly raising someone else's children as your own.
Sure, one could just plumb vaginas with reckless abandon, hoping to spread one's seed as far and wide as possible, and just accept that the children you raise with your cohabitant are likely not all your own...but hey, some other schmuck is stuck with your kids.
But what if you want to guarantee not only that you have as many kids as possible, but also that they are raised in a fashion you find acceptable? Impregnate 20 homeless women gets your genes out there for pennies on the dollar, sure...but what is the chances that those kids are born healthy, or that they survive long enough to procreate too?
Consider that extreme with the extreme of monogamy, where you guarantee your only (and only your) children come from one woman and you raise them as you see fit and provide enormous physical, mental, and financial faculties to those select few kids.
Ryan would probably, faced with this argument, retreat back to pre-history, and argue that small foraging groups had communal children, and that the children were probably all kept in a "day-care" style group overseen by a small number of adults while the remaining adults foraged and hunted. In this way a parent oversaw their own children, as well as other children, all for the combined good of their group.
But Ryan can't retreat to pre-history. His book isn't an argument for pre-historic polygamy; its a treatise on the increased happiness individuals of modern society could achieve via polygamy that he believes was prevalent in pre-agricultural humanity.
But as of yet, from Ryan or from anyone else, I have not heard a single convincing argument that pre-historic humans were largely polygamous. So why should I believe that monogamy is against our nature when it isn't clearly established what our original nature was?