Book Review: Sex at Dawn, Part II

(See part 1 here.)

Primitive Warfare

As part of the sex-as-social-glue hypothesis, SaD asserts that pre-state societies were universally peaceful. The book argues that in a hunter-gatherer society with no possessions and no fixed resource base, rival groups would have nothing to fight about [p.183].

But this claim flies in the face of the evidence. As I wrote about in my review of The Better Angels of Our Nature, it’s inevitable that evolution would produce creatures with the capacity for violence, because otherwise a single violent mutant would quickly come to dominate a society of pacifists (another example of the evolutionary logic that Ryan and Jetha seem not to understand, as I discussed in part 1). Even if there were no stable resources in pre-state societies worth fighting over (debatable in and of itself), rival tribes may still fight to avenge an insult to their honor, or to preemptively ambush a feared rival, or out of sheer xenophobia when meeting strangers whose motivations are unknown. All these things have been widely observed in anthropological studies.

Since chimpanzees are hunter-gatherers in the truest sense, Ryan and Jetha’s reasoning would predict that they ought to be peaceful as well. This ought to be problematic, since chimp tribes have been observed fighting and raiding each other in the wild. To dispute this, SaD argues that some of the early chimp studies were flawed because primatologists provisioned them with caches of bananas, giving them access to a reliable food source they’d never had before, which became a point of contention. (Why wouldn’t a grove of wild banana trees, say, constitute a naturally occurring reliable food source?)

But the problem is that more recent studies in which chimps weren’t provisioned, like the Ngogo studies linked in the previous paragraph, still found violent intergroup conflicts. Ryan and Jetha, incredibly, beg off discussing these studies because they “lack the space” [p.189]. How can you not have the space to address important contrary evidence in your own book?

The book goes on to attack a talk given by Steven Pinker about death rates from warfare in primitive societies, claiming that some of the societies he described as hunter-gatherers actually had domesticated animals and village gardens. This is an unfair comparison, they insist, because the findings in SaD apply only to what they call “immediate-return” hunter-gatherer societies: those with no fixed territory, no cultivated plants or domesticated animals, nothing that requires long-term planning [p.185]. But as this review points out, many of the sexually liberal human societies that Ryan and Jetha cite as living examples of our promiscuous past aren’t immediate-return hunter-gatherers either, which by the book’s own logic ought to make them irrelevant:

It is interesting indeed that Ryan and Jethá approvingly cite some horticultural societies (all partible paternity cultures in South America; the Trobriand Islanders; Tahitians; Mohave) as affirming evidence of the sexually promiscuous nature of humans, and a purported lack of universal concern over paternity, while at the same time rejecting other horticultural societies as representative of ancestral humans in their discussion of warfare on the grounds that they are not foragers. They are attempting to have their cake and eat it, too.

Partible Paternity

A linchpin of SaD‘s argument is “partible paternity”: the belief found in some primitive tribes that a baby can have multiple biological fathers, and that for a woman to have sex with many men prior to giving birth may be good luck or even necessary. This is an interesting concept, and one I’m not familiar with myself. However, there’s reason to believe that Ryan and Jetha haven’t presented a complete picture here.

For example, the review article I cited above says that sexual jealousy and conflict are far from unknown even in these societies, that promiscuous mating isn’t a free-for-all but is based on family relationships, and that paternity is never considered irrelevant or unimportant. It also states that some of the human societies Ryan and Jetha cite as surviving examples of our primitive polyamorous state also have monogamous pair-bonds, particularly for high-status men. This includes their prize example, the Musuo of China (which are also an agricultural society, not hunter-gatherers).

Sexual Dimorphism and Sperm Competition

One of the great successes of evolutionary reasoning is in the area of sexual dimorphism. One reliable correlation is that in a species where males compete for exclusive access to females, the males tend to be larger and stronger. For example, male silverback gorillas keep harems that they defend from rivals through dominance and intimidation. Since mating is controlled by physical confrontations, male gorillas are very much larger and heavier than females, on average about twice as much. This is the fruit of an evolutionary arms race where only the biggest and strongest males got to breed.

But arms races can also play out in a different arena, as in the phenomenon of sperm competition. In species where mating is more of a free-for-all, a female may mate with several males in a short span of time, and the sperm from each one will compete with each other to reach and fertilize the egg. Therefore, it’s in a male’s interest to produce as much sperm as possible, which is like buying more tickets in the evolutionary lotto.

Evolution never expends resources unnecessarily, so in species where there’s little chance of sperm competition, the male’s testicles are much smaller. A four-hundred-pound silverback male has testicles the size of kidney beans. Chimps and bonobos, which do mate promiscuously, have things the other way around: as Ryan and Jetha put it, a one-hundred-pound bonobo male has testicles as large as chicken eggs, significantly larger than a human’s [p.222].

And where do humans fit in this scheme? It turns out we’re basically in the middle: men, on average, are larger and stronger than women, though not to the extent seen in gorillas. Nor are men’s testicles as large, relative to their body size, as the more promiscuous chimps and bonobos. This fits well with the standard narrative that human beings are generally monogamous, with some mate competition and some sperm competition on the side.

Ryan and Jetha recognize that this is a problem for their argument. Their suggestion is that human testicle size may have undergone extremely rapid evolution in the last few thousand years [p.226], with selection causing them to shrink as we transitioned to a monogamous, agriculturally-based lifestyle, leaving no trace of our more promiscuous past. This is, of course, unfalsifiable. But they never address an obvious implication: shouldn’t larger testicles persist in those modern hunter-gatherer societies that they claim still live and mate in the ancestral way? I can see some hurdles with conducting a study like this, but if they’re serious, they ought to try: it could be a crucial test of their ideas.


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