Against the standard narrative of human sexual evolution

by WrongBot 5 min read23rd Jul 2010154 comments

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(This post is the beginning of a short sequence discussing evidence and arguments presented by Christopher Ryan and Cacilda Jethá's Sex at Dawn, inspired by the spirit of Kaj_Sotala's recent discussion of What Intelligence Tests Miss. It covers Part I: On the Origin of the Specious.)

Sex at Dawn: The Prehistoric Origins of Modern Sexuality was first brought to my attention by a rhapsodic mention in Dan Savage's advice column, and while it seemed quite relevant to my interests I am generally very skeptical of claims based on evolutionary psychology. I did eventually decide to pick up the book, primarily so that I could raid its bibliography for material for an upcoming post on jealousy management, and secondarily to test my vulnerability to confirmation bias. I succeeded in the first and failed in the second: Sex at Dawn is by leaps and bounds the best evolutionary psychology book I've read, largely because it provides copious evidence for its claims.1 I mention the strength of my opinion as a disclaimer of sorts, so that careful readers may take the appropriate precautions.


The book's first section focuses on the current generally accepted explanation for human sexual evolution, which the authors call "the standard narrative." It's an explanation that should be quite familiar to regular LessWrong readers: men are attracted to fertile-appearing women and try to prevent them from having sex with other men so as to confirm the paternity of their offspring; women are attracted to men who seem like they will be good providers for their children and try to prevent them from forming intimate bonds with other women so as to maintain access to their resources.

This narrative is remarkable for several reasons. In Chapter 2, Ryan and Jethá point out that it fits in neatly with much of Darwin's work, which famously drew upon Malthus and, to a lesser extent, Hobbes. The problem here is, of course, that Malthus's theory of population growth was wrong (see Michael Vassar's criticism and my reply). Like Hobbes, he looked at his society's current condition and assumed that prehistorical man lived in a similar state; the book calls this unfortunate tendency "Flintstonization" after the famously modern stone-age cartoon family. Those familiar with the heuristics and biases program may recognize this as an example of the availability heuristic.

The human population of the earth exceeded 1 billion individuals when Darwin was writing his works on human evolution, and his conclusions were drawn from the study of living individuals in densely-populated modern cultures; it is remarkable that these findings are claimed to be equally true of the small bands of immediate-return foragers2 that defined anatomically modern human existence between the time they emerged roughly 200,000 years ago and the adoption of agriculture 190,000 years later, during which period there were likely no more than 5 million human beings alive at any one time (to offer a very generous estimate).

Unfortunately, many prominent evolutionary psychologists seem to think it's obvious that these situations should be parallel, as can be seen in the ubiquity of justifications of the standard narrative based on just-so stories and studies performed on undergrad psychology majors. (Examples to follow momentarily.)

Another curiosity is that, "where there is debate about the nature of innate human sexuality [among supporters of the standard narrative], the only two acceptable options appear to be that humans evolved to be either monogamous or polygynous." (Ryan and Jethá, 11, emphasis theirs.) This has been amply demonstrated by a number of commenters on my recent post about the common modern assumption of monogamy. The idea that humans of both genders might be naturally inclined to have multiple partners didn't get much mention3, despite an embarrassing wealth of evidence supporting that position. But I'm getting ahead of myself; the anthropological and anatomical support for the multiple-mating hypothesis will be covered in my next two posts.


In Chapter 3, Ryan and Jethá focus on four major research areas that are used to support the standard narrative. These lines of research all rely on Flintstoned reasoning; taken together, they lead to the standard narrative's conclusion, which Ryan and Jethá summarize as "Darwin says your mother's a whore." (50) The four areas are:

The relatively weak female libido - Donald Symons and A. J. Bateman have both claimed (among numerous others) that men are much more interested in sex than women are. (Pay no attention to the multiple orgasms behind the curtain.) One of the most cited studies in evolutionary psychology purports to demonstrate this by comparing the responses of men and women when solicited by strangers for casual sex. But such studies do not distinguish between social norms and genetic predispositions, leaving evolution's role commensurately cloudy.

Male parental investment (MPI) - Robert Wright wrote in The Moral Animal that "In every human culture in the anthropological record, marriage... is the norm, and the family is the atom of social organization. Fathers everywhere feel love for their children.... This love leads fathers to help feed and defend their children, and teach them useful things." He is not alone in this view, but the argument is based on a number of dubious assumptions, especially that "a hunter could refuse to share his catch with other hungry people living in the close-knit band of foragers (including nieces, nephews, and children of lifelong friends) without being shamed, shunned, and banished from the community." (Ryan and Jethá, 54)

Sexual jealousy and paternity certainty - David Buss's research has demonstrated that, on average, (young, educated, modern, Western) men are more upset by sexual infidelity than women, while (young, educated, modern, Western) women are more upset by emotional infidelity than men. Or, at least, this is true when subjects are given only those two options; David A. Lishner repeated the study but also offered respondents the option of being equally upset by emotional and sexual infidelity. In his version, a majority of both men and women preferred the "equally upset" option, which substantially narrowed the gap between the sexes. The remainder of this gap can be further narrowed by the finding that women asked this question are more likely than men to assume that emotional infidelity automatically includes sexual infidelity. (This paragraph has been edited to fix a reasoning failure that was pointed out to me by a friend.)

Extended receptivity and concealed (or cryptic) ovulation - "Among primates, the female capacity and willingness to have sex any time, any place is characteristic only of bonobos and humans." (Ryan and Jethá, 58) While Helen Fisher has proposed that in humans this trait evolved as a means of reinforcing a pair-bond, "this explanation works only if we believe that males--including our 'primitive' ancestors--were interested in sex all the time with just one female." (Ryan and Jethá, 60, emphasis theirs.)


Chapter 4 expands on the role that the other apes play in the standard narrative. Arguments that evolutionary psychology should focus on the gibbon as a model of human sexuality are frequently attempted on the grounds that they are the only monogamous ape. But gibbons are the ape most distantly related to humans (we last shared a common ancestor ~20 million years ago), live in the trees of Southeast Asia, have little social interaction outside of their small family units, have sex infrequently and only for purposes of reproduction, and aren't very bright.

The chimpanzee model provides much more coherent support for the standard narrative: like modern humans, they use tools, have intricate, male-dominated social hierarchies, and are highly territorial and aggressive. The most recent common ancestor they share with humans lived approximately 6 million years ago, by most estimates. (I originally wrote "between 3 million and 800,000 years ago", which is untrue. Thanks to tpc for pointing that out.) There is just one unfortunate snag: "among chimpanzees, ovulating females mate, on average, from six to eight times per day, and they are often eager to respond to the mating invitations of any and all males in the group." (Ryan and Jethá, 69)

Helen Fisher, Frans de Waal, and other advocates of the standard narrative have claimed that the success of the human species is directly due to the abandonment of chimpanzee-style promiscuity, but they lack a convincing explanation for why this abandonment should have occurred in the first place. Worse yet, there is a particularly important piece of evidence that they are reluctant to acknowledge:

Given the prominent role of chimpanzee behavior in supporting the standard narrative, how can we not include the equally relevant bonobo data in our conjectures concerning human prehistory? Remember, we are genetically equidistant from chimps and bonobos. (Ryan and Jethá, 73, emphasis theirs.)

Oddly enough, bonobos have patterns of sexual behavior that are more like those of humans than any other animal. They hold hands, french kiss, have (heterosexual) sex while facing each other, and have oral sex. Compared to chimps, they're more promiscuous, more egalitarian, less violent, and less territorial. If it seems like this should be evidence for a multiple-mating hypothesis for humans, well, it is. The next post in this series will examine the anthropological evidence Ryan and Jethá use to support this view.


1: I have necessarily omitted much of the evidence that Ryan and Jethá provide in favor of their claims. Please feel welcome to request further information if there are any points you find particularly dubious; while I am not an expert in this field, I will at least attempt to pass on the sources cited.

2: Immediate-return foragers are those who eat food shortly after acquiring it and do not make significant use of techniques for its processing or storage.

3: I was admittedly among those dubious of such a conclusion.

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