Against the standard narrative of human sexual evolution

(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.