Two years ago I was persuaded by the book Sex At Dawn, at least on its “key claim, that forager females were sexually promiscuous.” While I didn’t buy authors’ free-love scenario, I thought our ancestors were much less tied to their sex partners than most folks realize:

A Hadza man hunts big game to look sexy, even though that retrieves less food. Except that when a women he has sex with has a kid he thinks is his, he’ll gather more but less-sexy food, to give this woman ~1/2 of her food for one year, ~1/4 for the next two years, and declining amounts thereafter. Now, yes, this may be more pair-bonding than in chimps or bonobos. But it is also far less than the farmer ideal of life-long monogamy! Many men today reluctant to marry for life would be ok with this level of commitment.

Lynn Saxon has a new book Sex At Dusk, quite critical of Sex At Dawn. She was a kind enough sent me a copy, which I’ve just read. Searching, I’ve only find positive reviews of it (here, here, here, here).

Saxon doesn’t write as well, and especially fails to summarize well. Even so, she does successfully undercut many Sex At Dawn arguments. In humans, sexual jealousy is a universal, females are picky about sex partners, penises aren’t over-sized, testes are small, sperm production slow, and the evidence doesn’t suggest a great deal of sperm competition. Female chimps have little extra-group sex, bonobos don’t usually mate face-to-face, and many Sex At Dawn quotes are misleading, given their context.

On sound during sex, Saxon offers evidence that female primate cries during sex aren’t simple invitations:

A recent study of female chimpanzees found no support for the sperm competition hypothesis: females did not produce calls when mating with low-ranked males so it was not about inviting other males to join the party, and calls did not correlate with fertility and the likelihood of conception. … Females called significantly more while mating with high-ranked males, but suppressed their calls if high-ranked females were nearby. …

[Researchers] found that [bonobo] females were more likely to call with male rather than female partners but the patterns of call usage were very similar in that females called more with high ranked partners (as in chimpanzees), regardless of the partner’s sex. With a female partner compilation calls were consistently produces only by the lower ranking of the two females. … In bonobos the increase in calls during the alpha female’s presense. (p.279-280)

Yes, it looks like chimp and bonobo sex calls are more brags than invitations. Even so, brags make little sense when it is common knowledge who has sex with whom. So a habit of similar bragging sex calls by human females would suggest that humans often didn’t know who was having sex with whom, suggesting a lot of promiscuity.

A key question, to me, is what percentage of our forager ancestor kids were fathered outside pair-bonds. That is, what fraction of kids were born to mothers without a main male partner, or had a father different from that partner. This number says a lot about the adaptive pressures our ancestors experienced related to various promiscuous and polyamorous arrangements today. And hence says a lot about how “natural” are such things.

Alas, none of these authors give a number, but my impression was that Saxon would estimate less than 20%, while the Sex At Dawn authors would estimate over 50%. Even 20% would be consistent with a lot of human promiscuity adaptations, such as female sex brag calls. I asked Saxon directly via email, however, and she declined to give a number – she says her main focus was to argue against Sex At Dawns‘ “paternity indifference” theory (that humans don’t care which kids are theirs). Which is fine – that is indeed a pretty crazy theory.

So where does the evidence sit on promiscuity? Our closest living relatives, chimps and bonobos, are quite promiscuous. Yes, their pair bonds much weaker than ours, and pair-bonding usually greatly reduces promiscuity. But few pair-bonded animals live in big social groups where hidden extra-pair sex is so easy to arrange, and humans live in even bigger groups than chimps or bonobos. For humans, we have lots of clear evidence of outside-pair sex, mate-guarding to prevent such, bragging sex cries, and desires for sexual variety. And humans do seem to spend a record fraction of their time thinking about and doing sex.

Since humans usually have clear overt norms against extra-pair sex, but strong urges to arrange covert sex, promiscuity estimates comes down in part to estimates of how well humans actually enforced their norms. My homo hypocritus theory suggests a lot of covert norm violation, and so a lot of promiscuity. So I’ll estimate the key promiscuity number in the 20-30% range.

Btw, here’s another fascinating quote from the book:

Hrdy writes about a startling interview with an old hunter in which he reminisces to a time when just the sound of his footsteps on the leaves of the forest floor struck terror in the hearts of old women. He was the socially sanctioned specialist in eliminating old women deemed no longer useful: coming up behind an old woman he would strike her on the head with his axe. (p.216)