Sex At Dawn Is Right

I like to think of myself as courageously seeking out important truths, however uncomfortable. But like most would-be-courageous folk, I don’t really know what I want until I get it. I was excited to read the contrarian Sex at Dawn, which suggests sexual promiscuity is our forager heritage. But that pretty sparkler was really a grenade – its uncomfortable truths shook me to my core.

To hear a different view, immediately afterward I read Evolutionary Biology of Human Female Sexuality [EBoHFS], which supports a standard view of foragers as long-term pair-bonders. By comparison, EBoHFS is more academic: dry, clinical, verbose, and careful to define terms and consider many possibilities. It reviews an immense number of studies, and appears to takes a cautious middle ground.

Sex at Dawn, in contrast, frustrates academic sensibilities. It is passionate, partisan, even snide. It doesn’t systematically review evidence pro and con, or points of view, and it takes long detours to settle scores with opponents. It is long on anecdotes and examples, relative to systematic data. Its authors are confused on how economists use “selfishness”, and on basic Malthusian population theory; they aren’t really to be trusted on theory. Furthermore, their relationship advice seems flippant.

But on their key claim, that forager females were sexually promiscuous, I am persuaded: they are basically right. EBoHFS hardly offers any contrary evidence, it just keeps repeating that evidence is ambiguous, while embracing the usual story by default. (Sex at Dawn also gets forager peacefulness right – see Chapter 13.)  Searching for expert critical reviews, the closest I found were this and this, which mainly just complain it is all very complex and no simple generalizations apply.

The basic facts are these. Recent humans mostly had long-term pair-bonds, while our two closest primate relatives, chimps and bonobos, are quite sexually promiscuous. Yes, they hardly mate at random, and may return often to favorites. Even so:

“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. … A recent study … showed that more than half the young (seven of thirteen) had been fathered by males from outside the female’s home group.” [p70]

Bonobos females are even more promiscuous. In fact, to find biological analogues to recent human monogamy, EBoHFS looks to various kinds of birds; mammals won’t do. “Monogamy is not found in any social, group-living primate.” [p64]

The big question then is when did the biologically-rare (3% of mammals) phenomena of (near) monogamy arise in our lineage, millions of years ago with the rise of humans, or ten thousand years ago with the rise of farming? And since our data on modern foragers suggests that farming at least greatly reduced promiscuity (especially for females), the big question is really whether lightning struck once or twice, i.e., if there were one or two big unprecedented moves away from typical social-primate promiscuity. Occam’s razor suggests one lightning strike.

If human sex were like chimp and bonobo sex, how should we expect it adapt to human changes, like larger brains, group sizes, and lifespans, and more egalitarian sharing? Dunbar says brain size, group size, and grooming time fraction correlate, and says language let humans better “groom” our record size groups. If sex was part of “grooming”, we’d expect humans to spend a record fraction of time on sex. We should also expect more adaptations to longer term relations (like pedophilia).

So what is the data? Humans spend more time having sex than any known species. Human sex shares many otherwise-rare features with the promiscuous bonobos, who hide their fertile days and have sex all month long, in many positions including missionary where they gaze into each other’s eyes and kiss deeply. Bonobos share food with sex, and use sex for social bonding, such as via homosexuality. Human sex has many other features understandable as adaptations to promiscuity, including large external testicles, a record size penis designed to scoop away other semen, men preferring high male-to-female ratios in porn, long frequent sex, and women being louder and lasting longer than men.

Monogamy vs. promiscuity is a rare area where academics and cultural elites tend to favor the conservative/farmer side of the forager vs. farmer divide. While I find myself balking at the idea of embracing sex partners with forager promiscuity levels, I accept that this preference was culturally imprinted on me.

Many Sex At Dawn quotes below the fold:

The only American scholar to have been cited by each of the three intellectual giants of his century, Darwin, Freud, and Marx, many consider [Lewis Henry] Morgan the most influential social scientist of his era, and the father of American anthropology. … Morgan … hypothesized a far more promiscuous sexuality as having been typical of prehistoric times. “The husbands lived in polygyny, and the wives in polyandry.” [43] …

No animal spends more of its allottted time on Earth fussing over sex than Homo Sapiens. … [In contrast,] pair-bonded “monogomous” animals are almost always … having sex … infrequently, quietly. [85] … Female primates are highly attracted to novelty in mating [p.96] … The total number of monogamous primate species that live in large social groups is precisely zero. … The few monogamous primates that do exist (out of hundreds of species) all live in the treetops. Primates aside, only 3 percent of mammals and one in ten thousand invertebrate species can be considered sexually monogamous. [97] …

Modern men’s testicles are smaller than those of chimps and bonobos, yet they put those of polygynous gorillas and monogamous gibbons to shame … The argument that modern men’s testicles would be as big as the chimps’ if we’d evolved in promiscuous groups is founded on a crucial and erroneous assumption: that human testicles haven’t changed in ten thousand years. … Testicular tissue in humans, chimps, and bonobos (but interestingly, not gorillas) is controlled by DNA that responds unusually rapidly to environmental changes. [227] …

[Why is it] male ejaculate that puts the money in money shot. … Images and videos showing one woman with multiple males are far more popular on the Internet and in commercial pornography than those depicting one male with multiple females. … Why does being cuckolded consistently appear at or near the top of married men’s sexual fantasies.? [231] …

“In primates which live in family groups consisting of an adult pair plus offspring the male usually has a small and relatively unspecialized penis.” …”Adult male humans have the longest, thickest, and most flexible penises of any living primate.” … The unusual flared glans of the human penis forming the coronal ridge, combined with the repeated thrusting action …creates a vacuum .. [that] pulls away any previously deposited semen. … Men last far longer in the saddle than bonobos (fifteen seconds), chimps (seven seconds), or gorillas (sixty seconds), clocking in at between four and seven minutes, on average. [235] …

The very existence of the external human scrotum suggests sperm competition in human evolution. Gorillas and gibbons, like most other mammals that don’t engage in sperm competition, generally aren’t equipped for it. A scrotum is like a spare refrigerator in the garage just for beer. … Anyone who’s been kicked in the beef fridge can tell you this is a potentially costly arrangement. [237] …

Researchers found “marked differences in testis size among human races. … This range is far beyond that average racial differences in body size would predict. [241] … Coming in at hundreds or thousands of copulations per child born, human beings out copulate even chimpanzees and bonobos. … When the average duration of each copulation is factored in, the sheer amount of time spent in sexual activity by human beings easily surpasses that of any other primate – even if we agree to ignore all our fantasizing, dreaming, and masturbating. The evidence that sperm competition played a role in human evolution is simply overwhelming. [242] …

When it comes to sex, men may be the trash-talking sprinters, but it’s the women who win all the marathons. … If men and women evolved together in sexually monogamous couples for millions of years, how did we end up being so incompatible? [245] …

Why is it that from the Lower East Side to the upper reaches of the Amazon, women are far more likely than men to loudly announce their sexual pleasure for all to hear? … “In a wide variety of species, females vocalize just before, during, or immediately after they mate. These vocalizations,” Semple says, “are particularly common among the primates and evidence is now accumulating that by calling, a female incites males in her group.” … Before you conclude that female copulatory vocalizations just a fancy phrase for a little excitement, think about the predators possibly alerted by this primate passion. [256] …

As with the complex penis and external testicles in the male, the elaborate filtering design of the human [female] cervix points toward promiscuity in our ancestors. [265] … Primate species with orgasmic females tend to be promiscuous. [268] …

[Researchers] exchanged that season’s newborn sheep and goats (the baby sheep were raised by adult goats, and vice versa). Upon reaching sexual maturity a few years later, the animals were reunited with their own species and their mating behavior was observed. The females adopted a love-theone-you’re-with approach, showing themselves willing to mate with males of either species. But the males, even after being back with their own species for three years, would mate only with the species with which they were raised. [272] …

Whether they were watching men with men, women with women, … their genital blood was pumping. But unlike the men, many of the women reported that they weren’t turned on. … This disconnect between what these women experienced on a physical level and what they consciously registered is precisely what the theory of differential erotic plasticity predicts. [273]