[Evolutionary psychology] is trying to take on an incredibly challenging task: understand what of human behavior is adaptive and why. We can better circumvent the conditions that lead to violence, war, and hatred if we know as much as we can about why we are the way we are. What motivates us, excites us, angers us, and how can evolutionary theory help us understand it all?
Because of this, there are consequences to a bad evolutionary psychology interpretation of the world. The biggest problem, to my mind, is that so often the conclusions of the bad sort of evolutionary psychology match the stereotypes and cultural expectations we already hold about the world: more feminine women are more beautiful, more masculine men more handsome; appearance is important to men while wealth is important to women; women are prone to flighty changes in political and partner preference depending on the phase of their menstrual cycles.
It may not always be evident, but biologists who get all shirty when we see the latest evo-psych study splashed across the headlines generally agree with the most basic premise of EP: that humans are evolved, biological organisms, and that our present behavior is a result of our evolutionary history. What drives us up the wall is the refusal of EP research to apply understanding developed over decades of work by evolutionary biologists—the discovery that there’s more to evolution than natural selection, that natural selection often acts on many traits simultaneously, and that there may be many ways to acheive the same level of reproductive success.
With modern genetic tools and a modern evolutionary perspective, biologists—including Kate, who studies human reproductive biology in an evolutionary context—have learned a lot about how natural selection and other evolutionary processes shaped current human diversity. Some of the best examples so far are in relation to diet (drinking milk and cultivating corn) and adaptation to low-oxygen conditions at high altitude; but there’s no reason the same methods can’t tackle other features of human nature, given sufficient quantities of the right data. Yet we rarely see EP studies based on the kind of data that could actually provide answers to the questions they ask. (And then, all too often, we see EP studies that are unmoored from basic biology altogether.)
So: The complaint that most evolutionary biologists have with EP isn’t that it’s asking the wrong questions, or asking questions it has no right to ask. It’s that EP is using the wrong tools to answer those questions. And, to the extent that we agree that those questions are important, it’s upsetting to see someone claim to have answered them using only surveys of undergraduates. We’re like plumbers expressing our exasperation with a guy who insists on intalling a new toilet using a nail file and a hot glue gun: dude, go buy a wrench!
If this sounds ridiculous to you, well, it did to me, too. I reacted right off the bat with Tweeted snark, and had some back-and-forth with Bering in which, I have to say, I didn’t acquit myself especially well. I try to do better than engaging in scientific debate while steam is coming out of my collar, and, once I’d cooled down a bit I attempted an apology (which recieved a perfectly polite response), and I then resolved to sit down and actually figure out the merit of Davis and Gallup’s hypothesis.
Spoiler alert: Further study did not make D&G’s hypothesis any more plausible. But my reasons for disbelieving it after doing the background reading aren’t what you might expect.
Preeclampsia and semen familiarity
First, a little more detail about preeclampsia. It’s a condition linked to pregnancy-induced hypertension—not so much a single complication as a cluster of symptoms that seem to be connected to an immune reaction against the “foreign” tissue of a fetus. As the name implies, it can escalate to eclampsia, which involves seizures and a coma, and can seriously endanger both the woman and her fetus; but preeclampsia can create dangerous complications [$a] even if it doesn’t get to that stage, causing premature delivery, restricted fetal growth, and loss of the pregnancy.
One of the first things I found out, following up on papers cited in Bering’s post and D&G’s original description of the hypothesis, is that the “semen familiarity” idea is actually quite widespread in the medical literature and, as far as I can tell, reasonably well supported. Various studies have found that preeclampsia is more common in first pregnancies; more common in women who have switch sexual partners between pregnancies [PDF]; and more common in women who become pregnant via artificial insemination from an unknown donor [$a] than in those who are artificially inseminated with their current partner’s sperm. That last study has a rather small sample size, but it’s about as close as you could get to an outright experimental test of the “semen familiarity” effect, I think.
However, if sperm familiarity is one factor contributing to the risk of preeclampsia, it’s not the only thing, and its role isn’t quite universally accepted. It’s a risk factor in the epidemiological sense, not a direct, clear-cut cause. Women who are overweight, who are diabetic, who smoke, or who have hypertension when they become pregnant have an elevated risk of preeclampsia independent of any effect of sperm familiarity. A relatively recent review article notes that there’s some support for an alternative hypothesis that longer waits between pregnancies is a stronger determinant; women who change partners often also tend to wait longer between pregnancies because, well, it can take a while to make that kind of switch. Studies that account for time between pregnancies have found that, in fact, switching partners can be associated with a somewhat reduced risk of preeclampsia.
Rolling the die against rape
But so if preeclampsia is indeed made more likely by unfamiliar semen, how much of a selective advantage could this tendency incur? (Assume, for the moment, that there’s a genetic underpinning to the tendency to respond to unfamiliar sperm by developing preeclampsia; natural selection can’t act on any trait that isn’t passed from parent to offspring with reasonable reliability, no matter how useful or detrimental that trait might be.) As I’ve noted in other contexts, a very basic result in population genetics is that, for natural selection to over come the effects of genetic drift and mutation [$a], it has to have some minimumn strength; any selective advantage at all isn’t enough for a gene variant to spread via natural selection.
To determine whether selection favoring preeclampsia as a response to unfamiliar sperm might be strong enough to overcome drift and mutation, we’ll have to do some back-of-the envelope calculations. Here, I can draw on some data from the medical literature, but this is all pretty crude, so grab your salt-cellar.
First, how much more likely does unfamiliar semen make preeclampsia? In that above-mentioned comparison of artificial insemination by unknown donors versus familiar partners, which was published in 1997, women recieiving donor sperm were about 1.85 times more likely to develop preeclampsia than those who recieved sperm from their partners; a much larger study from 1999 [PDF] reports that preganancies resulting from sperm donation had about 1.4 times the risk of preeclampsia seen in comparable natural pregnancies.
That baseline risk is about anywhere from 2 to 7 percent of pregnancies. So if we take the higher end of both estimates (the baseline probability of preeclampsia, and the factor by which unfamiliar sperm increases it), we’re talking about an effect that elevates the probability of developing preeclampsia up to about 13 percent. That’s not zero, but (to take Trisha Greenhalgh’s advice to heart) let’s try to think about that in concrete terms: it’s less than the probability of rolling a six with one toss of a die.
Then, consider that not all preeclampsia cases result in loss of the pregnancy. The current risk of fetal death associated with preeclampsia is about 1 to 2 percent of cases; so now figure that you have to toss that die more than enough times to roll six 100 times—more than six hundred tosses—to be reasonably sure of ending just two rape-related pregnancies this way. Put it another way: a (purely hypothetical) gene variant responsible for making a woman likely to develop preeclampsia when she encounters unfamiliar sperm would help her avoid carrying a rapist’s baby to term with something less than one chance in 300.
(One caveat: of course, I’m working from present-day risks of pregnancy loss due to preeclampsia, and of course preeclampsia would’ve been more likely to result in pregnancy loss—and also maternal death—before the advent of modern medicine. But I wasn’t able to find similarly precise estimates of those risks predating modern medicine, and in any event Davis and Gallup, and Bering, discuss the hypothesis in terms of its implications for modern society.)
Strong enough for selection?
Now, let’s compare that educated guess to the estimated strength of natural selection acting on two adaptations biologists have studied much more closely in humans: the capacity to survive in high-altitude, low-oxygen conditions, and the ability to digest milk sugars as adults. These are each cases where a useful genetic variant has spread through a population, which means selection overcame drift and mutation; although I don’t believe that either adaptive variant has “fixed,” or spread to the entire population.
In the first case, a gene variant found in people living on the high Tibetan plateau is associated with reduced risk of death for the children of women carrying the variant. A 2004 study of Tibetan women found that those without the variant gave birth to about 4.5 children, and an average of 2.5 of those children died before the age of 15; women carrying the beneficial gene variant had about the same number of live births, but only an average of 0.5 children who died. In other words, carrying the high-altitude gene variant meant they had twice as many children surivive to age 15.
In the second case, a 2009 study used population genetic data to estimate the strength of selection on the gene variant responsible for lactase persistance, the ability to digest milk sugars as an adult, in European populations that have historically raised cattle for milk. The estimated selective benefit of being able to digest milk was about 1.8 percent. That is, people in those European populations who couldn’t digest milk had about 98.2 children for every 100 children born to people who could digest milk.
Stack those selective effects alongside that proposed for preeclampsia as a response to rape: less than a one-in-six chance for a two percent chance of losing an unwanted pregnancy, or somewhat less than three out of a thousand rape-related pregnancies ended prematurely. And, as Kate Clancy notes in her excellent discussion of the Akins fiasco, preeclampsia characteristically occurs late in pregnancy—so, in the rare cases when it does end an unwanted pregnancy, it does so after a mother has already invested months of resources in supporting the fetus.
As Clancy points out, an adaptation to prevent pregnancy by rape would be much more effective if it caused miscarriage well before preeclampsia could even come into play—and, indeed, Davis and Gallup proposed, at the end of their book chapter, that earlier miscarriages might also be related to semen familiarity. They cite no data to test that hypothesis, and I haven’t found any published since their book chapter. But as Clancy describes quite clearly, we have reaonable evidence that rates of pregnancy from rape are similar to rates of pregnancy from consensual sex, and that would seem to close the book on the question of anti-rape defenses in early pregnancy.
In other words, if women have evolved some sort of physiological adaptation to avoid getting pregnant as a result of rape—whether via elevated risk of preeclampsia or another means—the actual benefits conferred by such an adaptation are so miniscule as to stretch the definition of “adaptive” to meaninglessness. But I can think of another well-known adaptation that does allow women to end unwanted pregnancies with a high degree of reliability: human intelligence. Women have been using abortifacients and other means to end pregnancies, sometimes well before preeclampsia typically occurs, since the dawn of recorded history, and modern medical technology from hormonal birth control to emergency contraception to, yes, abortion itself makes this simpler and safer than it’s ever been.
Contrary to Jesse Bering’s quippy title, Darwin’s morning after pill isn’t some mysterious power of a woman’s reproductive tract; it’s the big brain that millions of generations of evolutionary history gave her.◼
As noted in the main text, all calculations herein are back-of-the-envelope estimates, and subject to the foibles of my limited numerical skills; if you see something wrong with them, let me know in the comments!
Beall, C. M., K. Song, R. C. Elston, and M. C. Goldstein. 2004. “Higher offspring survival among Tibetan women with high oxygen saturation genotypes residing at 4,000 m.” Proc. Nat. Academy Sci. U.S.A. 101:14300. DOI: 10.1073/pnas.0405949101.
Dekker, G., and P.-Y. Robillard. 2007. “Pre-eclampsia: Is the immune maladaptation hypothesis still standing?: An epidemiological update.” Journal of Reproductive Immunology 76:8-16. DOI: 10.1016/j.jri.2007.03.015.
Gerbault, P., C. l. Moret, M. Currat, and A. Sanchez-Mazas. 2009. “Impact of selection and demography on the diffusion of lactase persistence.” PLoS ONE 4:e6369. DOI: 10.1371/journal.pone.0006369.
Haldane, J. B. S. 1927. “A mathematical theory of natural and artificial selection. Part V: Selection and mutation.” Proceedings of the Cambridge Philosophical Society 23:838-844. DOI: 10.1017/S0305004100015644.
Hoy, J., A. Venn, J. Halliday, G. Kovacs, and K. Waalwyk. 1999. “Perinatal and obstetric outcomes of donor insemination using cryopreserved semen in Victoria, Australia.” Human Reproduction 14:1760-1764. DOI: 10.1093/humrep/14.7.1760.
Li, D.-K., and S. Wi. 2000. “Changing paternity and the risk of preeclampsia/eclampsia in the subsequent pregnancy.” American Journal of Epidemiology 151:57-62. Full text PDF.
MacKay, A. P., C. J. Berg, and H. K. Atrash. 2001. “Pregnancy-related mortality from preeclampsia and eclampsia.” Obstetrics & Gynecology 97:533. Full text PDF.
Robillard, P.-Y., and T. Hulsey. 1996. “Association of pregnancy-induced-hypertension, pre-eclampsia, and eclampsia with duration of sexual cohabitation before conception.” The Lancet 347:619 DOI: 10.1016/S0140-6736(94)91638-1.
Skjaerven, R., A. J. Wilcox, and R. T. Lie. 2002. “The interval between pregnancies and the risk of preeclampsia.” New England Journal of Medicine 346:33-38. DOI: 10.1056/NEJMoa011379.
Smith, G. N., M. Walker, J. L. Tessier, and K. G. Millar. 1997. “Increased incidence of preeclampsia in women conceiving by intrauterine insemination with donor versus partner sperm for treatment of primary infertility.” American Journal of Obstetrics and Gynecology 177:455-458. DOI: 10.1016/S0002-9378(97)70215-1.
I mean, I don’t want to be making an ad-hominem argument here, but I tend to think that the point of popular science writing is for the audience to benefit from a writer’s perspective and expert judgement. And Jesse Bering’s judgement is in pretty serious question. (Don’t justtakemywordforit!) He might very well be a great psychologist—that field is beyond my expertise to assess—but it’s pretty clear that Bering’s knowledge of evolution begins and ends with an exceptionally superficial understanding of natural selection, and, more often than not, he rallies that superficial understanding (but not much actual scientific evidence) for the defense of some pretty damn’ regressive ideas.
Plus which, “plunger penis” isn’t exactly news: the paper Bering seems to be citing is from 2003, and Jared Diamond discussed the ways in which the human penis stands out (heh) in comparison to those of other apes in The Third Chimpanzee, which was first published in 1992. Wasn’t this covered in Sex at Dawn?
All I’m saying is, read that new book with a saltshaker handy.◼
Gallup, G. G., R. L. Burch, M. L. Zappieri, R. A. Parvez, M. L. Stockwell, & J. A. Davis (2003). The human penis as a semen displacement device. Evolution and Human Behavior, 24, 277-89 DOI: 10.1016/S1090-5138(03)00016-3
In case you think I was kind of an asshole in my response to that study about female vulnerability and sexual explitability (and, fine, I was; but I’d like to think I was channeling my natural asshole-ness for a righteous and scientifically important cause), here are some non-scientists taking many of the same issues with that very study, and coming up with even more basic questions about its logic:
A Biologist went down to the coffee shop one day, because the walk out to the edge of the University campus provided some brief respite from the laboratory. Along the way the Biologist encountered an Evolutionary Psychologist, who was also going to the coffee shop, and they fell to walking together.
As they entered the coffee shop, they found it crowded with undergraduates, for it was almost Finals Week. Accordingly, they joined the long queue of prospective customers waiting to place an order. Said the Evolutionary Psychologist to the Biologist, “My dear colleague, do you not see this crowd of fertile young people as I do, engaged in a dance of mate selection and competiton that predates our ancestors’ descent from the trees?”
And the Biologist replied, “I don’t believe that our ancestors had access to steamed milk and espresso. Or free wi-fi.”
“You are being amusingly obtuse!” chortled the Evolutionary Psychologist. “The environment may have changed somewhat since the days of our Darwinian origins, I will allow, but ova remain much dearer than sperm cells.”
“That much is certainly true,” said the Biologist. “But I am not sure how much it matters to the coffee-shop flirtations of undergraduates, almost none of which will result in procreative intercourse.”
“Ah,” said the Evolutionary Psychologist, “Perhaps this is a subject wherein my own field has surpassed the expertise of yours, my dear colleague. For instance, we have recently discovered [PDF] that men are more attracted to unintelligent, inattentive women—precisely what one would expect if men have been naturally selected to seek out easy opportunities for impregnation. And this search is doubtless underway all around us at this very moment.”
“That is a remarkable and possibly misogynistic hypothesis,” said the Biologist. “I am most curious to know how it was tested.”
“O! It was most elegantly done,” said the Evolutionary Psychologist. “Some of my colleagues simply asked a small class of undergraduate psychology students—males, of course—to examine photographs of women which were previously selected for their various appearances of vulnerability, and tell whether the photographs indicated vulnerability to sexual exploitation, suitability for a one-night stand, and suitability for a long-term relationship.”
“I see,” said the Biologist.
“Most surprisingly,” continued the Evolutionary Psychologist, “My colleagues discovered that the young collegiate males felt that women who looked drunk, or were standing in compromising postures, or indicating vulnerability in any of a dozen different ways, were both more vulnerable to sexual assault and more suitable for a brief sexual dalliance—but not more suitable for matrimony.
“So you see, my dear Biologist, it is not we Evolutionary Psychologists, who proposed the hypothesis of sexual exploitability, that are misogynists—the only misogynist here is Natural Selection itself, which confirmed our hypothesis.”
“I must beg your pardon, dear colleague,” said the Biologist, “but I am afraid I do not understand the basis for your conclusion. In order for this discovery to have any bearing on reproductive success, is it not the case that most human reproduction would need to occur via coerced intercourse?”
“I must confess that this seems to be what the data indicate,” replied the Evolutionary Psychologist. “But we must not conclude therefrom that all men are rapists! By no means, dear colleague. I think it is quite plain that this result demonstrates no more then that all men are potential rapists.”
“But I remain perplexed!” said the Biologist. “Surely rape is an inefficient way to reproduce, since babies traditionally require a good deal of care after impregnation, and women have long known how to un-plant unwanted seeds.”
“That,” said the Evolutionary Psychologist, “is an important question to be resolved by additional study! But of course it need only be the case that the occasional coercive impregnation could increase a man’s reproductive success, however slightly, for Natural Selection to grab hold.”
“I suspect,” said the Biologist, “that you attribute greater efficiency to Natural Selection than this evolutionary force truly possesses, my dear colleague. But even if drunken collegiate hook-ups were a viable avenue for procreation, you must concede that there would needs be some genetic basis for the tendency to reproduce in this fashion, if Natural Selection is to act upon it. Do you truly believe this to be the case?”
“What a peculiar question!” exclaimed the Evolutionary Psychologist. “I thought that you Biologists were well aware that, in the absence of evidence to the contrary, it is quite safe to assume that any and all aspects of human nature have a heritable genetic basis. Would you truly require the demonstration of heritability in order to conclude that an observed trait or behavior is adapted by Natural Selection?”
“Indeed we would,” said the Biologist. “Such a demonstration, in the case of a tendency to sexual coercion, would be considered most remarkable in its own right, in the scholarly journals of my discipline.”
“What a boring and backward discipline you practice!” said the Evolutionary Psychologist. “Truly, it is no wonder that your field has seen no great advance this last half-century, even as we Evolutionary Psychologists dissect the very nature of humanity.”
“Your ambitions,” said the Biologist, “are indeed remarkable.”
At this juncture, the two colleagues found that they had reached the front of the queue, placed their orders, and went their separate ways.◼
Goetz, C., Easton, J., Lewis, D., & Buss, D. (2012). Sexual exploitability: Observable cues and their link to sexual attraction. Evolution and Human Behavior DOI: 10.1016/j.evolhumbehav.2011.12.004
Over at Neuroanthropology, Greg Downey’s launching an ambitious project: making evolutionary psychology less … shitty.
More specifically, and more politely, Downey thinks (as I do) that evolutionary biology can tell use some valuable things about human nature; but he’s concerned (as I am) that the most visible representatives of an academic field which takes the evolution of human nature as its central question often apply an impoverished understanding of evolutionary biology to telling titillating (and usually unsubstantiated) adaptive fairy tales. Which fairy tales all seem to take place in a sort of dark Lake Wobegon, where all the women are weak and choosy, all the men are strong and horny, and children are barely more than notches on the bedpost of natural selection.
Against the strong man/choosy woman story, Downey proposes the “long, slow sexual revolution.” The central idea is that, as our ancestors’ intelligence increased toward modern humanity, their interest in, understanding of, and uses for sex and sexuality changed:
The idea of the ‘long, slow sexual revolution,’ I think, provides a simple and balanced umbrella for pulling together contradictory elements of our sexuality, gender relations, and reproductive strategies. Everyone knows that the more recent ‘Sexual Revolution’ didn’t erase pre-existing sexual mores and patterns, but rather mixed with them, producing a conflicted, sometimes-unpredictable pattern of sexual expression. Starting with a ‘sexual revolution’ rather than the Men-are-from-Mars-Women-are-from-Venus story means less erroneous leaping to stereotypes to undo when we teach or communicate about human evolution. [Emphasis sic.]
In one of many insightful points, Downey draws in Emily Willingham’s recent post on family planning before the Pill—humans have had the intelligence, and the means, to use sex for more than making babies since (probably) before the dawn of recorded history.
That’s really only the jumping-off point of a post that delves deep into the problems of evolutionary psychology and what might be done about them. And it’s the first part in a promised series! So go read the whole thing, and keep an eye out for future installments. I’ll certainly be watching with interest. ◼
I have a history with Jesse Bering’s evolutionary psychology writing, and I do, in fact, have better things to do over the holidays than deal extensively with his latest offense against evidence-based reasoning. But this one is pretty egregious: Bering pretends to be an advice columnist counseling a (hopefully imaginary) “hebephile” that there is a perfectly good adaptive explanation for lusting after “very young girls,” even if our insufficiently evolution-conscious society frowns on it. Oy.
Bering cites a previous column arguing that attraction to young adolescents could be adaptive because youth correlates with fertility. Said column is conspicuously devoid of biological data. However, five minutes with Google found me an abstract that puts the age at which women’s fertility is up to full adult capacity at about six years after their first periods. Given an average age of menarche at 12.5 years, that means it should be most adaptive to lust after, um, 18- to 19-year-olds.
Of course, there are also all sorts of environmental and cultural factors to consider—that second paper I cited above is a study suggesting that increased obesity may lead to earlier onset of puberty. There’s also the question of whether there’s a genetic basis to finding a particular age cohort attractive, and whether the expected gain in reproductive output associated with attraction to women at exactly their age of peak fertility is enough to overcome genetic drift. Modern biology has data and understanding to apply to all these questions, but Bering can’t seem to be bothered to mention any of it.
If we get a response from Bering, I expect it’ll to be in line with his tweeted answer to critics and his previously demonstrated inability to do anything other than double down on whatever he’s already said. My assessment, which isn’t new, is that Bering’s writingstronglysuggests he’s not interested anything so boring as what we can deduce from actual evidence. Especially if it would get in the way of a nice, juicy headline.
I certainly can’t prevent Bering doing whatever brings in the page-views, but I do wish he’d stop calling it “science.” ◼
It’s … actually kinda plausible. But I think that, if Guillaume manages to overcome his vexation, he might also note that there’s probably some sort of marginal fitness benefit associated with landing a regular gig at Slate. ◼