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Jerry Coyne seems like a nice guy. He’s a biology professor, written some good books on evolution, done some great work pushing back against creationists…. but he also writes things like this.

One distressing characteristic of the Left, at least as far as science is concerned, is to let our ideology trump scientific data; that is, some of us ignore biological data when it’s inimical to our political preferences. This plays out in several ways: the insistence that race doesn’t exist (and before you accuse me of saying that races do exist, read about what I’ve written here before: the issue is complex), that there are no evolutionarily-based innate (e.g., genetically based) behavioral or psychological differences between ethnic groups, and that there are no such differences, either, between males and females within humans.

I can take issue with nearly all of that (did you know the concept of gender pre-dated second-wave feminism and was the result of academic research?), but let’s pretend I don’t. Let’s buy into Coyne’s premise that “the Left” rejects scientific evidence in favor of ideology, without exception, and see where it takes us.

Thus, to claim, as does P.Z. Myers in a new post, that higher testosterone levels in males have minimal influence on their aggressiveness compared to the effects of culture, is a claim based not on data—which show that he’s wrong—but on ideology. And so he and his commenters try to refute the testosterone-effect notion using anecdotes: some males aren’t aggressive, Myers himself is not aggressive (!), aggression is due “mostly” to cultural difference (the “patriarchy”) rather than to biological differences, and so on.

That’s a rather uncharitable summary of the comments; a contrary anecdote refutes a broad assertion, correlation is not causality, and I can see at least one citation (albeit poorly done).

Well, I’m not an expert on testosterone, but what I do know is that levels of that hormone are not only correlated with aggression within and among the sexes, but that injecting it into both men and women also makes their behavior and psychology more aggressive. Thus the correlation at least partly reflects causation.

Again, let’s just roll with this. If Coyne is in the right, he should have no problem coming up with citations to support this view.

But let’s look at some data showing prima facie that there are biological differences in behavior between males and females, and that those differences reflect the working of natural selection—in the form of sexual selection—in our ancestors. To do this, we’ll use body size as an index of behavior.

Whoa whoa whoa, Coyne just moved the goalposts! Rather than discuss the testosterone-aggression link, he’s now talking about sexual dimorphism. This marks him as a hypocrite right off the bat, as he’s one citation behind “the Left.”

But let’s go down this rabbit-hole too. What’s the evidence for behaviour differences due to sexual dimorphism?

It’s well known that in virtually all species of primates (there are a few exceptions in lemurs), and in other groups such as pinnipeds, males are larger than females. That is not cultural, but genetic; if you rear gorillas in any habitat, the males are going to grow up larger than females. You can see the data among species yourself in a 2006 paper by Adam D. Gordon (reference and free link below), showing an almost universal trend for the male/female body mass to be larger than 1 in primate species (in every species there are of course some small males and large females, but we’re talking about averages).

Ok, but that’s evidence for dimorphism. It isn’t evidence for behavior due to dimorphism. In fact, that citation doesn’t even argue that dimorphism is primarily due to sexual selection! From Coyne’s citation, emphasis mine:

Regardless of why intrageneric and supergeneric patterns differ in the relative influence of sexual selection, it is clear that we cannot account for much of the contemporary variability in [Sexual Size Dimorphism] by sexual selection alone. This is not a new idea, nor is the observation that because forces other than sexual selection account for much of the variability in SSD we should exercise care in reconstructing fossil behaviors based on skeletal dimorphism alone (cf. Plavcan and van Schaik, 1997b; Plavcan, 2000).

Gordon, A. D.  2006. Scaling of size and dimorphism in primates II: Macroevolution.  Int. J. Primatol. 27:63-105. pg. 89-90.

So what, you say? Consider this theory invoking natural selection: it takes a lot of energy to turn a fertilized egg into a baby. The smaller you are, the less energy you need to survive, and thus any surplus food you collect can go towards a growing a fetus. If this theory holds, people with wombs would be naturally selected to be relatively smaller than those without, because otherwise they probably wouldn’t have access to a surplus. A pregnancy could starve them right out of the gene pool. Notice there’s no behavioral component here, so there’s no need for a change in behavior based on your body type. We get sexual dimorphism without behavioral dimorphism!

Coyne and others argue instead that dimorphism is sexually selected. Males and females behave differently, which causes them to prefer mates who were bigger/smaller than themselves. Over time, this causes an explosion of sexual dimorphism much like the peacock’s tail. If sexual selection were the primary driver of sexual dimorphism in primates, it’s almost certain to come with a behavioral component too.

But it isn’t, as Coyne’s own citation concedes. We shouldn’t expect a strong link between behavior and dimorphism, and shouldn’t be surprised if we find none.

We can’t ignore the 900lb gorilla in the room, though: how do you measure dimorphism? Emphasis mine:

patterns of dimorphism differ throughout the dentition, skull, mandible, and skeleton, and such patterns differ between species (Gordon, Green, and Richmond 2008; Plavcan 2001, 2002, 2003). For example, within M. sphinx, possibly the most size-dimorphic primate alive (ratio of male to female body mass 2.69; Gordon 2004), dimorphism in skull dimensions ranges from a low of 1.06 (male mean divided by female mean) for postorbital breadth to a high of 2.5 for zygomatic arch thickness (Plavcan 2002). Orbital dimensions differ dramatically in dimorphism in this species (as in most other primates), with orbital height dimorphism of 1.07 and biorbital breadth dimorphism of 1.73. The implication is that one cannot simply compare variation across traits and assume that they will yield similar signals of size dimorphism for any particular species (Plavcan 2002, 2003).
Plavcan, J. Michael. “Body size, size variation, and sexual size dimorphism in early Homo.” Current Anthropology 53.S6 (2012). pg. S412.

On second thought, let’s ignore it anyway. The consensus in the field seems to be that sexual dimorphism doesn’t map cleanly to behavior or vice-versa. Again, with my emphasis:

The issue of estimating the magnitude of dimorphism is important because dimorphism constitutes critical evidence of behavior and life history in extinct species and thus has weighed heavily in discussions of the evolution of hominin behavior (e.g., DeSilva 2011; Gordon 2006a; Lovejoy 1981, 2009; Martin, Willner, and Dettling 1994; McHenry 1994; Moore 1996; Plavcan and van Schaik 1997a). Dimorphism is one of the only anatomical traits that is directly causally related to social behavior that is preserved in the fossil record (Plavcan 2004a). But dimorphism is a complex phenomenon, and a simple one-to-one correspondence between behavior and the magnitude of dimorphism does not exist (Plavcan 2000a). Dimorphism reflects separate causal factors influencing male and female traits whose expression is potentially limited by the genetic correlation between males and females (Gordon 2006a, 2006b; Greenfield 1992; Lande 1980; Leigh 1992; Lindenfors 2002; Martin, Willner, and Dettling 1994; Plavcan 2011; Plavcan, van Schaik, and Kappeler 1995).

Plavcan [2012]. pg. S412.

But a complicated relationship isn’t the same as a non-existent one. It’s possible that in general there’s no link between sexual dimorphism and behavior, but in the specific case of human beings there is. Alas, that isn’t likely to be true: only extremely sexually dimorphic species show clear causality with behaviour…

in no study is the magnitude of size dimorphism uniquely associated with one or the other breeding system, competition level, sex ratio, or operational sex ratio (OSR; Plavcan 2000a). While very strong dimorphism is invariably associated with polygyny, skewed OSRs, and intense male competition, monomorphism is not uniquely associated with any particular mating system. This means that a lack of dimorphism alone in a fossil sample cannot be used as evidence for monogamy, a humanlike mating system, polyandry, or any other mating system (Plavcan 2004a). [pg. S413]
Plavcan (2012)

… and human beings have unusually low levels of sexual dimorphism compared to our ancestors and fellow primates.

Comparing sexual dimorphism of homo sapiens sapiens to its ancestors. Plavcan, J. Michael. “Body Size, Size Variation, and Sexual Size Dimorphism in Early Homo.” Current Anthropology 53, no. S6 (2012): Table 6. doi:10.1086/667605.Sexual dimorphism among primates. Smith, Richard J., and James M. Cheverud. 'Scaling of Sexual Dimorphism in Body Mass: A Phylogenetic Analysis of Rensch's Rule in Primates.' International Journal of Primatology 23, no. 5 (2002). Table 1, pg. 1099Speaking of which, our ancestors diverged from African branches of Homo Erectus approximately 1.2 million years ago.[1] In the span of a million years, then, we went from fairly sexually dimorphic to barely dimorphic at all. This is hard to explain via sexual selection, because you’d expect runaway positive feedback that kept inflating dimorphism until physics put a cap on it. It fits perfectly with the natural selection hypothesis, though, because that was roughly when our ancestors learned to use fire.[2] We had an easier time feeding ourselves, and thus didn’t have to worry as much about preserving a surplus in case of pregnancy. That’s accelerated with our switch to agriculture and increased tool use. Given enough time, our species’ sexual dimorphism may even disappear.

So the link between behavior and sexual dimorphism is probably weak or nonexistent in human beings.

That leaves Coyne’s claims about a link between testosterone and behaviour. Here at least, I’m on familiar ground.

In non-human animals, the relationship between testosterone and aggression is well established. In humans, the relationship is more controversial. … The present analyses are based on 45 independent studies (N=9760) with 54 independent effect sizes. … Correlations ranged from −0.28 to 0.71. The mean weighted correlation (r=0.14) corroborates Archer’s finding of a weak positive relationship.

Book, Angela S., Katherine B. Starzyk, and Vernon L. Quinsey. “The relationship between testosterone and aggression: A meta-analysis.” Aggression and Violent Behavior 6.6 (2001): 579-599.


A critique is presented of the meta-analysis of testosterone and aggression by Book, Starzyk, and Quinsey [Aggression and Violent Behaviour 6 (2001) 579], and the results of a reanalysis of their data are reported. … A reanalysis that corrected these problems produced a lower mean weighted correlation (r=.08 instead of the reported r=.14). The conclusions from our categorical comparisons were different from those of Book et al.: Neither of their positive findings (a decline with age; lower correlations in morning than afternoon samples) were confirmed. We found significant differences for sex, age, offender status, and source of hormone measure, all of which are different from those in the original analysis.

Archer, John, Nicola Graham-Kevan, and Michelle Davies. “Testosterone and aggression: A reanalysis of Book, Starzyk, and Quinsey’s (2001) study.” Aggression and Violent Behavior 10.2 (2005): 241-261.

That isn’t a strong link at all, either. Here’s a collection of 15 datasets pulled from a Gaussian distribution. One column of this was left alone; one column was skewed according to Archer [2005]’s correlation coefficient; and the remaining column was skewed by Book [2001]’s coefficient. Can you tell me which column is which? Click to make it larger:One column is skewed by r = 0.08, one by r = 0.14, and the third is unskewed. Can you tell which column is which? This isn’t the effect size most people think of when they say testosterone effects behavior. We asserted the testosterone-behavior link before we had statistical tests that could detect an effect this sensitive, so we had to believe this link was large and easily spotted. Since the link is actually small or non-existent, our belief must have been unfounded. It must have been invented by our prejudices and biases, rather than arrived at through reasoned analysis.

Nor is this a new thing, either. Back in 1974, Elenor Maccoby and Carol Jacklin published what quickly became the “bible” of human sex differences. It was so large that it had to be published in two volumes, and much of those thousand pages came from summaries of every relevant study the authors could get their paws on. Hundreds of pages of short summaries made for dry reading, though, so the authors also wrote up a distillation of the major findings of those studies.

One of them was an interesting observation: multiple studies had noticed that boys tended to become less aggressive after puberty started, when their testosterone levels were dramatically higher. [3] So if more testosterone = more aggression, why does it go down when testosterone goes up? Why do boys get less aggressive just as they become able to mate, if competition between them determines who gets the girl?

Coyne not only can’t find a citation for the testosterone-behavior link, he’s promoting ideas that were refuted 40 years ago!

Sadly, he’s not done. After getting some pushback for promoting stories instead of hard science, he fired off a massive post which he claims gives all the proper citations. I’ve only had time to skim it, and it looks like history repeating; there’s no citation for the testosterone-behavior link, but instead a whole new bouquet of assertions, at least one of which is no better than the testosterone-behavior line. He didn’t do a good job of tying his assertions to his citations, though, so if I want to critique his post I’m going to have to painstakingly find those ties myself.

Ugh. And this guy is a professor?


[1] Stringer, C. (2012). “What makes a modern human“. Nature 485 (7396): 33–35. doi:10.1038/485033a.

[2] Francesco Berna, Paul Goldberg, Liora Kolska Horwitz, James Brink, Sharon Holt, Marion Bamford, and Michael Chazan. “Microstratigraphic evidence of in situ fire in the Acheulean strata of Wonderwerk Cave, Northern Cape province, South Africa.Proceedings of the National Academy of Sciences, April 2, 2012 DOI: 10.1073/pnas.1117620109

[3] Maccoby, Eleanor E., and Carol Nagy Jacklin. The psychology of sex differences. Vol. 1. Stanford University Press, 1974. Pg. 351-2.

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