I get pushback on the sexual-selection theory for sexual dimorphism

December 18, 2016 • 12:33 pm

Last week I published a post intended to show that the profound sexual dimorphism for human size (and musculature) reflected sexual selection in our ancestors, a form of selection that can be explained only by an evolved difference in behavior: in humans, as in many other species in which females invest more in reproduction than do males, males (who often make little reproductive investment, sometimes only sperm) must compete with each other for access to females. The behavioral difference is a marked tendency to be promiscuous, compared to the greater choosiness of females. That behavioral difference is in turn a direct result of an evolved difference in gamete size and reproductive investment between males and females.

Competition among males for females can involve either direct male-male “battles” (as in elephant seals, gorillas, and chimps, as well as stag beetles and deer), or female choice of males based on of their ornamentation (as in African widowbirds, peacocks, and lions). I think the size dimorphism of humans is more likely a result of male “battling” for dominance and access to females than simply female preference for large males, though of course both factors can be involved. But regardless of whether the sexual selection involves inter-male competition—what Darwin called “the law of battle”—or female preference, it implies a behavioral difference between the sexes, and one involving the traits most crucial for evolution: those directly involved in sexual reproduction.

I also adduced four other bits of evidence predicted by the sexual selection hypothesis, which you can see at my earlier post. Those predictions were made before the data were collected, and they were confirmed. There are many other data supporting the sexual selection theory, and I’ll discuss them tomorrow. One I’ll mention now is that the measured variance in reproductive success among human males is higher than among human females, particularly in hunter-gatherer tribes. That is, in such groups some males leave a lot of kids (and thus their genes) and many others leave none, while, in contrast, the variation in offspring number among females is much lower. The difference in variance between males and females, by the way, is directly correlated with the degree of sexual dimorphism in those groups: the greater the difference in variance among males than among females—and thus the more polygynous the society—the greater the sexual dimorphism for body size. That relationship is a prediction made by the sexual selection theory.

Now, however, Holly Dunsworth, a biocultural anthropologist at The University of Rhode Island, has taken issue with the long-accepted theory of dimorphism (it’s not mine; Darwin was the first to suggest it!), and goes after me in a blog post called “In man’s evolution, woman [sic?] evolve too.” (That post was also picked up and supported by Jesse Singal in a column in New York Magazine, which makes the same errors as Dunsworth).

Dunsworth offers her own thesis, which, she says, puts more emphasis on female evolution.  I suspect her own hypothesis is in fact ideologically driven, and also neglects the possibility, which I did indeed raise, that female preference has evolved.  Apparently anthropologists bridle when what evolves in females during sexual selection is psychology rather than morphology! I also see that Dunsworth has emitted a very long string of tweets about my piece, which suggests some obsessiveness about the sexual-selection hypothesis and male-male competition. I don’t engage in Twitter battles, which are unproductive, but will make my positions clear on this site.

At any rate, here is Dunsworth’s own theory of why human males are bigger than females (I’ve put her theory in bold).

It’s not that Jerry Coyne’s facts aren’t necessarily facts, or whatever. It’s that this point of view is too simple and is obviously biased toward some stories, ignoring others. And this particular one he shares in this post has been the same old story for a long long time. [JAC: Yes, because it’s supported by lots of diverse evidence and makes predictions that have been met!]

What about the other side of the body size sexual dimorphism story?

What about the women?

Selection could well be the reason they stop growing before men and why they end up having smaller bodies than men, on average.

Perhaps men can make babies while growing, but perhaps women can’t. Energetically, metabolically. So reproduction wins over growth. We reach sexual maturity and stop growing. Is that just a coincidence?

Why doesn’t this (and other tales) fit alongside the big-aggressive-males-take-all explanation for sexual dimorphism? #evolution

Not only is it absent, but selection on women’s bodies be the driving force (if such a thing could be identified) and, yet, it’s as if women don’t exist at all in these tales except as objects for males to fight over or to fuck (but *thankfully* there’s that female choice!).

Knowledgeable people aren’t objecting to facts, as Coyne suggests. They’re objecting to biased story-telling and its annoying and harmful consequences, which Coyne doesn’t acknowledge or grapple with in his piece. [JAC: I do indeed acknowledge that we must be mindful of the misuse of biological facts, and not use what we deem “natural” to make social policy. Did Dunsworth even read what I wrote?]

I’ll respond to her hypothesis tomorrow (she calls mine a “story,” a snarky way of denigrating it since there’s ample evidence supporting the sexual selection hypothesis), but right now I want to make three points:

  • Dunsworth, who says that I am a sucker for unsupported just-so stories in evolutionary psychology, doesn’t seem to realize that I have a long history of criticizing adaptive evolutionary-psychology stories unsupported by evidence (go here, for instance).  In fact, evolutionary psychologists used to be mad at me, considering me overly critical. But there are some aspects of evolutionary psychology, like that of human sexual behavior mirrored by sexual dimorphism in body size, that are more scientific, for they make testable predictions that have been met. It would be churlish and intellectually blinkered to ignore both this hypothesis and the evidence that supports it, equating this to more speculative adaptive hypotheses that I’m warier of.
  • Dunsworth’s own “story” really is closer to a story, as it’s contradicted by the known facts about human reproduction. I’ll let the readers figure out what those facts are.
  • Finally data on the nature and traits that are sexually dimorphic in humans have, as noted above, been predicted by the sexual-selection hypothesis but not by Dunsworth’s “growth and reproduction tradeoff hypothesis.” So not only is her hypothesis contradicted by data already known, but is countered by many facts about sexual dimorphism in body size, not only in humans, but also in our primate relatives and other animals. Comparing the sexual selection theory with the tradeoff story, it’s clear that the former is the best explanation for the facts.

I conclude that Dunsworth knows nothing of my history of writing on evolutionary psychology and, further, is remiss in her own scientific approach, offering a story that’s amenable to her ideology because it allows females to evolve (mine does too!), and also a story that not only fails the empirical tests, but can’t predict the observations that sexual-selection theory can. Too, there seems to be more than a touch of intellectual mendacity in the way both she and Singal blithely ignore the supported predictions of the sexual-selection theory. Believe me, it’s more than “just a story.”

I’ll have more to say on this tomorrow, but am throwing it out here now for the readers to chew on.

Male and female gorilla. Guess which one’s the male?

124 thoughts on “I get pushback on the sexual-selection theory for sexual dimorphism

  1. One sex evolves and the other doesn’t? Did I get that right? I can see that for variation on Y chromosomes, but other than that…

    1. Well clearly there were pre-existing evolved differences between males and females in their reproductive systems, as we can see from distant outgroups, but yes, when male-male competition acts during sexual selection, it’s possible for males to evolve much more than do females. Similarly, it’s possible for females to evolve but not males, as when there are distinct selection pressure on females (to protect offspring when fathers aren’t around, and so on)–and those changes need not reside on the sex chromosomes.

      1. But given that evolution is “just” changes in distribution of genes in a population over time, how do these genes just vary in half the population? (Except for Y chromosome genes.)

        1. They don’t vary in half the population, they vary in both males and females but are expressed in a sex-limited way. After all, man carry the genes for breast and vaginas, and women carry the genes for penises and beards. The genes just aren’t expressed, presumably because of hormones or some other sex limited form of transcription or translation control.

          1. Yeah, I know… I’m just entertaining my inner pedant. Apologies for the distraction, it really was off-topic.

        2. Their frequency doesn’t vary between the sexes, but the pattern of expression of those genes does. Sometimes a gene in one sex will be almost completely suppressed, others will be expressed differently to produce variations between the sexes. (Some species don’t have sex chromosomes; sex is determined by environmental conditions such as the temperature during development, so all sex differences result from differential expression.)

          Natural selection (including sexual selection) can only operate on genes that are expressed, so would be neutral against genes within the sex that does not express them.

  2. Why would she argue against such well-demonstrated facts….my guess her real purpose is to raise her own profile in an ideologically driven community that, relying mostly on confirmation bias, is more concerned about conclusions than the facts/logic supporting those conclusions. Now she appears in this blog. Success!

    1. Yes, there is a big disparity between the numbers of readers of that website (run by several Penn State and Rhode Island folks) and here, and she notes that in her post, saying that ‘high profile’ folks like PCC(E) have to be extra careful. (Her post has but two comments, one by herself and the other by another professor who writes for that site.) But I’m more concerned with teaching readers the facts, and how to discriminate among hypotheses, than about Dunsworth’s motivations.

        1. Perhaps, but when entering into a discourse with someone who makes statements such as “… facts aren’t necessarily facts, or whatever.”, be they ID’ers, YEC’s, or regressive leftists, I’m not sure any facts, or “facts” matter; they can be disregarded in favor of pet hypotheses that fit one’s ideology.

      1. Your concern with teaching readers the facts and how to discriminate among hypotheses is much appreciated. Thank you! I’m looking forward to your next post on this subject.

  3. Leaving all the important argument and refutation process of science aside – and coming down to earth – I get the impression that someone has deeply – almost emotionally – committed themselves to an idea – or specifically, a conclusion. In other words, I can’t imagine “Jerry Coyne” caring in the slightest if, through his reasoning, reached a different conclusion than he did. It’s not the destination but the journey that matters. Why do I seem to see this everywhere nowadays?… don’t answer that.

  4. I look forward to your more thorough debunking tomorrow, even though her “hypothesis” seems to debunk itself based on commonly known facts.

    I really appreciate how you go into this from a fact and data based perspective. A great deal of people who argue against people like Dunsworth, even though they may be right, argue from just as much an ideological standpoint as she does as opposed to a science and data based one and I find your site to be a great way to further my own knowledge without simply confirming my notions with rhetoric. Stay Awesome.

  5. “Perhaps men can make babies while growing, but perhaps women can’t. Energetically, metabolically. So reproduction wins over growth. We reach sexual maturity and stop growing.”

    Perhaps I am misunderstanding her point here, but females of many species, humans included, become reproductively mature before reaching their maximum size, although in many species, I believe,there is a minimum size required before sexual maturity. They do not stop growing when they begin to reproduce. In tortoises and turtles, for example, the number of eggs increases as the size of the mother increases over time but there is a size minimum required to begin egg production, and as I recall, captive animals who have their dietary requirements met (and then some) often reach sexual/reproductive age/size sooner than wild counterparts. Now, perhaps growth of younger mothers is slowed while pregnant, I don’t know, but I’m not aware of people or other animals who stop growing simply because they’ve reached sexual maturity or reproductive age, or did I misunderstand her just-so story?

    1. Yes, that’s one point: both males and females become sexually mature and capable of reproduction about the same age (12-13), though of course we’re not sure about that in our ancestors. And they keep growing after that, though again, the condition of our ancestors isn’t known here.

      1. AFAIK she’s also just wrong when claiming women stop growing sooner. Both male and female humans stop growing at about the same age (18ish).

        So, her hypothesis relies on two fact-claims, both of which turn out to be observationally untrue. Not a good start.

        Also AFAIK pre-puberty girls are either the same size or slightly bigger than the boys. Whatever is causing the difference, it (a) occurs after the onset of puberty, and (b) induces a chance in the rate of growth over what is, on average, the same time of growth for both men and women.

        Sounds kinda biological to me. It would be somewhat remarkable for there to be a cultural characteristic that was common to all human cultures and only affected growth in such a specific way.

    2. This is exceedingly common. But in birds and mammals which expend a lot of energy in rearing I think that it is also the case that the younger females are less successful at getting their young to survive to independence. I generalize here, because of course in many birds and mammals the males also help in rearing, but it is reasonable to assume that the male is also young in a good proportion of cases.
      Now comes the interesting problem in trying to explain why a trait that seems to be strongly selected against (a high mortality of young from parents that reproduce at a young age) should be so pervasive in the animal kingdom. I can put forth a few ideas, but here is one: Not every trait that emerges in the surroundings of natural selection needs to be the result of natural selection.

      1. I don’t see much of a problem explaining low reproductive success among young parents. Mortality isn’t the whole story; those who wait till they’re older are passing up reproductive opportunities, and there will be selective pressure against that as well.

        So the optimal time to begin reproducing is when those two opposing pressures are balanced, and the fitness benefit of success starts to outweigh the cost of failure. On this view, we should be surprised if first-time parents had the same success rate as seasoned veterans, because that would mean they waited too long.

        1. Yes, both to you & tgdavies below. Your reasons are not only reasonable, I think they are more plausible than mine. I simply did not want to put up the whole story.

        1. Also in relatively long lived species there may be a learning component in breeding success. Breeding attempts by naive parents may have low success but the parents gain valuable experience so they are better at it when the next attempt comes round.

  6. I read somewhere that human male physique has a display function as well as other utility. The conjecture was that men with well developed shoulders should affect a warning to competing males apart from the use of these muscles in battle.
    In addition it has been advanced that male beards are designed to increase the apparent head size and thus present a more formidable display to competitors. Maybe the male gorilla’s crown serves the same purpose.

    Along these lines females may evolve preference for larger, more hirsute men as a way to increase protection. I would think this aspect should be testable by asking women what they find attractive in men. Would any ladies like to venture an opinion?

    1. I read somewhere that human male physique has a display function as well as other utility.

      That would be in “QG” magzine, or one of the multitude of “mens grooming and health” magazines that litter the shelves between the frankly pornographic mags on the top shelf and the “home, cooking and make up” mags at waist height?
      Personally, one group of those magazines is suspiciously homoerotic, while the other group displays a deep fear of the cheating of males who fail to provide for the children – as far as I can tell from the covers. Ah, such transparency of what is considered important and fear-inducing to their target audiences.

      1. Well, no. I’ve never read those rags. No telling where though. Isn’t it a bit unnerving to remember something but be unable to dredge up the exact reference. Oh, well.

        1. Some people live their whole lives like that, or even don’t understand the idea of listing one’s sources.

    2. Regarding beards, I’ve got my own hypothesis, strictly unscientific and based mainly on hearing Joe Rogan yell “right on the button!” too many times: beards serve as padding for the chin, which as we all know, is one of the most susceptible points for knockout strikes in fistfights.
      In other words, another workaround for evolution’s shoddy engineering 🙂

  7. Gorillas evolved so that males are much larger than the females. Men are somewhat larger and stronger than women. A huge majority of human societies permit a man to have multiple wives. As Dr. Coyne states here, the variance in offspring numbers for males is much greater than that of females.

    It isn’t strictly physical prowess that gives some men a reproductive advantage over others. Humans are thinking animals, and their brains are more important than their biceps. Does sexual selection theory predict some difference in the brains of men and women? Does it explain the difference in mathematical ability?

    1. Geoffrey Miller wrote a book about that called The Mating Mind, but that was (as I recall, and I may be wrong) mostly about how art was the equivalent of a penis, used to secure females, and the book was flawed. There are clearly some mental differences that lead to the behavior differences above, but for other stuff there’s not much difference (intelligence, math ability, etc.) except for spatial orientation, which MAY be the result of male hunting in our ancestors (now that is less well supported than the sexual dimorphism).

      1. I wrote a 2013 paper introducing the analogical peacock hypothesis that builds on Geoffrey’s work and integrates it with the social brain hypothesis. Geoffrey largely accounted for a lack of sexual dimorphism in mental aspects of human cognition due to assortative mating and mutual mate selection. I added to this the need for alignment of cognitive abilities that result from having to be strongly communicatively linked. In order to communicate in the way humans do we must be strongly mentally aligned – a necessity for perspective taking, mind-reading and theory of mind skills. I argue that this leads to a lack of strong mental sexual dimorphism in humans while allowing sexual selction to be an important driver in the evolution of human cognition. That mind-reading is so important in human communication and that it requires strong linguistic and cultural shared knowledge necessitates strong alignment and permits only very limited differences in mental states. However, as we exist within this game we are very intrgued by and attentive to the very smallest of differences. This mental alignment for communicative purposes does not preclude other selection factors retaining pushes towards sexual dimorphism – notably in more physical aspects. The section at the end of this paper snappily entitled “Mutually Entangled Encephalization” deals with this most clearly.


          1. I would like to contribute to the discussion and I think the paper is an important addition here as it is directly relevant. I am wary about getting too drawn into these discussions as they too often descend into flame wars that help few people, and send everyone back to their respective trenches. Although, from what I have seen of your moderation and advice in the last day I am convinced you are working hard to avoid this. I am a social psychologist with feet in both evolutionary psychology and the social worlds, so I see both sides often. I also think that there is a way through the all too often ideological mess that this debate so often descends into.

          2. In short, I argue that mind-reading abilities became a sexually selected mental trait in humans as it serves as a proxy for socio-political astuteness and ability to rise up through the ranks of a social hierarchy. Once it becomes a sexually selected trait then classic arms races lead to a need to display. To be able to display mind-reading abilities obviously requires strong knowledge of what is in the mind of others and how they function. Rather than a sexually selected positive feedback process there is a negative feedback process at work here, aligning minds and making them more alike – at least within a shared value system of some cultural and social hierarchy.

            In broader terms, that are relevant to the current discussion, one moralistic reading of this view would be that if you can communicate with someone – or if you could do so if you were linguistically capable of doing so – it is probably worth considering them to be your equal. The content of what they say and manner in which they say it may be more or less disagreeable to you, but that would be according to your personal taste, and to the degree to which they have shown themselves capable of reading your mind. You can then seek to elevate their social status (seek a potential romance, or friendship) or walk away from them as not worthy of the social effort.

            I think this is a view that would be tenable to most in the more traditionally social domains. Although I am not so naive as to think it will be acceptable, as it is all too easily dismissed as another evolutionary psychology “just so story.”

            I also have no expectation for people to buy into this view without some direct empirical testing of the hypothesis and evidence generated as a consequence of it. This is why I have not been really attempting to tout this paper to date. It is also why the word “hypothesis” is firmly and explicitly placed within the title. I am currently doing some of that work, although, as ever, it is a much slower process than I would like. I do think the mechanism has been outlined in sufficient detail in the paper to allow for the generation of experimentally testable hypotheses in a number of domains. So it could all be wrapped up quite quickly should the data show itself to be unfriendly to me.

            Even then I think there would be some remaining value in the ideas – value that is relevant to the current discussion – in showing how there may be pathways from an evolutionary way of thinking that do not necessarily point to a view of humans as abhorrent to current social values. It is this fear of finding out that there may be some naturalistic reason that justifies or legitimises the worst aspects of human behaviour that drives the ideological wilful ignorance of evolution in many social science quarters – even in spite of knowledge of the naturalistic fallacy.

    2. Does sexual selection theory predict some difference in the brains of men and women? Does it explain the difference in mathematical ability?

      At the upper end of the mathematical ability spectrum, probably yes. I don’t know if this theory has a particular name attached to it or how thoroughly it’s been investigated, but I summarize it as “men tending towards extremes.” In a bell curve distribution of a given attribute, the male curve will have (slightly) thicker tails than the female one. To take your example, men are are more likely to be mathematical geniuses, but also more likely to be mathematically illiterate (I don’t know if there’s some sort of condition that’s been identified for that).

      This probably has something to do with sexual dimorphism. It’s also a third rail in our discourse about human nature. A Harvard economist was sacked for explaining the prevalence of male physicists (or some other STEM field) in these terms a few years ago.

      1. Perhaps mathematical ability is not worth focusing on. In my experience, most people regardless of sex, are not very good at math, and probably most would never be, even given every advantage to become so.

        1. However, what Zardo says also applies to intellectual ability in general. Men are more likely to have both geniuses and complete idiots (greater variation), while women cluster more around the middle on average.

          1. Excellent point. It should also be noted that people with high intelligence are more prone to mental illness, as a group.

            All of the variability we’ve discussed in this thread are clearly indicated and predicted by the higher variability in male genes and the sexual dimorphism in brain structure.

      2. … men are … also more likely to be mathematically illiterate (I don’t know if there’s some sort of condition that’s been identified for that).

        In any normal distribution, a given portion of the population will be found under the left-and tail and a similar portion under the right-hand tail. We can label the outliers as having “conditions,” I suppose, but labeling them as such doesn’t suggest causation.

        The common label for mathematical illiteracy is (especially since John Allen Paulos’s book of that name) “innumeracy.”

    3. Does sexual selection theory predict some difference in the brains of men and women? Does it explain the difference in mathematical ability?

      Only if women are irresistibly drawn to men who are proficient at maths.

      Sadly, not.

      1. However, I imagine there are many women who are more attracted to those with a higher general intelligence (rather than limiting it to just higher proficiency in math (or “maths,” as Brits say)). It could also be that higher intelligence results in a greater likelihood of financial success or, in the distant past, success in survival itself.

      1. “Besides, an increased intellectual capacity would be of benefit to both sexes. So no.”

        This isn’t a good argument. One could just as well say that increased musculature is a benefit to both sexes. From an evolutionary point of view these things generally come as a trade-off (e.g., muscles and big brains require more energy expenditure) so what matters are selection pressures that lead to differences in the ‘optimum’.

        Obviously, the overall human population is larger than Lucy so there was presumably some benefit to all humans being larger than A. Afarensis, but male humans are still typically bigger than females, quite probably due to the behavioral factors Prof. Coyne discussed. Similarly, human intelligence gains compared to our ancestors clearly apply to both sexes, but there could in principle be a sexual difference there as well if there were different selections between the sexes affected by intelligence.

        I don’t think that last part is necessarily true as it’s not so obvious how intelligence plays into male vs. female behavior and the differences are rather more subtle than with body size. But it can’t be obviously dismissed because “both sexes would benefit.”

    4. I wonder if mathematical ability has provided *any* fitness advantage other than in the last millennia or two.

      1. “Mathematical ability” didn’t evolve in order to solve differential equations or develop abstract algebra. It was initially probably an ability to think abstractly – to plan and predict, that gave those having it an advantage.

  8. In movies today we see a lot of small girls to outperform big muscular men in fights. And look sexy at the same time! This something tell us.

    Somewhere I have read (in Diamond or Dawkins) that small female size has an advantage. That their body needs less resources to survive exactly because of much bigger female investment in reproduction. Males, as “cheaper” in reproduction, can die out in great numbers in case resources become too scarce. So maybe there are reasons for males to become big and females to stay small.

    In the matter of male aggressiveness probably we have to be more careful. Females can be very aggressive to protect their babies. This aggressiveness comes from somewhere if not testosterone. My female cat that lives around my house (don’t like in, just to eat, comes to my window to signal “I want to eat”) is a very aggressive and intimidating animal even to bigger male cats. I have seen her to attack male cats that had “an argument” near the place had his kittens without any provocation and make them run, leaving “in the field” parts of their fur.

    1. Females can be very aggressive to protect their babies.

      I watch a live cam channel that features wild Brown Bears on the Brooks River in Alaska. The bears there make their primary living off salmon, and the prime fishing spot is at Brooks Falls.

      I once watched a female at the falls with three cubs charge and chase off the three biggest, most dominant male bears in the area in the space of ten minutes (by watching the cams, one can learn to recognize specific bears and know their rank in the dominance hierarchy). Although, it should be noted, being killed by male bears is a major reason for cub mortality.

        1. There are actually three or four different cams, all solar powered, so they don’t work during the winter (bears are hibernating any way). I’ve also seen gigantic moose, wolverines, a wolf catching a salmon, and various other fauna.

          They site currently shows highlights well worth watching. Perhaps even the incident I described, but I don’t know.

  9. Competition among males for females can involve either direct male-male “battles” (as in elephant seals, gorillas, and chimps, as well as stag beetles and deer), or female choice of males based on of their ornamentation (as in African widowbirds, peacocks, and lions..

    Although we perhaps ought to add that competition among females for ‘prime’ males also happens. Most mammal females don’t engage in physical violence against each other (such behaviour would be ‘selected out’ as too risky for future progeny) but they do engage in status displays and other behaviours. You could argue that (and I’m proposing a just-so story here) that females compete with each other through ornamentation (artificial ornamentation in humans).

    1. I have heard that women dress for other women or maybe it should be to compete with other women. Have no data to confirm this.

      1. That peculiar odour is the whiff of the TNT/RDX bursting charges in the mines surrounding your current position. I shall throw you a pogo stick to hop your way out of the minefield – but I’d rather not watch.

    2. Indeed. I think the competition in many ways is as hard, if not even more brutal, in its ways between females for prime males and or resources and attention, than between males.

      It is just fought by different means.

      Joyce F. Benenson wrote a very (to my mind) thought provoking book recently about just this, Warriors and Worriers: The Survival of the Sexes (2014)

      What really hit me, were her thoughts about the (possible) evolutionary consequences of these “battles” in (possible) psychological predispositions in todays females.

  10. Not sure who posted this peer reviewed article in Science Advances on Twitter, but it is very much germane to the topic. The title is, “Darwinian Sex Roles Confirmed Across the Animal Kingdom” by Janicke et al (2016), under the DOI: 10.1126/sciadv.1500983 .


    Two questions: how does sexual selection theory handle when investment sizes reverse (eg seahorses), and species where the dimorphism in size reverses (eg: anglerfish, black widow spiders).

  11. “It’s not that Jerry Coyne’s facts aren’t necessarily facts, or whatever.”
    …or whatever?
    this has the hallmarks of giving you Prof(E) the brush off. It’s now not what you wrote and the subject matter but more what I think of you and your piece, she seems to be ticked off at what it all means. The evidence is not for her, it’s a “whatever”!
    How would she explain polyandry or would that be more to her liking, so no explanation is necessary… or whatever.
    Sexual selection is fascinating and proven. How life copes and the complex variance in behaviours in humans and fellow creatures puts us and them, all in the same journey by selection and evolution. No ideology can snap that connection.

    “biased story-telling and its annoying and harmful consequences”
    if she had not noticed the worst has been with us for centuries and played out daily. We are i think, working it out and grappling with our evolutionary past and it is vital we know what we are dealing with no matter how annoying the facts are.

    1. The reference to ‘story-telling’ and ‘facts or whatever’ are tell-tale signs of post-modernist thinking. In a PM world,’facts’ are just part of the narrative. You tell one story, she tells another. Either is equally valid, as they are just narratives. Objective reality is an illusion. What is important is the struggle for power that telling these stories represents. And your (Jerry) story is seen as ‘biased and harmful’.

  12. I believe the key sentence is this:

    Holly Dunsworth wrote: Not only is it absent, but selection on women’s bodies be the driving force (if such a thing could be identified) and, yet, it’s as if women don’t exist at all in these tales except as objects for males to fight over or to fuck (but *thankfully* there’s that female choice!).

    And this hinges entirely not on the facts, but the perspective of how the situation is being described: she feels it sounds wrong, and it reminds her of something sexist, and gives away her ideological reasoning.

    But the exact same situation can be described from a female perspective as well, where females get to decide, pick and choose what the men have to offer, and she then, as an empowered prehistoric woman ogling at men, can decide which ond she likes best her for her offspring.

    The odd thing is the psychological projection commonplace in certain disciplines. The softer they are, they more they accuse others of being ideology-driven. Not they are motivated by ideology, of course, but everything that tends to be lower on the pyramid of emergence (with physics at the bottom, and sociology at the top). The most ideology driven are — of course — physicists and their sexist equations they tend to write down in “rape manuals”.

    1. Thanks for linking to one of my favorite (ridiculous) quotes ever. I know what it is without clicking, but it makes me chuckle just thinking about it.

      1. That brings me to an idea: Jerry could one (slow) day ask readers to post the most ridiculous seriously-meant quotations in or around science. Probably narrowed down some more, since Real Peer Review (on Twitter) is already a treasure trove of nonsense.

  13. “So reproduction wins over growth. “
    Isn’t this like saying that brain and heart are competing for oxygen, therefore, somehow heart is smaller. NO?
    (I am not a biologist, so I am aware that my question might not make sense. :)).

    1. “Isn’t this like saying that brain and heart are competing for oxygen, therefore, somehow heart is smaller. NO?”

      Not exactly, I think. I guess that the hypothesis she is putting forward is that when a woman is gestating she does not have resources to both continue growing and to supply the foetus so she stops growing. Men do not have this ‘dilemma’. The woman could defer breeding until she had reached a larger size but in doing so would achieve a lower lifetime reproductive output (unless achieving larger size gave rise to higher reproductive success later in life to outweigh the earlier wait). It is not a completely implausible hypothesis, a priori, but unfortunately for her the facts don’t appear to support it (i.e there is evidence ha in may species incuding humans, females continue growing after they start reproducing) whereas they do support the hypothesis advanced by Jerry. As Richard Feynman said “It doesn’t matter how beautiful your theory is, it doesn’t matter how smart you are. If it doesn’t agree with experiment, it’s wrong”.

      1. …there is evidence ThaT in maNy species incLuding humans, females continue growing…

        apologies for the missing letters – sticky keyboard at work!

  14. Patricia Brennan who because of her having a glory tunnel(synonym gotten from a loooooong list from: https://namingschemes.com/Vagina_Synonyms) was more receptive in focusing on duck’s vaginas and not just their penises as most researchers historically had done, thereby discovering some female evolutionary adaptations in the battle of the sexes:

    So yes women can identify aspects more readily than males in certain research leading to breakthroughs. However Dunsworth’s inductive grousing pales compared to Brennan’s evidence-based reduction.

  15. What I think is behind sexual selection is the female selecting for maladaptive traits outright (as in intersexual selection, i.e. encumbering tails, flashy colors) or less obviously (as in intrasexual selection, large antlers, or ramped up, deadly testosterone)to keep the population of philandering males in check. Those species that require the male for child rearing don’t have dangerous impediments, even though they would be a honest signal of the male’s fitness. If the peahens required the cocks for brood rearing, they wouldn’t select males who were prone to dying encumbered by a large tail. If you are going to abandon your offspring I am going to make it hard for you to have another brood with another hen down the road, so my chicks will be more fit, the hen seems to be selectively saying.

      1. I suppose, though we are a slightly dimorphic species which may explain why dangerous guys are so darn attractive especially in matrilineal societies where brothers, not husbands, raise their sister’s offspring. Penguin hens who need the male to raise a chick select a male with a lower pitched call indicating more fat stores in the larynx and in general. They don’t value gaudiness.

    1. As Jerry outlined in his original post variance in reproductive output is much greater in males than in females especially in polygynous populations. In such populations, for example your peacocks, virtually all healthy females will mate successfully but some healthy males will achieve many successful matings and others will achieve few or none.
      Given this, it is much more likely that the benefit to your peahen of mating with a male with a magnificent tail is not that he may die before he gets the chance to mate with another peahen (he probably wont) but rather that her own sons will inherit his magnificence and will have a good chance of themselves being successful breeders when they mature, thereby ensuring that lots of copies of her own genes (including those fixing her preference for gaudy tailed males) are carried forward into subsequent generations.

      1. I agree; however I think the lack of paternal input on child rearing in peafowl allows sexual selection to proceed and produce the ridiculous impediment. The male doesn’t contribute to brood rearing but he does contribute faster muscles or a more cunning central nervous system by having the impediment. It explains why fish are rarely dimorphic because neither parent is contributing and why truly monogamous birds with co parenting strategies are much less dimorphic.

        1. Yes. A lot of the evolution of behavioural strategies is about trade-offs. In the case of parental care versus polygamy peacocks perhaps lie at one end of the spectrum and seabirds such as penguins and albatrosses at the other. The male peacock probably doesn’t gain much benefit from helping to rear his young as they can feed themselves more or less straight from hatching so for him he is much better off going off to look for other females (and the female bird cannot do much about it). For the seabird nesting on land or ice but having to travel long distances out to see to find food to feed the chick the opposite is true. Without input from the male the female would not be able to rear the chick by herself so a male who doesn’t help rear his own chicks will probably achieve zero breeding success so the selection pressures drive the behaviour and associated morphology in a different direction. (I am sure there are quite a few sneaky extra-pair copulations in pengun colonies though so some males end up unwittingly rearing someone else’s chicks!). In between the two ends of the spectrum there are species where a male is not absolutely constrained to help rear his chicks in that lone females can succeed in rearing some young to maturity but the improved rate of success achieved by the male contributing outweighs any advantage he might achieve through a strategy of serial mating with and abandoning successive females.
          I am not sure it is true to say that sexual dimorphism is rare in fish – there are quite a few examples of it and various mating strategies associated with the dimorphism. Fish also show some interesting examples of gender not being genetically determined with some species changing sex as they age.

          1. I am getting above my pay grade; however, if the peahen were a strict Darwinist she would select males with perfectly camouflaged coats and tended towards interest in rearing offspring. That she instead selects for just the opposite indicates that the male is superfluous in chick rearing; just as many chicks survive with him as without him in the constraints and resource availability of their jungle environment. The traits she selects for are a win-win for her and her brood. The females obtain fast-twitch muscles and clever minds; the males do as well until they hit maturity. At that point the males either hit the jackpot and produce a lot of offspring or they die sooner rather than later and improve the survival of her breeding offspring.
            Yes there is some dimorphism in fish but tends not to be the dangerous and perplexing dimorphism seen in some birds.

  16. either direct male-male “battles” (as in elephant seals, gorillas, and chimps, as well as stag beetles and deer)

    Jerry, you obviously attended a relatively civilised selection of bars and discos in your youth, if you exclude the human male from the list of direct male-male battlers. Ha-ha-but-serious.

    or female choice of males based on of their ornamentation

    Same locus, with chest-wigs and medallions.

    a biocultural anthropologist

    [Mutters to self, deconstructing the words to et the Latin roots out … this is a profession who make models of people out of yoghurt? Live yoghurt?
    OK, it’s a low shot, but I like the mental image. More seriously … one of the things I’ve always likes about trace fossils is the way that they’re fossils of living organisms, not dead organisms. And as such they give one of the few routes into fossilised psychology instead of fossilised osteology.

    Selection could well be the reason they stop growing before men and why they end up having smaller bodies than men, on average.

    My brain comes to a screeching halt on that. Now, I know that I’ve never wasted much of my time memorising the ins and outs of the size development of the anatomically modern human – it’s something I’ve never had need to know. but I’m pretty sure that it’s not what she implies of males and females developing similarly until a point at which the females stop growing and the males continue. I’m pretty sure that there are multiple growth spurts at different ages, and in some of them females are (typically) ahead of males in the same chronological cohort, and in others the males are ahead, and the whole story is considerably more complex than she implies.
    I too detect the odour of someone who has gone off-track from the hard surface of evidence, to find that the field has been freshly fertilised. Deeply, organically fertilised.

  17. Hi, interesting article. But I have a question, how does the sexual-selection theory of sexual dimorphism explain why human sexual dimorphism has diminished in the past million years.

    Like the Bononobo, humans have moderate sexual dimorphism, and I’ve seen it argued that perhaps human ancestors are much closer to Bonobobo’s than to chimps or Gorillas that exhibit large sexual dimorphism compared to humans.

    It would be interesting to know why sexual dimorphism diminished in human evolution if the sexual-selection theory is correct.

    1. Assuming you are correct in that dimorphism has diminished, let me guess the answer to your question – perhaps the evolution of the human brain toward more abstract and conceptual capacities and a more complex social environment with language lead to a set of adaptive forces that reduced the influence of gross anatomy. Today reproduction and mate selection in humans is diverse over many cultures and influenced by diverse social forces that would not have existed a million years ago.

      1. I think these changes to human societies has happened to late in evolutionary terms (basically in the last couple of 1000 years for the most part) to have had any decisive effects, yet.

        And the dimorphism in height and upper body strength is (if I understand correctly) a human universal, and of (roughly) the same relative size in Europeans, as in !Kung foragers, Mae Enga Highlanders in Papua New Guinea as Yanomama in the rain forest in Venezuela.

        But it would be interesting to travel half a million years forward in time to observe the (possible) changes…

        1. “it would be interesting to travel half a million years forward in time”

          If you do find a way, be sure to drop us a line and let us know what happens. 😎

      2. From my cursory laymen research on the topic, it seems that peloanthropologists (and not all agree) think that sexual dimorphism among early hominids like A afarensis, was much larger than among neanderthals or humans. And that the changes occurred much earlier (by about a million years) than previously thought.

        Not only that but there is evidence that evolution also favored larger females, since compared to our early ancestors females were much smaller.

        It would be interesting, as you suggest to know to what extent did early cultural developments influence our evolution.

        1. “It would be interesting, as you suggest to know…”

          Whenever I consider the subject, it’s frustrating as hell. 8(

    2. Has it?

      I am not so sure, since I think it is important to take into account, how conflicts is conducted and the outcome determined.

      Your argument would be stronger, if it was conceivable that our human ancestors and for example gorillas fought in the same way, but there are many reasons to believe that we didn’t.

      Gorillas (for example) fight (if I understand correctly) by basically running against one another in a way that utilizes shear physical momentum to overpower their opponent. Size, (i.e. volume, weight) is the determining factor here.

      But, after our ancestors got an erect pose, freeing their hands, it is possible humans stopped fighting in the same ways as gorillas or chimpanzees, and started using the same tools they used for hunting, i.e. clubs, spears, and stones.

      And here is the thing, what is critical when wielding these kinds of weapons, is rather upper body strength in combination with agility.

      And, the difference in upper body strength between human males and females, is close to 3 standard deviations, which translates to an approx 100% difference, which happens to be the same difference as between male and female gorillas i physical size…

      I think it is important don’t to get trapped in one-dimensional thinking here 😉

  18. First, as ever, thank you for this. The posts I most enjoy are where you hammer woo creeping into biology.

    Her whole thing about stories/narratives is a tell for Foucault style twaddle.

    Still growing teens can give birth for one thing.

    1. Good ol’ po-mo. Remember, we’re just constructing narratives. Facts are socially constructed…”or whatever.”

      And the fact that her whole idea hinges on the completely untrue assumption that women stop growing once they become able to reproduce is more icing on the very tasteless cake.

  19. Dunsworth seems to be positing a metabolic theory for size differences rather than a behavioral one.
    The trouble is that we are free to examine other species to compare how it holds up. The sexual dimorphism theory fits in very well with how we see the animal kingdom operate. Species that have harems (gorillas, some deer etc) have much greater size differences than species that have long term monogomous partnerships.
    Exceptions to the rule that males are the aggressive patriarchs in such harems – such as hyenas – only prove it the other way around since it is female hyenas that are the aggressors.
    In humans females become capable of bearing children at an age where they are much more similar in size to their male counterparts. In addition, both males and females stop growing at about the same age.
    If Dunsworths theory was correct wouldn’t there be an advantage for males to keep growing until they were in their forties (since that would give them the size advantage)? Or at least keep growing when women have stopped growing.
    Why do human males and females have growth spurts at the same time and also stop growing at the same time if there was a purely metabolic reason connected to childbirth that keeps women smaller?

  20. Holly wrote:

    …it’s as if women don’t exist at all in these tales except as objects for males to fight over or to fuck (but *thankfully* there’s that female choice!).

    Yea, we can’t see any ideology (also beginning with F) there at all.

    Scientific Prediction: she will accuse those who disagree with her ideological anthropology (shall we also look at the record of ANTHROPOLOGISTS abusing science over the decades? LMAO) of sexism and oppression of women in science in the west where 58% graduates are women. She will post tweets disagreeing with her as proof of “harassment”.

    1. She isn’t exactly a clickbait-hunting scientific ignoramus like Jesse Singal (his current article is – I kid you not – “Why Straight Rural Men Have Gay ‘Bud-Sex’ With Each Other”).
      Dunsworth regularly publishes in some good journals. She seems to have strayed outside her area of expertise here, however, and failed to read up on what her target has written.

  21. Hey Jerry,

    Id take it that you have read PZ Myers take on your original blog post about sexual dimorphism in humans (http://freethoughtblogs.com/pharyngula/2016/12/18/natural-selection-is-not-the-only-mechanism-of-evolution/).

    I dont really frequent PZs site very much anymore, though i used to. Now I can of findn a lot of his blog posts “meh”, or just too enthranchent in some of his ideological views (e.g. feminism, SJWs, controversies in the atheist community, and some of his political views). I listen to Sam Harris´s podcasts, and occasionally read his blog posts. In one of the podcasts (i cant remember which one, though a fairly recent 1) he says describes Myers as an “Internet troll”, and has in a much older podcasts said that he doesnt bother dealing with PZ anymore.

    1. One weakness of scientific blogging compared to the old fashioned method of’publishing a paper’, is that the former operates without the peer-review quality filter.
      It used to be easy to dismiss creationist arguments with a simple request to publish their evidence that overturns accepted evolutionary theory and then come back to us.
      Science blogging of the ideological persuasion should be treated in the same way.
      If PZ Myers thinks he can overturn accepted science on this question he is free to do so in the same way we would expect challengers of other accepted scientific theories – namely publish the evidence in a peer reviewed journal.
      It’s funny that Myers regards Jerry’s article as coming from “the conservative side of science”!
      I thought Jerry was a lefty!
      On a serious note, I remember Massimo Pigliucci making the point, a few years ago, that science is inherently conservative (with a small “c”). In other words, it should take a lot of evidence to build up a scientific consensus and, subsequently, a lot of additional evidence to shift that consensus.
      The ‘conservative side of science’, in other words, is how science has come to be practiced, not because it serves one particular group (The Patriarchy, Old White Men, the rich, the Illuminati etc) but because it enables the gradual accumulation of knowledge and the winnowing out of incorrect ideas about how the world functions.

    1. Just to follow up on this, the fact that sometimes males can be smaller than females (anglerfish are another example) does not contradict the standard sexual selection theory, as the theory predicts differences in behavior and bodysize between the sexes based on the degree of resource investment in the gametes and in contributions to raising the offspring. Whereas her hypothesis seems to indicate that there is something more resource intensive about female reproduction, and therefore we should generally expect males to be larger than females across the board in the animal kingdom.

  22. I read in Steve Jones’ “Darwin’s Island” that the gene (or lack of expression thereof) that causes hairlessness also makes humans so much weaker* than our close relatives, the chimps (irritatingly he gives no reference). Indeed, a 40 kg female chimp is several times stronger than our biggest bodybuilder.
    If true, and if the sexual dimorphism in humans was maintained through physical battle between males, how come they lost their hair and strength in the first place? I do not contend -not at all- that it invalidates the sexual selection theory, but it is an intriguing question.

    (The loss of large canines maybe related to using arms/fists as weapons described by Richard Wrangham in his “Demonic Males”, where chimp Hugo gets rid of a ‘large canined’ baboon with a well aimed single blow to the abdomen)

    As for beards, I think they might have had a protective function, protecting jugular and carotid. Like the lion’s and baboon’s manes.

    * [echoes of Samson and Delilah? (I nearly wrote Samsung 🙂 )]

    1. Indeed, a 40 kg female chimp is several times stronger than our biggest bodybuilder.

      I’m skeptical. Olympic weightlifting records are in the range of 200kg. Several times that would be on the order of a ton.

      Wikipedia has a paragraph on why these claims are often exaggerated.

      1. Ok, ‘several times’ is exaggerated. But even if ‘only’ twice the musclestrength, the intriguing question remains the same: if sexual dimorphism in humans was maintained by physical battle between males, why lose so much strength (and hair) in the first place?

      2. If the Wikipedia ref is correct (or if I read it correctly) , twice the musclefibre strength, would still mean a human would need about four times the muscle mass (muscle cross section gives the strength, volume the mass) for comparable strength.
        A 40 kg chimp would still be stronger than a 100 kg beefcake. Not very wise to fight a chimp, I’d say.

        1. Human muscles aren’t twice as long as chimp muscles; they’re roughly the same length. For a given length, mass and volume scale directly with cross-sectional area.

          1. doubt whether the same lenght, even roughly.
            But still no idea why humans lost hair and strength, but kept dimorphism,

  23. Comparative studies of primates generally attribute sexual dimorphism in body mass to male-male competition; however, as noted several times, the amount of variance in size dimorphism that can be explained by male-male competition is often quite low, usually under 50%, suggesting that other factors, such as selection on female size/fecundity/growth, are at play. Mike Plavcan’s paper in 2012 (in the journal Human Nature) seems to be the most balanced treatment of the evolution of sexual size dimorphism in humans.

  24. This is a rather trivial point, but my reading of Dunsworth’s use of the word “story” to describe the hypothesis that human sexual dimorphism is driven by evolved behavioral variation in reproductive strategy – and her less convincing alternative – is not a matter of denigration, per se.

    Instead, I would suggest it’s an expression of that postmodern impulse – common among anthropologists – to frame all discourse in terms of narrative. The explanation for her use of “story” is thus both more mundane and more insidious, because it reflects an ideologically motivated viewpoint that frames the world in terms of competing narratives, whose value flows from the respective social capital behind them, rather than available corroborating or falsifying evidence. In other words, Dunsworth is reflecting the dominant thinking of the larger ideological province from which opposition to evidence-based hypotheses about biological gender tends to derive.

  25. ‘Knowledgeable people aren’t objecting to facts, as Coyne suggests. They’re objecting to biased story-telling and its annoying and harmful consequences, which Coyne doesn’t acknowledge or grapple with in his piece’ – typical confirmation bias. It’s like people who read Richard Dawkins’ ‘The selfish gene’ and accuse him of social Darwinism despite him telling that ‘it would be horrible’ over and over. Ideologies are predetermined in all their stance.

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