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Robert Kurzban

The Evolutionary Psychology Blog

By Robert Kurzban

Robert Kurzban is an Associate Professor at the University of Pennsylvania and author of Why Everyone (Else) Is A Hypocrite. Follow him on Twitter: @rkurzban

To Which Organisms, If Any, Does The Logic Of Adaptationism Apply?

Published 20 March, 2011

Will Wilkinson, who I note I respect a great deal, recently blogged about a reply to a post by Jesse Bering – why does this seem oddly familiar? – in which he (Bering) wrote about homophobia. Wilkinson claims that the hypothesis Bering discusses “lacks merit in the way that hypotheses in evolutionary psychology so often do.” (his italics.)

I don’t want to talk about homophobia or the research in question. I only want to talk about the claim made by Jeremy Yoder, who Will points to and quotes at length in his post, in which Yoder teaches Bering and his readers about what constitutes evidence that a trait is adaptive. Here is what he says on the topic, and please notice that there is no mention of homophobia or hemophilia or anything like that, but rather simply a broad claim about what it means to say that a trait is adaptive (his emphasis and quotes):

When evolutionary biologists say a trait or behavior is “adaptive,” we mean that the trait or behavior is the way we see it now because natural selection has made it that way. That is, the trait or behavior is heritable, or passed down from parent to child more-or-less intact; and having it confers fitness benefits, or some probability of producing more offspring than folks who lack the trait. Lots of people, including some evolutionary biologists, speculate about the adaptive value of all sorts of traits—but in the absence of solid evidence for heritability or fitness benefits, such speculation tends to get derided as “adaptive storytelling.”

The key bit is in the last sentence, that a claim of adaptation requires either 1) evidence for heritability or 2) evidence of fitness benefits. (I’m not sure if he intends that “or” literally. From the material above, it sort of looks like he intended “and” there.)

On the first part, it could be that biologists have changed the way that they use the word “heritable,” but it historically hasn’t meant “more-or-less intact,” it refers to the extent to which differences among individuals are due to differences in genes. (If Yoder is right, someone needs to update Wikipedia. And all the biology textbooks.) Further, adaptations generally show very low heritability; when a trait is under selection, genetic variation is reduced by the process of selection. So if one showed evidence of heritability – as one would find for a trait such as hair color – it’s not clear at all one would want to use this to support a claim of adaptation. (It’s possible he meant “inherited” here. But I’m not familiar with this “more-or-less intact” gloss of heritable, so I’m not sure what to say about it.)

His second criterion is fitness benefits. This is probably a lot easier to measure in moths than humans – and the problems with inferences derived from measuring reproductive success has been discussed at length elsewhere (Symons, 1992), so I won’t do so here – but the point is that it seems that by pointing to heritability and reproductive success – and citing Gould’s spandrels paper – as the sole sources of evidence in favor of claims of adaptation, he’s denying Williams’ adaptationist logic, that form (or behavior) can be used to make inferences about adaptive function.

I wonder a little if he’s sincere about this. If he genuinely thinks that you have to have heritability data or reproductive success data to make a claim about adaptation, then he certainly would have a problem with the seahorse work that I blogged about recently. Recall that the claim there about the function of the seahorse body shape was made with a little mathematical model, with no data on heritability or reproductive success at all. Also, not long ago I was reading a blog in which the author casually made an adaptationist claim that a certain species of moth had “specialized mouthparts” designed to carry pollen – clearly an adaptationist claim – and it seemed to me the claim was based on a (quite reasonable) inference from the shape of the trait, rather than any data on heritability or differential reproductive success of individuals with different mouthpart shapes, though I admit I didn’t read carefully, and it’s possible I misunderstood.

Generally, though, if one takes him at his word, it does seem as though Yoder is denying that you can use the clear lens and photosensitive retina as evidence for the view that eyes are for seeing. (If he does deny this, then he disagrees even with Gould, who was happy to admit that “eyes are for seeing” without data on heritability.)

I want to repeat that I am taking no stand at all on the content of the Bering post. My discussion here is solely on the logic of making claims about adaptation.

Given how many claims of adaptation are made on the basis of structure or behavior in the non-human literature (and the non-human blogosphere), just as a statistical matter it seems as though opposition to the logic of adaptationism should be plentiful in these contexts. But instead, for some odd reason, it seems – to me, anyway, and I confess my sample is biased – that objections to the logic seem to appear with some regularity for one just particular part of the natural world, the computational mechanisms designed around human social behavior.

From Yoder’s quote above, I think that Wilkinson and Yoder think that the researchers on the seahorse work – and those who do work based on Williams’ adaptationist logic – similarly “missed the memo” about the problems with the logic of adaptationism. If they think the reasoning in that paper is wrong, and one can’t support adaptationist claims in this way, I’m sure we’ll see them post similarly scathing critiques of work in the non-human domain that recruits adaptationist logic. It’s probably just a coincidence that when people get exercised about the reasoning behind inferring function from form, it just happens to be in the context of one tiny part of the natural world, the computational mechanisms that underlie human social behavior.


Symons, D. (1992) “On the use and misuse of Darwinism in the study of human behavior” in Barkow, J., Cosmides, L. & Tooby, J. (eds) The Adapted Mind: Evolutionary psychology and the generation of culture (New York: Oxford University Press)

  • http://vertebratesocialbehavior.blogspot.com Clara B. Jones

    I found the attached of much interest as a reflection of how evolutionary psychologists and other evolutionary biologists infer that their observations may be adaptive. Ideed, at a certain level, all of the examples in this piece may be viewed and examples of evolutionary psychology (i.e., the adaptive value of intelligence in relation to sexual selection):


  • Martin

    You might want to pay a visit to Larry Moran over at http://sandwalk.blogspot.com/
    Larry often cites (and obviously likes) Gould and doesn’t really like evo psychologists, being a professor of biochemistry as he is, and often derides their ‘just-so-you-know-what’. I’d love to see a reasoned argument between him and a evo psychologist over how someone actually distinguishes between an adaptation and a spandrel and, for instance, non-adaptive complexity/outcome of genetic drift or whatever.

    On that note, do you know of any good literature that tackles the issue (to reiterate, the issue being ‘how to tell an adaptation?’)

    • Marco DG

      I’ll point you to some “classic” relevant papers:

      Adaptationism in psychology

      Andrews et al. (2002). Adaptationism – how to carry out an exaptationist program. Behav Brain Sci, 25.

      Schmitt & Pilcher (2004). Evaluating Evidence of Psychological Adaptation. How Do We Know One When We See One? Psychol Sci, 15.

      Durrant & Haig (2001). How to pursue the adaptationist program in psychology. Philos Psychol, 14.

      Buss et al. (1998). Adaptations, exaptations, and spandrels. Am Psychol, 53.


      Ketelaar & Ellis (2000). Are Evolutionary Explanations Unfalsifiable? Evolutionary Psychology and the Lakatosian Philosophy of Science. Psychol Inquiry, 11 [See also the authors’ response in the same issue].

      Conway & Schaller (2002). On the Verifiability of Evolutionary Psychological Theories: An Analysis of the Psychology of Scientific Persuasion. Pers Soc Psychol Rev, 6.

      Good general reference

      Tooby & Cosmides (2005). Conceptual Foundations of Evolutionary Psychology. In Buss (ed), The handbook of evolutionary psychology. Wiley.

  • http://vertebratesocialbehavior.blogspot.com Clara B. Jones

    I would suggest the following two readings from Westneat, D.E., & Fox, C.W. 2010. Evolutionary behavioral ecology. New York: Oxford.

    Fox & Westneat (Chapter 2: Adaptation)
    Hunt & Hodgson (Chapter 4: What is fitness, and how do we measure it?)

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