Tag Archives: philosophy of science

More on Buller and Evolutionary Psychology

This is an ongoing series of meditations on evolutionary psychology inspired by my recent reading of David Buller’s piece in Scientific American.  I have been thinking quite a bit in the last year about the relationship between evolutionary psychology, human behavioral ecology, and evolutionary genetics, and maybe these ruminations will help me get my thoughts clear on these difficult topics.  Caveat utilitor: these are not fully formed ideas but the blog is a useful device for organizing my sketches.

I found an interesting  critique of Philosopher of Science and evolutionary psychology critic David Buller‘s book, Adapting Minds. Edouard Machery and H. Clark Barrett wrote an extended, critical review of Buller’s 2005 book in the journal Philosophy of Science.

I must admit that I find myself torn on some of these debates. I am sympathetic to many of the criticisms voiced by Buller, but think that some of the rebuttals are quite compelling as well. For example, Buller is highly critical of work on child homicide by Martin Daly and Margo Wilson of McMaster University.  Daly and Wilson, in a series of famous studies, suggest that child homicide (a rare event) is much more likely to be perpetrated by step-parents (including boyfriends).  The explanation for why this might be relates to the existence of an anti-cuckoldry mechanism in men’s brains. Given the enormous obligate investment — generally on the part of two parents — entailed in the successful recruitment of human offspring, cuckoldry represents a potentially enormous fitness cost for human men.

In one study of child homicides in Canada between 1974 and 1990, Daly and Wilson calculated a risk-ratio that child homicides are perpetrated by step-parents vs. (putative) biological parents of 123.7.  Buller suggests that such results might simply arise because of ascertainment bias in the reporting of child homicide.  Specifically, he suggests that the cause of death listed on a child’s death certificate is far less likely to be homicide if the act was perpetrated by a biological parent. In support of this argument, he cites a paper by Crume et al. (2002) which compared cause-of-death as listed on the death certificate with the cause determined by a interagency multidisciplinary child fatality review team.  This team reviewed child deaths in the state of Colorado and found that a substantial number of likely homicides were not reported as such.  They were then able to investigate which attributes of (alleged) perpetrators made ascertainment more or less likely.  They found that homicides committed by non-relatives (including boyfriends) were 8.41 times more likely to be recorded as such than were those committed by parents. Of 152 death at the hands of parents only 65 were correctly ascertained while 87 were not.  For the 51 deaths attributable to non-relatives, 44 were correctly ascertained while seven were not.  This yields an odds ratio of (44*87)/(65*7)=8.41 that non-relatives will be correctly ascertained compared to parents (the OR changes to 8.71 following multivariate adjustment — it is this number that is discussed in the various papers). This seems pretty damning (and suggests there are major problems detecting fatal violence against children).  However, one point from this paper that Buller does not note in his critique (at least his 2005 paper in Trends in Cognitive Sciences) is that the odds of ascertainment for non-parent relatives — including step-parents — is not significantly different from unity. That is, the group that includes step-parents is as likely to be ascertained as biological parents.  My understanding is that Daly and Wilson’s analysis applies to step-parents as well as boyfriends.  The theory certainly predicts this.

My sense is that Buller is reaching a little too far in this critique. While I would hardly consider myself an expert on the topic, I have always thought quite highly of Daly and Wilson’s demographic work on homicide.  One of my students is currently relying heavily on their Chicago mortality study published in BMJ.  That is something I do have some expertise in and I think it is excellent.  What I want to know is this: what is the counterfactual to the Daly & Wilson work?  How many child deaths would need to be re-classified in order to have ascertainment bias be sufficient to account for their observed differences?  Daly & Wilson (2007) do just this sort of counterfactual calculation.  They assume that step-fathers are always caught, whereas biological fathers are never caught.  According to their calculation, such a scenario would imply that there were 500 unaccounted-for paternal murders to yield the observed rates.  This is where the problem comes in.  There simply aren’t 500 deaths each year to children under five in Canada in that period that aren’t due to congenital defects or infectious disease.  Mortality among the young is rare in developed countries. Clearly, not all of the effect that Daly & Wilson report can be attributed to ascertainment bias.  There seems to be some there there.

I think that this over-reaching is a shame.  The critiques that Buller levels in his recent Scientific American piece are serious and deserve to be taken seriously. Here, I specifically mean the idea that an analysis of the Pleistocene will yield significant clues for understanding the design of the human mind and that evolutionary psychology will be much use in helping us understand unique and universal human traits.  The tone of this debate (on both sides) seems to preclude serious consideration of these important concerns.

As I mentioned in my previous post, I find the latter problem particularly troubling because it suggests that there are some things we can never know about human evolution in a scientific way.  Depending on the question, one possible solution to this problem is something Marc Hauser used to talk about in Science B-29 at Harvard.  The problem was how to use evolutionary tools to explain the unique phenomenon of human language.  While human language is clearly a unique, derived trait — and therefore in a difficult position with respect to scientific explanation — there are features of human language (e.g., those described by Hockett in his design features of human language) that are shared across multiple species, making them amenable to the comparative method.  If we limit ourselves to specific autapomorphies — as Buller apparently wants us to when it comes to Human Nature — then we are sunk.  If we can find features of our cognition that are shared across species and look, as Darwin first suggested, at convergent solutions to similar problems across species, then we may have some hope of understanding the unique whole of human cognition. Of course, we can’t do this for cognitive features that have arisen since the Pleistocene because we only have one remnant of the hominin clade left (us).

Regarding our ability to understand the design of the human brain based on our knowledge of the environment of Pleistocene hunter-gatherers, Machery and Barrett (2006: 236) write that Pleistocene hominins experienced a “reduction in sexual dimorphism in body size due to increased pair bonding and male investment in offspring and corresponding reduction in male-male competition.” While I happen to agree with this point (and have two new papers either submitted or in prep elaborating my take on this particular phenomenon), it is, in fact, conjectural.  There is nothing to stop us from forming hypotheses about the mechanisms or functional consequences of human behavior that result from this conjecture, and there might be substantial value in doing so.  Nonetheless, I think it’s important to note that it is hardly certain that the cause of the reduction in sexual dimorphism among Pleistocene hominins (something we are pretty sure of) was pair bonding.  I’m afraid to say that I am not the least bit confident that we will ever know this for certain.

Why do we think that Pleistocene hominins were “pair bonded”?  We know that sexual size dimorphism is a correlate of mating system.  Polygynous mammals tend to be sexually dimorphic.  The more polygynous, the more dimorphic.  Presumably, this arises through intra-male mating competition, where size matters for the outcome of agonistic encounters.  As detailed in our 1999 paper, the best paleontological evidence we have suggests that there was a substantial reduction in both sexual size dimorphism and dimorphism in canine teeth (another strong correlate of polygyny among Primates) with the emergence of the genus Homo.  This reduction in sexual dimorphism is attributed by many authors, ourselves included, as a signal of a change in mating system toward increased monogamy.  Does monogamy necessarily mean pair-bonding?  Not necessarily. (again, I do think it’s true in this case and hopefully, I will finish the paper in which I discuss the details of this argument soon)  There is also the issue that humans are not much different in terms of sexual size dimorphism from chimpanzees, whose mating system is completely promiscuous.  Our teeth may rescue us here.  Chimpanzees are quite sexually dimorphic in their canine teeth.  But how do you weigh the importance of canines as a weapon in a species that makes tools, including weapons that allow it to kill from a distance?

My point here is that there is a good deal of uncertainty about basic aspects of Pleistocene hominin behavior.  This uncertainty is unlikely to ever be completely resolved.  As a result, I’m not convinced that looking for clues about human behavior and the design of the human brain in the behavior of Pleistocene hominins is necessarily the most efficient of productive avenue for understanding our psychology. I don’t take the absolutist position that Buller seems to take that there is nothing to be learned about the present by studying the deep past (i.e., it is more than “pure guesswork”).  I like the iterative approach of working between hypothesis generation and empirical test that Machery and Barrett describe and think that it sounds an awful lot like the process that most scientists employ in their work and it sounds like the way individuals adapt to dynamic environments.

I’ll end this ramble with a question: Do you have to be an evolutionary psychologist to believe in Human Nature?  Buller seems to think so and to think that it’s a bad idea.  I don’t think of myself as an evolutionary psychologist, but I do think there is such a thing as Human Nature.  I am struck by the fact that despite the dizzying array of cultural diversity that is manifested by our species, a smile is a smile, embarrassment is embarrassment, and a look of consternation is a look of consternation.  We might find different things amusing, mortifying, or distressing but pretty much people everywhere experience these emotions and, because of our theory of mind, recognize them in others.  The work of Eckman, Eibl-Eibesfeldt, and Fernald, to name a few, is pretty compelling in this regard.  Do we have a cheater-detection module that was engineered in the Pleistocene?  Maybe.  Honestly, I don’t care that much, but I do think that denying the existence of Human Nature is done at our collective peril.


Buller, D. J. (2005). Evolutionary psychology: the emperor’s new paradigm. Trends in Cognitive Sciences, 9(6), 277-283.

Crume, T. L., DiGuiseppi, C., Byers, T., Sirotnak, A. P., & Garrett, C. J. (2002). Underascertainment of Child Maltreatment Fatalities by Death Certificates, 1990–1998. Pediatrics, 110(2), 1-6.

Daly M, Wilson M (2007) Is the “Cinderella effect” controversial? A case study of evolution-minded research and critiques thereof. In C Crawford & D Krebs, eds., Foundations of evolutionary psychology. Mahwah NJ: Erlbaum.

Machery, E., & Barrett, H. C. (2006). Essay Review: Debunking Adapting Minds. Philosophy of Science, 73, 232-246.

Wilson, M., & Daly, M. (1997). Life expectancy, economic inequality, homicide, and reproductive timing in Chicago neighbourhoods. British Medical Journal, 314(7089), 1271-1274.

Buller on Evolutionary Psychology

Relentless critic of evolutionary psychology, David Buller recently wrote a piece in Scientific American outlining the critique he has developed over the last several years against this particular flavor of human evolutionary studies.  The author of Adapting Minds lists four ideas from contemporary evolutionary psychology (EP) that he suggests are fallacious:

  1. Analysis of Pleistocene Adaptive Problems Yields Clues to the Mind’s Design
  2. We Know, or Can Discover, Why Distinctively Human Traits Evolved
  3. “Our Modern Skulls House a Stone Age Mind”
  4. The Psychological Data Provide Clear Evidence for Pop EP

In my graduate seminar on Evolutionary Theory for the Anthropological Sciences, we read Buller’s more technical (2005) critique of EP.  I find myself largely in agreement with his criticisms and, importantly, when I disagree with him, I think it is for interesting reasons.  

The first of these critiques is, in my opinion, the most far-reaching and damning. The Pleistocene, the geological epoch that lasted from around 1.8 million to 10,000 years before present, takes on the role as a mythical age of creation for EP. You see, the Pleistocene represents out species “Environment of Evolutionary Adaptedness” (EEA), a concept derived from developmental psychology and particularly John Bowlby, the father of attachment theory.  In the words of Tooby and Cosmides (1990: 386-387), the EEA “is not a place or a habitat, or even a time period.  It is a statistical composite of the adaptation-relevant properties of ancestral environments ecounted by members of ancestral populations, weighted by their frequency and fitness-consequences.”

The key question, as Buller notes, is what would such a statistical composite look like for humans?  The insight that is regularly trotted out is that humans (hominins really) were everywhere hunter-gatherers until about 10,000 years ago — and were mostly hunter-gatherers for some substantial period after that! So, what do we know about hunter-gatherers?  Much to our collective loss, most of what we know about hunter-gatherers comes from the study of highly marginalized populations.  This is because states, with their potential economic surpluses, large populations sizes, and hierarchical social organization (read: efficient militaries) pushed hunter-gatherers into marginal habitats throughout the world as they moved across the landscape.  Nonetheless, the hunter-gatherer populations that we know about are a remarkably diverse lot.  A terrific reference cataloging some of this diversity is Robert Kelly’s (1995) The Foraging Spectrum.  In my specific area of interest (i.e., biodemography), Mike Gurven and Hilly Kaplan have recently written a very interesting paper on the diversity of hunter-gatherer patterns of mortality.  In this figure, taken from Gurven and Kaplan’s paper, we can catch a glimpse of the variability just in hunter-gatherer demography.

Humans are clearly quite different from chimpanzees.  The point of Gurven and Kaplan’s paper is that the existence of elderly within our societies is not simply an artefact of the modern industrialized world.  Old-age is as much a part of the human life cycle as is childhood.  Given the long potential lifespans of people in all the sampled populations, there is nonetheless a remarkable diversity in life expectancy (the average number of years lived by a person in the population) portrayed in this figure, considering that these are all groups without access to modern medicine.  There are people who live in arid lands of Sub-Saharan Africa (!Kung, Hadza), South American forests (Ache, Tsimane, Yanomamo) and South American grasslands (Hiwi).  Life expectancy at age 5, (\stackrel{\circ}{e}_5) varies by as much as 30%.  The basic point here is that even in something as basic as age-specific schedules of mortality and fertility, different hunter-gatherer groups are very different from each other (note that the Ache and !Kung differ in their total fertility rates by a factor of nearly two).

In all likelihood, our Pleistocene ancestors, like the sample of hunter-gatherer societies discussed in Kelly (1995) or Gurven and Kaplan (2007), lived in diverse habitats, engaged in diverse economic activities within the rubric of hunting and gathering, had diverse social structures, met with diverse biotic and abiotic environmental challenges to survival and reproduction, and dealt with diverse hostile and harmonious relations with conspecifics outside of their natal groups or communities. In other words, it’s hard to imagine what neat statistical generalizations about hunter-gatherer lifeways — and the selective forces they entailed — could emerge from such diversity. People lived in face-to-face societies.  People had to integrate disparate sources of information to make decisions about fundamentally uncertain phenomena. There was probably a sexual division of labor, though not necessarily the same one everywhere. There were women and men. Probably some other things too, but not that many.  Robert Foley (1996) has a nice critique of what he sees as an extreme simplification of the Pleistocene hunter-gatherer lifeways under the rubric of the EEA.  

Another related problem with the EEA line of thinking is this idea that somehow selection stopped when humans developed agriculture.  10,000 years, while brief in the grand scheme of things, is still not exactly evolutionary chump change.  That span represents anywhere from 350-450 human generations.  This is, in fact, plenty of time for selection to act.  We know from genome scans done in the lab of Jonathan Pritchard, for example, that there is extensive evidence for rapid, recent selection in humans. New, complex psychological mechanisms?  Probably not, but we should nonetheless not fall into the trap of thinking that somehow evolution stopped for our species 10,000 years ago.

Buller’s second fallacy (“We Know, or Can Discover, Why Distinctively Human Traits Evolved”) is a deeply difficult problem in human evolution. I’m afraid that my current thinking on this problem leads me to the same pessimistic conclusion that Buller and his colleague Jonathan Kaplan come to: There are just some things that we can’t know (scientifically) about human evolution.  This arises from the fact that our species is the only member of our genus and we are separated from our sister species by nearly six million years. As Dick Lewontin first noted in 1972, despite our dizzying cultural and social diversity, we are an amazingly homogenous species genetically.  I suspect that what this means is that the standard conceit of EP (one that Buller is highly critical of), that humans are everywhere the same critter, is probably true.  Unique (and universal) phenomena present science with a particular explanatory challenge. Buller is spot on when he criticizes EP for wanting it both ways.  On the one hand, EP sees a robust and universal human nature (an idea to which I am sympathetic, by the way).  On the other, EP sees strong selection driving the evolution of diverse psychological mechanisms.  The unpleasant reality is that if selection on psychological mechanisms were, in fact, that strong and pervasive, we should expect contemporary heterogeneity in the expression of such adaptations across different populations.  This is a topic that University of Illinois anthropological geneticist Charles Roseman and I have talked about quite a bit and have a very slowly gestating manuscript in which we discuss this and other ideas.  I know of no convincing evidence that such variation exists and for this and other reasons, I remain a steadfast skeptic of the idea that natural selection has shaped all these important psychological mechanisms independently and with precision to the tasks to which they are supposed to represent engineering solutions.

Buller’s argument for fallacy #3 (“Our Modern Skulls House a Stone Age Mind”) is, I think, a little unfair.  The major argument he makes on this point is that some of our psychological mechanisms did not, in fact, arise in our Pleistocene hunter-gatherer ancestors, but are of a more ancient, primate (or even mammalian) nature.  Honestly, I doubt that this point would elicit many complaints by anyone of the so-called Santa Barbara school. Sometimes critics — myself included — make a little too much of the it-all-evolved-in-the-Pleistocene bit.  I think this is one example of that.  Tooby and Cosmides have themselve argued that the EEA can be thought of as working at a variety of time scales.  The emotional systems described by Jaak Panksepp and used by Buller in his critique — Care, Panic and Play — are all pretty basic ones for a social species.  Indeed, the emotional system of panic almost certainly pre-dates complex sociality.  The EEA argument, as laid out by John Tooby and Irv DeVore (1987) and then by Tooby and Cosmides (1990), is essentially one of evolutionary lag: complex adaptations to past environments are carried forward into the present.  When a system retains its function, the scale of such lag can be large.  Think about bilateral symmetry or the tetrapod bauplan. I think that a fair assessment of Santa Barbara style EP reveals that there is nothing contradictory about the existence of primitive (in the sense of pleisiomorphic) emotional systems in contemporary humans.

Another small point of departure between Buller’s critique and my own thinking on the matter is his discussion of David Buss‘s work on sexual jealousy.  Now, I should be perfectly clear here.  I happen to think that the whole sex-differences in sexual preferences thing is the most overplayed finding in all of evolutionary science.  In class, I refer to this work as Men-Are-From-Mars Evolutionary Psychology.  The basic idea is to take whatever tired sexual stereotype that you’d hear in a second rate stand-up comedian’s monologue, or read about in airport bookstore self-help tracts and dress it up as the scientifically proven patrimony of our evolutionary past.  Ugh.  No, the part of Buller’s argument with which I disagree is his apparent take on decision-making. Buller writes, “According to Pop EP, many cultural differences stem from a common human nature responding to variable local conditions.”  I guess I’m not so clear as to what’s wrong with such a statement.  Isn’t that really what he argues in the previous paragraphs when he suggests that women and men have a fundamentally similar reaction to sexual jealousy?  On this he writes, “Instead both sexes could have the same evolved capacity to distinguish threatening from nonthreatening infidelities and to experience jealousy to a degree that is proportional to the perceived threat to a relationship in which one has invested mating effort.”  An evolutionary psychology that took seriously environmental (including cultural) variability and combined it with some preferences over risk and uncertainty and a generalized calculus of costs and benefits: Now that would be interesting!  Of course, I’d call that behavioral ecology.

Regarding fallacy #4 (“The Psychological Data Provide Clear Evidence for Pop EP”) more generally, I think that Buller is right on.  The evidence for many of these so-called psychological adaptations is pretty weak.  There is general contempt for population genetics among the smarter (and there are smart ones) evolutionary psychologists with whom I have talked and general ignorance among the less gifted.  I think this contempt and/or ignorance is expressed to the detriment of scientific progress in EP.  Buller’s point that cross-cultural differences are sometimes greater than inter-sexual differences in the psychological traits that are putative adaptations for sex-specific reproductive strategies, while not specifically substantiated, is pretty devastating.  This is where population genetics comes in.  Thinking about within vs. between population variance is a very important step in understanding the evolutionary forces at work.

The complex organ that is the human brain is certainly the result of selection.  As George Williams reminds us, selection is the only evolutionary mechanism that can produce this type of complexity. So, like Buller, I agree that there must be an evolutionary psychology.  Our various complaints are with the evolutionary psychology that Buller labels “Pop EP.”  It’s all too easy to be critical. Developing scientific theories for phenomena as complex as those surrounding the evolution of our species is a difficult task and takes ingenuity, courage, and, of course, thick skin. Among the various practitioners of EP of whom Buller is particularly critical, I think that John Tooby and Leda Cosmides are smart people who manifest all these qualities.  A fallacy of contemporary discourse — one that is all too easily seen in anthropological meetings —  is that people who disagree intellectually must hold each other in contempt or otherwise dislike each other.  I disagree with much of current EP but I also think there are some interesting ideas among practitioners of EP, once we get beyond the trite Men-Are-From-Mar/Women-Are-From-Venus stereotypes.

Detailing where I think the action is in an interesting evolutionary psychology is at the very least another long blog post.  Some areas that I think are promising and/or under-studied include: detailed analyses of cultural transmission dynamics, understanding how people integrate diverse types of information to form decisions with fitness consequences, and understanding how people weigh risk and uncertainty.  I have a lot more to say on these topics, so I think it will have to wait for future posts…


Buller, D. J. (2005). Evolutionary psychology: the emperor’s new paradigm. Trends in Cognitive Sciences, 9(6), 277-283.

Foley, R. (1996). The adaptive legacy of human evolution: A search for the environment of evolutionary adaptedness. Evolutionary Anthropology, 4, 194-203.

Gurven, M., & Kaplan, H. (2007). Longevity Among Hunter-Gatherers: A Cross-Cultural Examination. Population and Development Review, 33(2), 321–365.

Kelly, R. L. (1995). The Foraging Spectrum: Diversity in Hunter-Gatherer Lifeways. Washington DC: Smithsonian Institution Press.

Lewontin, R. C. (1972). The apportionment of human genetic diversity. Evolutionary Biology, 6, 381-398.

Voight, B. F., Kudaravalli, S., Wen, X., & Pritchard, J. K. (2006). A map of recent positive selection in the human genome. PLoS Biology, 4(3), e72. Epub 2006 Mar 2007.

Tooby, J., & Cosmides, L. (1990). The Past Explains the Present – Emotional Adaptations and the Structure of Ancestral Environments. Ethology and Sociobiology, 11(4-5), 375-424.

Tooby, J., & DeVore, I. (1987). The reconstruction of hominid behavioral evolution through strategic modeling. In W. Kinzey (Ed.), Primate Models of Hominid Behavior. Stony Brook: SUNY Press.