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.