Can Evolution Explain All Human Behavior?
Possibly but it is difficult to prove
This blog entry is about one of my many pet peeves. Essentially, there is much literature examining correlates of sex differences in a specific behavior that claim to provide a test of one evolutionary theory or another. Possibly my favorite example (or is it my least favorite?) relates to the notion that the male advantage in spatial abilities and the female advantage in object location memory in humans has an evolutionary basis. This whole idea arose from the hunter-gatherer hypothesis presented by Silverman and Eals (1992) to account for their findings of a female advantage in object location memory. This hypothesis is based on the notion that archeological and paleontological data suggest the presence of an early division of labor in human males and females in the Pleistocene period, with men generally viewed as the hunters whereas women were the gatherers.
Silverman and Eals argued that skills that promote successful hunting (e.g., ability to orient one’s body in relation to objects and across distances, perform mental transformation of objects, etc.) are generally in favor of males in modern humans; modern sex differences on spatial tasks can be accounted for by the hunter role held by males way back in pre-historic times. However, Silverman and Eals hypothesized that aspects of spatial abilities relevant to foraging, such as object location memory, should be in favor of women. Not surprisingly, their research supported this “prediction” and women’s advantage in object location memory has been replicated many times since then (Voyer, Postma, Brake, & Imperato-McGinley, 2007).
Why did I put prediction in quotation marks in this last sentence? The reason is that it is a common criticism of evolutionary explanations that they are post-hoc (i.e., after the fact). Essentially, we look for a given behavior in modern times and dig back in the evolutionary past to see if anything might account for present findings. By definition, this is “after the fact”, therefore what we have is not a prediction but a post-hoc explanation. This point was nicely made by Cornell (1997) when he showed that an evolutionary model could account plausibly and with similar arguments for a pattern of gender differences that is the opposite of what we observe in modern times (i.e., a hypothetical society where women are physically stronger and more sexually promiscuous than men).
In his excellent book, philosopher of science David Buller (Buller, 2005) suggested that the hypotheses that arise from evolutionary models are untestable in practice even though they might be testable in principle. What does this mean? Essentially, we know what kind of evidence is needed to test these hypotheses (for example, direct observation of our pre-historic ancestors), and this makes them testable in principle. Unfortunately, we do not have access to these data and this makes these hypotheses untestable in practice. A statistical technicality also adds a further complication in this hypothesis test.
Specifically, the research methodology for testing the evolutionary hypotheses typically uses by definition a quasi-experiment: a type of design that does not allow causal conclusions because we do not manipulate any variables directly (see Furlong, Lovelace, & Lovelace, 2000 or any textbook on research methods on this point). In other words, data that fit with predictions based on evolutionary models do not prove the validity of these models or allow one to reject other models. From this perspective, evolutionary hypotheses can explain everything but they cannot refute other explanations either. Essentially, evolutionary models appear to provide circular arguments that seem untestable in practice. Although it is possible that a given model is true, we cannot prove its conclusions.
What should you take out of this post? Obviously, it would be foolish to reject evolution as an explanation of modern human behavior. We have too much data suggesting that the human species evolved since the first evidence of human presence on Earth. The aspect that is frustrating for a scientist is that we have no way to prove what specific aspects of evolution account for a particular behavior. As we continue to adapt to our changing environment, perhaps we will evolve a way to figure an answer to this issue in 100,000 years or so. However, it is much more likely that we will grow a much needed third arm first!
In the meantime, you can find out much more on this debate by going to this Wikipedia entry: https://en.wikipedia.org/wiki/Criticism_of_evolutionary_psychology(link is external)
Whereas some counter-arguments to criticisms raised by Buller can be found at this web page: http://www.cep.ucsb.edu/buller.htm
Buller, D. J. (2005). Adapting minds. Cambridge, MA: MIT Press
Cornell, D. G. (1997). Post hoc explanation is not prediction. American Psychologist, 52, 1380.
Furlong, N., Lovelace, E., & Lovelace, K. (2000). Research methods and statistics: An integrated approach. Fort Worth, TX: Harcourt.
Silverman, I., & Eals, M. (1992). Sex differences in spatial abilities: Evolutionary theory and data. In J. Barkow, I. Cosmides, & J. Tooby (Eds.), The adapted mind: Evolutionary psychology and the generation of culture (pp. 533-549). New York: Oxford University Press.
Voyer, D., Postma, A., Brake, B., & Imperato-McGinley, J. (2007). Gender differences in object location memory: A meta-analysis. Psychonomic Bulletin and Review, 14, 23-38.