What is the inferential strategy that evolutionary psychologists use? There are two such. Sometimes they begin by identifying a behavior exhibited by contemporary humans, and set about to identify its evolutionary function. On other occasions they speculate about the sorts of recurrent challenges our prehistoric ancestors faced; they then speculate about how these were responsible for psychological adaptations; and finally, they extrapolate from this to explain features of contemporary psychology. Both kinds of inference rely on practitioners being able to correctly individuate modules.
Evolutionary psychologists have to have a way of knowing which module causes which behavior. Furthermore, since their explanations rest on the claim that the modules that are supposedly hard-wired in human brains are the very same modules that were selected for, and that these modules explain both ancestral and contemporary behaviors, it is clear that evolutionary psychologists need to have some method for inferring which prehistoric modules are identical to which contemporary ones. Unless this can be done it is difficult to see how one can determine the evolutionary functions of contemporary behaviors. However, to the best of my knowledge, there is no discussion about this important methodological point in the literature.
It is a difficult matter to determine whether a trait was under selection, and what it was under selection for doing. In straightforwardly biological cases, scientists use
comparative methods, optimality models, and so on, to determine that selection has taken place, and that the items under consideration have retained their ancestral functions (Sober 2008; Orzack & Sober 1994a, 1994b, 1996, 2001). But evolutionary psychologists are faced with special challenges to establishing that the human mind is comprised of many evolved computational structures and that at least some of them have retained their evolutionary functions. For this to happen, at least three things need to be accomplished: 1) one must identify the evolved modules, 2) one must provide independent support for the claim that each module is responsible for the production of certain contemporary behaviors, and 3) one must give evidence for the claim that there are functional, non-trivial similarities between contemporary and ancestral behaviors.
The first requirement is necessary because evolutionary psychologists argue that behaviors in the present are caused by cognitive systems which operate today as they did in the past. Recall that each module was supposedly selected for because of its specific fitness enhancing effects. So for example, the mate-procuring module in contemporary humans is so characterized because of its fitness effects in the past.
Because it has that specific function it will not be sensitive to situations that require the module that produces behaviors that result in (say) people avoiding poisonous plants.
Identifying poisonous plants has fitness-enhancing benefits, but one will not get those benefits unless there is a module that is sensitive to the appropriate range of stimuli.
The upshot of this demand is that unless evolutionary psychologists can identify
modules they are not going to be able to say that a particular behavior is underwritten by a specific module with the evolved function of producing behaviors of this sort, and their explanatory project will have difficulty getting off the ground. The second point calls for evidential support. Suppose that we grant evolutionary psychologists the claim that behaviors are caused by special-purpose modules. Such a concession does not necessitate oneʼs accepting their further claims that those modules are 1) the same as ancestral ones, and 2) that contemporary behaviors are caused by them. It is an empirical matter whether the modules that caused behaviors in early humans are the same as those causing behaviors in humans now. Unless this is established, one could claim that special-purpose modules are acquired ontogenetically rather than inherited genetically.
The final point concerns the grounds for matching contemporary modules with ancestral ones. This has got to be based on functional similarity, and this similarity cannot be trivial. The similarity (between a contemporary and an ancestral module) will not be trivial if the function is one that the ancestral module was selected for performing and if the contemporary module has the same function in virtue of its descent from the
ancestral module. This rules out cases where a contemporary module has function F due to learning, and this function happens to be the same as that of an ancestral module. It also rules out cases of functional similarity due to convergent evolution.
Functional similarity because of selection, which is what evolutionary psychologists want, would likely first require structural similarities between modules, but it is not
obvious that the cognitive architecture that modern humans have is the same as that possessed by our prehistoric ancestors.
All three conditions must be satisfied for the framework to work. It is clear that points 2 and 3 involve de facto judgments that contemporary behaviors and psychological phenomena are related to ancestral behaviors and psychological phenomena in a homology-like way in virtue of being underwritten by the same modules. I use the expression “homology-like” because although it is tempting to describe the similarity relations obtaining between ancestral modules and contemporary modules as
homologies, they are not homologous in the generally accepted sense of the word. In standard biological usage, “homology”25 pertains to similarities across taxa in virtue of common ancestry. For example, bird wings and human arms are homologous to the extent that their structural similarities are due to common descent from reptile forelimbs.
The sort of similarity that evolutionary psychologists wish to establish is better
characterized in terms of similarity due to descent of contemporary phenotypes from ancestral ones. Since homology is standardly understood as a “horizontal” relation (across taxa), I dub the sort of similarity that is the focus of this paper “vertical
homology.” More specifically, the sort of relation with which I am concerned requires that the function of an ancestral item is conserved over time (there is similarity or
25 I am not changing the meaning of “homology.” Rather, I am drawing some finer distinctions to better represent the sorts of relations that are present in evolutionary psychological accounts of human behavior. Although I will use “homology” to keep things simple, it is used to represent similarity within a taxon rather than across taxa.
commonality of function due to descent). Call this strong vertical homology to
distinguish it from cases where a contemporary item is similar to an ancestral item in virtue of the formerʼs descent from the latter, but without the contemporary item having the same function as the ancestral item.