Here is Leiter's (or Gould's) argument against EP (from Leiter's post):
[I]t is extremely reasonable, given what we know, to express doubts about evolutionary psychology and its selectionist hypotheses about differences between the sexes, since none of these hypotheses (as in none) have been confirmed by standards that approach those in biology [This is the Lloyd part. - Chris]. The fundamental difficulty is that there exist important non-selectionist evolutionary mechanisms (for example, genetic hitch-hiking or genetic drift), so that one can not, as evolutionary psychologists do, treat the selectionist explanation as the default one [This is the Gould part. - Chris]. This is just bad science. This point is also the stuff of baby biology textbooks [This is the Leiter-style gratuitous jab. - Chris] ; herewith Stearns & Hoekstra (OUP, 2000), p. 8:: “much of the variation in DNA sequences [over time] is neutral with respect to selection.” The challenge for evolutionary biologists studying, e.g., sex differences, is to figure out what role selection, if any, is really playing. Evolutionary psychology is silent on this problem. (There is a separate problem, of course, pertaining to the role of non-biological factors in observed sex differences.)He then goes on to give examples of some traits in nonhuman animals that did not evolve as selection-induced adaptations. The problem with this critique, of course, is that it doesn't say anything about any of the claims, arguments, or fundamental tenets of EP. It's not true, in practice, that Evolutionary Psychology assumes that "the selection explanation" is the "default one." This is how Buller correctly summarizes the picture of EP that Leiter and Gould are operating with:
This pattern of reasoning begins with the observation of a purported adaptation and then attempts to reconstruct its evolutionary history. (p. 90)But, as Buller notes (also correctly), this is not what Evolutionary Psychologists do. I can't think of any instance of an Evolutionary Psychologist using this line of reasoning, and I don't think we can expect Leiter to give us one (Gould never does). Instead, what Evolutionary Psychologists do is the following (also from Buller, and on the same page):
If early humans faced such-and such an adaptive problem in the EEA, then our species should have evolved this or that proximate behavior-control mechanism to solve that problem; so, if modern humans posses such a proximate mechanism, it is an adaptation.In other words, Evolutionary Psychologist hypothesize (i.e., they don't assume) that if certain adaptive conditions existed in the environment of evolutionary adaptiveness (EEA), then certain "proximate behavior-control mechanisms" should have evolved to deal with those problems. The existence of the proximate mechanisms are predictions derived from the hypotheses about the adaptive problems in the EEA. They then reason about the modern-day cognitive/behavioral manifestations of those proximate mechanisms, and go out and collect data to look for those cognitive/behavioral manifestations. If they find data that supports the existence of those manifestations, then their predictions are confirmed, and their hypotheses supported.
Now there are all sorts of problems with this line of argument. For one, it involves what is little more than speculation about the adaptive conditions of the EEA, along with the counter-to-fact belief that those adaptive conditions were static over an extended period of time in human evolution (and not, in fact, influenced by our adapting to them!). It also requires some pretty speculative reasoning about what sort of proximate mechanism we should have evolved to deal with those statically-conceived adaptive problems. In addition, in practice, it always involves some pretty sketchy reasoning about the modern-day manifestations of some pretty abstractly described proximate mechanisms. What this leads to is bad reasoning leading to speculative hypothes, resulting in poor empirical investigations that yield data that don't support those hypotheses. It's not surprising, then, that for virtually every EP empirical investigation, there are either many equally plausible alternative interpretations of the data, or there is in fact a wealth of data that is inconsistent with the EP interpretation (see Buller's chapters on mating, marriage, and parenthood for examples on these topics, and this post and the references for a discussion of the empirical evidence for social exchange theory).
What a critique of EP requires, then, and what Leiter (echoing Gould) does not provide, is a discussion of the problems with the reasoning by which Evolutionary Psychologists come to the conclusion that a certain proximate mechanism, if it exists, is an adaptation, and more importantly, a case-by-case look at the empirical support for the various EP hypotheses.