Berwick and Ahouse Respond
Steve Pinker suggests that when stripped of vehemence, we all agree that the brain is not just a uniform gray custard, that "processing" lurks behind the mental, and that we share a naturalistic commitment to biology and evolution, including natural selection. But beyond this catechism, we differ more than Pinker's rhetorical narrowing indicates. He suggests that we really disagree only about "which mental processes are functional adaptations." That is a basic point of contention: we think Pinker is much too quick to explain the evolution of mental processes by assigning them adaptive functions. But many of the other difficulties in How the Mind Works (HTMW) follow from his promiscuous adaptationism: reporting premature consensus where there is none; uncritically describing behaviors as genetic, implying that they were optimal solutions to an diffusely described past and thus naturalizing them as inevitable; underestimating the sheer difficulty of adaptationist arguments; identifying "innate mental faculties" with "traits"; and embracing a "Darwinian fundamentalism" that effectively equates evolution with natural selection.
Nothing in Pinker's reply urges us to step back from these conclusions. Does Pinker now demonstrate that any behavioral disposition, even "sexual desire" has a coherent evolutionary history or segregates just like the gene for wrinkled peas? No. Does he now establish that only one selective explanation-his- exists for any (hypothetical) trait, paying attention to the space of selective alternatives, physical possibilities, and empirical contingencies? No. Does he probe the selectional regimes 100,000 years ago? The assumption that genetic equilibrium was attained then, the commandeering pace of cultural evolution since? No, none of these things. We did not ask that he turn his book into a technical treatise-we pointed out that he provides none of the technical details.
Panselectionism and Other Distortions
To us Darwinian fundamentalism is a form of irrationalism that, left un-checked, erodes the very theory of evolution it embraces. We are repeatedly told that functionality, design, and adaptedness are explained by only one known physical mechanism: "Natural selection remains the only theory that explains how adaptive complexity, not just any old complexity, can arise" (p. 162); "Because there are no alternatives, we would almost have to accept natural selection as the explanation of life on this planet even if there were no evidence for it" (p. 162); "[natural selection] alone explains what makes life special" (p. 155, our emphasis). But appeal to this doctrinal ("indispensable") principle is coherent only if there is a criterion for functionality, design, and adaptedness that is independent of the actual existence of traits-otherwise, we can "explain" all adaptations, or their negations, equally well. And to this point we have no such independent criterion. HTMW suggests that "reverse engineering" is the independent measure-but we challenged this underpowered metaphor in our review and Pinker did not respond to this either.
Traits and adaptations are simply not obvious. Is it really so clear, as HTMW says, that "sexual desire" is an adaptation to beget children? We already noted in our review that it is controversial why sex originated-increasing genetic variance? parasite defense? mutation elimination?-let alone sexual desire. We simply do not yet know. Why do we see in three dimensions? The answer, to "see in depth," is a near tautology. And it is probably not to recognize objects, contrary to the "consensus" that is presented in HTMW for the now modified "2H-D sketch."1
On the other hand, take HTMW's "obvious" nonadaptation, music. Darwin conjectured music was adaptive, and devoted a whole section to it in The Descent of Man and Selection in Relation to Sex (pp. 496-501). If the answer is not obvious in cases where the traits are relatively easy to measure, then what of our mental capacities and the more plastic, behavioral repertoires associated with them? Allowing that they may well be adaptive, which we do, is not the same as insisting that virtually all are adaptive and that we know why, which is how it is presented in HTMW.
Pinker's response to our description of his position as panselectionist is to complain about distortion and quote a series of one-line caveats that (supposedly) demonstrate that he is aware of our points, anticipated them, and agrees with us. But consider his principal example of distortion. We quoted him as saying: "Why don't women give virgin birth to daughters who are clones of themselves instead of wasting half their pregnancies on sons who lack the machinery to make grandchildren and are nothing but sperm donors?" He responds that, taken in context, the point of the remark was that "sex is puzzle for all organisms; humans are presented just as an example." But this was precisely what we rejected: humans are not a good example for understanding the payoff schedule that could select for sexual reproduction, and neither are any other mammals, from aardvark to zebra (nor would any number of many other clades be a good example, e.g., birds). Our point was that structural constraints on evolution might provide the answer to Pinker's question about sexual reproduction. But the notion that there are structural constraints explaining a trait's state is apparently so remote from Pinker's concept of evolution that even when we raise the possibility, he restates (with emphasis) his misunderstanding.
As to the one-line caveats: We noted in our review that Pinker's text includes such hedges and comments, but they are never taken to heart and will not make the rest of the book go away. He really does have to defend his uncorsetted story-telling (the longest chapter, "Family Values," is his exegesis of evolutionary psychology-inspired morals). So we must ask readers to return to our review and see if the examples we challenge are really so unmotivated by the text. Or consider the passages on elephant trunks and evolution (pages 152-53); or cognitive "closure" and the limits of adaptations (pages 558-65). Though we welcome the caution in Pinker's response, our complaint remains: HTMW is a popularization that lacks the spirit of doubt that suffuses scientific practice.
One kind of caveat deserves special mention. This is his widely quoted defense of choosing not to have children thus subverting evolutionary imperative, "and if my genes don't like it they can jump in a lake." In a book whose unrelenting message is that our behaviors were optimized and branded into our genes on the savanna (100,000 to a million years ago) this sudden embrace of genic independence strikes us as merely diversionary, hardly a constitutive part of the view Pinker is defending, according to which "the biggest influence that parents have on their children is at the moment of conception" (p. 449). While conception is a necessary step on the path to a child, Pinker's claim, to be interesting, must be at least partly causal. This is why we find his objection to our accusation of genetic determinism curious. Here again, he drives the narrative forward by making claims that sound strong and controversial only to turn them bland and incontestable through qualification.
One final point on this topic: Pinker resents our implication that his evolutionary stories reflect no more than his biography. Fair enough. We do not know the details of his life or motivations, and were too quick to suggest that his stories only reflect his biography. We offered this hypothesis after having slogged through the book, and found ourselves unconvinced by his insistence that the conclusions are forced on him by data, unmediated by his predilections. Wherever they come from we still insist that there are richer and more interesting organizing myths.
Structure and Function
For those outside evolutionary biology, the stakes in the debate about the extent of functional adaptation may seem esoteric, so some background may be useful. Arguing from function to structure (the explanation for the heart's structure is its function, pumping blood) is an example of teleological thinking. Reasoning from "final causes" is disreputable when discussing systems that lack intention (primarily because it seems to be future states reaching back to form what will be). No one has a problem with saying a pot looks the way it does because the artisan wanted to make it that way, though even here the medium can push against the intentions of the creator. Darwin salvaged this form of thinking in biology through a clever insight, natural selection. After Darwin, function-centric teleological accounts became a kind of shorthand, able to unroll into a more elaborate hypothesis involving variation, heritability, and selection.
A counter-tradition and useful dialectical tension for functionalism is structuralism, an emphasis on conserved structures.2 To return to an earlier example: the explanation of sexual reproduction in humans need not appeal to the function of sexual reproduction in humans-rather, the explanation may well be a matter of constraints and options earlier in this lineage. The selectionist argument that allows us to speak teleologically about a trait requires that the population have variance in the trait. HTMW claims that all of the uniquely human traits emerged through selection on the savanna. But we do not know the variance in human populations with respect to behavioral traits 100,000 years ago, nor do we know the heritability of these dispositions. Unambiguous function assignment is difficult in any case, and from the distance of over a hundred millennia the data you would use to make tentative assignments is quite meager.
Structuralism faces a symmetrical challenge: identifying structures that are "the same" (homologies). These two perspectives provide an essential tension. In the late 1800s functionalism associated itself with evolution and structuralism was associated (by functionalists) with special creation. This has left an unfortunate residue, for as special creation was dismissed structuralism sank with it. Rather than a creative tension between these positions we have the odd situation where some evolutionists assign notions of historical, developmental, or structural constraints to footnotes and caveats. Fortunately, with the string of successes in developmental genetics in the last decade this is changing.3 The distance between a view that prefers a functional-structural dialectic and one that recognizes function as central and anything else as minor epicycles may help explain the rift between Pinker and us.
In part, the differences are matters of temperament: some are content to study the mind and its properties, others insist that we must first agree that the properties are adaptations (even if they can't deliver on the full account that adaptations demand). To us, studying the properties is the interesting thing, not asserting that they are adaptations and then studying them. Moreover, we think that the realization that brains and neurons are homologous structures has been methodologically much more productive (allowing us to do neurobiology by studying mice, crayfish, flies, octopus, and sea hares).
Finally, what are we to make of the suggestion that we (along with Stephen J. Gould, Steven Rose, and Philip Kitcher) are puppets hanging from Dick Lewontin's old and bitter fingers? Well, it is high praise indeed. If we reflect Lewontin's admirable writings, and this puts us on the dais (or in the dungeon) with other compelling writers, we welcome the association. When Steve Pinker asserts that we are all suspicious of attempts to connect psychology with standard evolutionary biology, he is right! We do not believe that nearly enough is known at present about psychology or evolution to link them. But it is possible to link a caricature of each, and this is what we found in HTMW.
Richard Feynman put it well. Our responsibility is to give all the detail to "help others judge the . . . contribution; not just the information that leads to judgment in one particular direction or another." The challenge in science writing, indeed science itself, is to tell a crackling good tale abiding the dramatic unities, while respecting scientific ambiguity, details, and unresolved tensions. Pinker's previous book The Language Instinct succeeded here precisely where HTMW fails, and for exactly this reason: the earlier book drew on a forty-year consensus about generative grammar patiently constructed by Noam Chomsky and colleagues, while HTMW forces one prematurely. So when we wrote "You need not believe a word of it" we meant just that: Evolutionary psychology does not force your hand (or your mind) here. You ought to judge for yourself. Pinker could have helped that judgment by presenting alternative explanations antithetical to his views in enough detail. But he didn't.
1 Pinker missed our point about Marr's 2H-D sketch. Of course some people still "believe" in it-though it is important to clear about just what they believe. All we needed to illustrate was a lack of consensus to make our point that his reporting was skewed. According to David Marr's former colleagues, Profs. Shimon Ullman, Tomaso Poggio, Berthold Horn, and Eric Grimson, who, together with Marr, came up with the notion of 'visible surface representation' known as the "2H-D sketch" it was meant to be an integrated representation for surfaces on the way to object recognition. HTMW's picture on page 260, which depicts the 2H-D sketch as a circle and then writes out separate, distinct slots for depth, slant, tilt, color, and surface identification is misleading. Visual surface reconstruction seemingly does matter for certain low-level visual tasks like figure-ground separation, but not, according to Ullman, Poggio, and Heinrich Buetlehoff, for object recognition, not even for 3-D. Further, there is really no hard biological evidence for such a surface representation-scientists simply have not found neurons that respond to surface orientation regardless of where it's coming from. Evidently, neurons behave differently. Moreover, the 2H-D sketch recorded local surface orientation, not really depth (Berthold Horn at the MIT AI Lab did much of this work at MIT in the late 60s and early 1970s, attracting Marr there in the first place). But our point was not to quibble about these details or history: it was simply to point out that nobody had gotten the integrated representation to work (which of course does not bar somebody else from trying to make it work), and that this integrated representation is not a consensus in the field.