Discovering the Functional Mesh: On the Methods of Evolutionary Psychology

Transcription

1 Discovering the Functional Mesh: On the Methods of Evolutionary Psychology PAUL SHELDON DAVIES Department of Philosophy, College of William and Mary, Williamsburg, VA , U.S.A. Abstract. The aim of this paper is to clarify and critically assess the methods of evolutionary psychology, and offer a sketch of an alternative methodology. My thesis is threefold. (1) The methods of inquiry unique to evolutionary psychology rest upon the claim that the discovery of the adaptive functions of ancestral psychological capacities leads to the discovery of the psychological functions of those ancestral capacities. (2) But this claim is false; in fact, just the opposite is true. We first must discover the psychological functions of our psychological capacities in order to discover their adaptive functions. Hence the methods distinctive of evolutionary psychology are idle in our search for the mechanisms of the mind. (3) There are good reasons for preferring an alternative to the methods of evolutionary psychology, an alternative that aims to discover the functions of our psychological capacities by appeal to the concept of a whole psychology. Key words: Evolutionary psychology, cognitive psychology, standard social science model, functional mesh, adaptive (evolutionary) functions, selection for, selection of, evolution by natural selection, evolution by random drift, computational theory, information-processing theory, neurological theory, information-processing tasks, psychological functions, how-possibly explanations, a whole psychology. The emerging field of evolutionary psychology attempts to take advantage of Darwin s crucial insight that there should be a functional mesh between the design features of organisms and the adaptive problems that they had to solve in the environment in which they evolved. By understanding the selection pressures that our hominid ancestors faced by understanding what kind of adaptive problems they had to solve one should be able to gain some insight into the design of the informationprocessing mechanisms that evolved to solve these problems. (Barkow, Cosmides, Tooby 1992, p. 9) Two of the most prolific advocates of evolutionary psychology are Leda Cosmides and John Tooby. Daniel Dennett has recently discussed their work and says this: I consider Cosmides and Tooby to be doing some of the best work in Darwinian psychology today... (Dennett 1995, p. 490) Dennett goes on to criticize Cosmides and Tooby, but only for their occasional bursts of exessive zeal and not for either the substance or the methods of their work. I believe, however, that Dennett s enthusiasm is unjustified. 1 My aim in this paper is to assess the methods employed by Cosmides, Tooby, and other advocates of evolutionary psychology, and offer suggestions toward an alternative methodology. Criticizing evolutionary psychology is no simple matter. The difficulty is not that evolutionary psychology enjoys especial power or plausibility, but rather that it is not yet a well-articulated form of inquiry. The first part of my discussion, then, is an attempt to clarify what evolutionary psychology is. My aim is to make explicit both its central methodological commitments and the argument given by Minds and Machines 6: , c 1996 Kluwer Academic Publishers. Printed in the Netherlands.

2 560 PAUL SHELDON DAVIES evolutionary psychologists for such commitments. These are the tasks of Sections I and II. These opening sections describe what Cosmides and Tooby call the functional mesh, namely the coordination or fittingness between ancestral organismic traits and ancestral environmental demands that resulted from evolution by natural selection. The core methods of evolutionary psychology involve an appeal to this functional mesh. Advocates of evolutionary psychology claim that the methods of cognitive psychology are significantly enhanced by, and hence should be supplemented with, an appeal to the functional mesh. More specifically the claim is that the methods of cognitive psychology should be supplemented with an appeal to the adaptive organismic functions that operate within the functional mesh. Section I explicates this important claim and Section II considers the evidence that supports it. I then turn to the central arguments of this paper. These occupy Sections III and IV. My thesis is that the an appeal to the functional mesh between ancestral organisms and ancestral environments cannot improve the methods of cognitive psychology. In particular, the thesis is that the attribution of adaptive functions that operate within the functional mesh is justified only after we have discovered the correct account of our psychological capacities. This means that the primary goal of cognitive psychology must be satisfied independently and in advance of the attribution of any adaptive functions. So the appeal to evolution by natural selection in our search for a true account of our psychological capacities is methodologically inert. It is quite clear that at least some evolutionary psychologists Cosmides and Tooby, for example are committed to the claim that psychological inquiry ought to be founded on or grounded in our natural selective history; the appeal to our selective history, they say, plays an integral role in the discovery of our informationprocessing mechanisms. But if the thesis of this paper is correct, then what is currently called evolutionary psychology is non-evolutionary at its core. Of course evolutionary psychologists tend to be more or less self-conscious about the extent to which their particular hypotheses are consistent with various claims about our natural selective history. But hypotheses may be consistent with selection without being grounded in or founded on selection, and mere consistency with selection does not amount to a methodology for the discovery of our cognitive mechanisms. Work within so-called evolutionary psychology has generated considerable excitement and some of this excitement is warranted. Tooby and Cosmides (1992) oppose themselves to the prevailing view within psychology the so-called standard social science model according to which biology and history are largely irrelevant to understanding our present information-processing mechanisms. Evolutionary psychology, offered as the better alternative to the standard model, appears powerful and plausible by contrast. While I agree with Cosmides and Tooby in their opposition to the standard model in opposition, at any rate, to the thesis that biology is irrelevant to psychological inquiry I do not agree with their support for

3 THE METHODS OF EVOLUTIONARY PSYCHOLOGY 561 evolutionary psychology. The evident naivete of the standard model in its views about biology should not blind us to the (very different) naivete of evolutionary psychology in its views of evolution by natural selection. It is the latter naivete I propose to criticize here, not the former, and my criticisms of evolutionary psychology should not be construed as a defense of the standard model. 1. The Aims and Methods of Evolutionary Psychology David Buss (1995) calls evolutionary psychology a theoretical paradigm for all of psychology, though he does not say what theoretical paradigms are. Buss also refers to evolutionary psychology as a metatheory and, while he also fails to explain what metatheories are, he seems to think that paradigms and metatheories are similar or the same. What matters, however, is that Cosmides and Tooby as well as several others seem to think that evolutionary psychology is something quite distinct from a paradigm or a metatheory. 2 The aim of evolutionary psychology, like the aim of cognitive psychology generally, is the discovery of the informationprocessing mechanisms that underwrite our psychological capacities: The eventual goal [of evolutionary psychology] is to recover out of carefully designed experimental studies high-resolution maps of the intricate mechanisms involved. Such an approach is intended to exploit the signal virtue of cognitive psychology: With its emphasis on mechanisms, cognitive approaches allow causal pathways to be precisely specified through reference to explicitly described algorithms and representations. (Cosmides and Tooby 1992, pp ) The aim is to reveal the various information-processing mechanisms that constitute our various psychological capacities. Whatever else it might be paradigm or metatheory evolutionary psychology is a species of cognitive psychology. What differs what makes evolutionary psychology distinctive and new are its methods. The methods of evolutionary psychology borrow quite heavily from the methods of cognitive psychology generally, but they do so with an evolutionary twist. Let us begin with a few basic distinctions from the methods of cognitive psychology. 3 David Marr, in his (1982) work on vision, distinguishes three levels at which we may theorize about the psychological capacities of any given kind of organism. These include (1) A computational theory: A systematic characterization of the relevant psychological capacities in terms of their functions or goals within the organism s interactions with its environment; (2) An information-processing theory: A systematic characterization of the information-processing mechanisms (algorithms and representations) that underwrite the functions of the psychological capacities characterized in (1); (3) A neurological theory: A systematic characterization of the neurological processes that underwrite the information-processing mechanisms characterized in (2).

4 562 PAUL SHELDON DAVIES Cognitive psychologists are not concerned with (3), at least not directly. 4 Their primary concern is with (2). Contemporary cognitive psychology takes the modern computer as its theoretical model. Computers are constituted from various hardware components that enable it to run a wide variety of software progrms, and the software programs enable it to perform a wide variety of information-processing tasks such as word processing, graphics, and more. Hardware, software, and the performance of various tasks have obvious analogues in human beings or so it is claimed. The human organism is construed as a complex information-processing machine. Our neurological processes are the analogue of hardware components and our psychological capacities capacities such as memory, language, jealousy, boredom, and more are the analogues of the capacities involved in the performance of various information-processing tasks. The analogue of computer software in humans is that set of information-processing mechanisms that underwrites our various psychological capacities. Our minds, like our computers, operate according to algorithms and representations that instantiate psychological capacities such as memory, language, jealousy, boredom, and more. This, at any rate, is the central working assumption of contemporary cognitive psychology, and the discovery of such algorithms and representations is its primary aim. Hence the concern with (2). David Marr s important methodological insight is that the best way to accomplish (2) is to begin with (1). The best way to discover the underlying informationprocessing mechanisms is to first discover the correct characterization of the information-processing tasks that the organism faces and then discover the functions of the psychological capacities with which the organism executes these tasks. The rationale, of course, is that a specification of the tasks involved should assist us in specifying the functions of our psychological capacities, and a specification of these functions should assist us in specifying the design features of the underlying mechanisms, where information about design should assist us in the construction and evaluation of experiments. Developing an account of the functions of our psychological capacities constitutes what Marr calls a computational theory of the capacities under investigation. We should, then, begin our search for a theory of the information-processing mechanisms involved in human cognition with a computational theory of the functions of our psychological capacities. 5 Information-processing tasks are the tasks proper to our various psychological capacities; they provide evidence of the functions that our psychological capacities execute. The information-processing tasks of our visual system, for example, include the representation of three-dimensional objects in the environment, and thus at least one function of our visual system involves the three-dimensional representation of such objects. So a computational theory level (1) in Marr s taxonomy of theories is a systematic characterization of the various psychological functions that our psychological capacities fulfill. We now may understand what is distinctive and new about the methods of evolutionary psychology. Evolutionary psychologists agree with Marr that the best

5 THE METHODS OF EVOLUTIONARY PSYCHOLOGY 563 way to discover the mechanisms that underwrite our psychological capacities is to first discover the functions that our psychological capacities fulfill; they agree, in short, that accomplishing (1) is the best way to approach (2). But evolutionary psychologists further assert that the best way to discover the functions of our present psychological capacities is to first discover the adaptive functions that the psychological capacities of our distant ancestors possessed. The further assertion is that the best way to accomplish (1) is to appeal to the functional mesh that evolved between ancestral organisms and ancestral environments, where the functional mesh refers to the adaptive fit between organismic traits and environmental demands that resulted from evolution via natural selection. We can, according to evolutionary psychologists, best discover psychological functions via adaptive functions. A crucial assumption here is that adaptive functions are functional properties acquired as a consequence of evolution by natural selection, properties that endow the relevant trait with the role of fulfilling certain standards or norms. The adaptive function of the human heart, for example, is to pump blood. This is so, at any rate, on the assumption that past hearts pumped blood, that such pumpings enabled some organisms to better satisfy certain environmental demands, and that the differential in satisfying environmental demands resulted in differential reproductive output. To say, then, that the heart has the adaptive function of pumping blood is to say that pumping blood is what hearts today are supposed to do; this is the standard imposed by the natural selective history of our species that applies to contemporary hearts. This, at any rate, is what some advocates of evolutionary psychology apparently believe about the adaptive functions of our various psychological capacities: Natural selection theory specifies how an organism should respond to different kinds of information from the environment. (Cosmides and Tooby 1987, p. 284; emphasis added) Another crucial assumption is that the psychological functions of ancestral psychological capacities the functions specified in a computational theory of those capacities are equivalent to a subset of the adaptive functions of those capacities. 6 Ancestral psychological functions are equivalent to those ancestral adaptive functions concerned with certain types of information-processing tasks: The intellectual payoff of coupling theories of adaptive function to the methods and descriptive language of cognitive science is potentially enormous. By homing in on the right categories ultimately adaptationist categories an immensely intricate, functionally organized, species-typical architecture can appear... (Tooby and Cosmides 1992, p. 68) The right categories what Tooby and Cosmides call adaptationist categories are categories defined by reference to adaptive functions. These are the categories discovered as a result of coupling adaptive function attributions with the attributions of psychological functions. Elsewhere the identification of psychological functions and certain adaptive functions appears more explicit:

6 564 PAUL SHELDON DAVIES When applied to behavior, natural selection theory is more closely allied with the cognitive level of explanation than with any other level...this is because the cognitive level seeks to specify a psychological mechanism s function, and natural selection theory is a theory of function. Natural selection theory specifies how an organism should respond to different kinds of information from its environment. It defines adaptive information processing problems that the organism must have some means of solving. Cognitive programs are solutions to information-processing problems. (Cosmides and Tooby 1987, pp ; emphases in the original) Natural selection theory, they say, defines adaptive information processing problems. I take this to mean that the attribution of an adaptive function to a psychological capacity is equivalent to the attribution of a psychological function to that capacity. Hence ancestral psychological functions constitute a subset of ancestral adaptive functions. To be fair, Cosmides and Tooby do not explicitly assert that psychological functions are adaptive functions and, to be sure, they do not articulate a clear theory of functions. Nevertheless several passages, including those just cited, support the view that psychological functions are equivalent to a proper subset of adaptive functions. More importantly, all advocates of evolutionary psychology are committed to the view that our psychological capacities are similar to those of our ancestors and that our ancestral psychological capacities simply are adaptive functions that emerged as a consequence of evolution by natural selection. This commitment lies at the very heart of their methods of inquiry. But even if this overstates the commitments of evolutionary psychologists, it is clear nonetheless that ancestral psychological functions must be closely related to ancestral adaptive functions. If not, it is difficult to see what all the talk of adaptive functions and evolutionary history has to do with the aims of cognitive psychology. For the sake of argument, then, I assume that the methods of evolutionary psychology rest on the claim that ancestral psychological functions are adaptive functions of a specific sort. 7 If, then, a theory of psychological functions is grounded in a theory of adaptive functions, we may describe the methods of evolutionary psychology this way: (MEP) The Methods of Evolutionary Psychology (i) the discovery of the functional mesh namely the discovery of the selective pressures that ancestral organisms had to face in order to evolve via natural selection and the discovery of the corresponding adaptive functions that enabled ancestral organisms to respond to such selective pressures enables us to (ii) formulate an evolutionarily-based computational theory of ancestral psychological functions, and this account of ancestral psychological functions enables us to (iii) formulate informative and testable predictions about the informationprocessing mechanisms that underwrite the psychological capacities of present human beings. The steps expressed in (MEP) are, I submit, the core methods of contemporary evolutionary psychology. There are, of course, further steps. These include the construction of experiments based on the predictions offered in step (iii), as well as the evaluation of the data that such experiments yield. But the step that makes these methods distinctively evolutionary in character is the move from step (i), namely

7 THE METHODS OF EVOLUTIONARY PSYCHOLOGY 565 claims about the adaptive functions of ancestral psychological capacities, to step (ii), namely claims about the psychological functions of ancestral psychological capacities. It is this move from (i) to (ii) that is the primary focus of concern here. This concludes my explication of these methods. I turn now to the main arguments offered in support of these methods Evidence for the Functional Mesh The methods expressed in (MEP) rest upon a claim concerning the alleged functional mesh between ancestral organisms and ancestral environments. The claim concerning the functional mesh rests upon a further claim concerning the evolution of complex phenotypic traits. Let us begin with the latter and work our way back toward the functional mesh. The first claim asserts that the only scientific explanation for the existence of complex phenotypic traits is in terms of evolution by natural selection. This is an empirical claim the evidence for which comes from cosmology. The argument is by elimination and is endorsed quite widely. Here are but two endorsements: Only a few causal processes have been proposed over the past two centuries to account for the origins of these complex organic mechanisms known as adpatations...the first is evolution by natural selection...the second is creationism. The third is seeding theory... (Buss 1995, p. 2) Evolutionary biology is fundamentally relevant to the study of human behavior and thought because our species is the product of naturalistic terrestrial processes evolutionary processes and not of divince creation or extraterrestrial intervention. (Tooby and Cosmides 1992, p. 50) 9 Seeding theory is the claim that in the past extraterrestrial organisms visited earth and planted seeds of life. This theory and creationism are both found wanting. We are left, then, with only one alternative. 10 It is conceded, however, that this argument applies only to complex traits, not simple traits. It is conceded, in particular, that Gould and Lewontin s (1979) criticisms of the so-called adaptationist programme provide a healthy corrective with respect to simple traits. For evolutionary forces other than selection, especially random drift, are candidate explanations for the proliferation of some organismic traits. Drift occurs when, in a relatively small population, evolutionary change results from chance factors. Imagine, for example, a population containing organisms that grow to one of two possible heights, short or tall. Suppose that, in some particular generation, while short and tall organisms exist in equal proportions, the short organisms fail to reproduce while the tall ones succeed, where this difference is a result of purely chance factors. The factors involved are chancy so long as they have no basis in the capacities of the organisms. It may be that on days when short organisms felt like mating, the weather turned foul and got in their way, whereas the weather was fine when the tall ones were in the mood. Since tall organisms have no more control over the weather than short ones, the difference in reproductive output is not due to differences in organismic capacities, and that is all that is meant in saying that the reproductive difference is due to chance. This, then, is a case of

8 566 PAUL SHELDON DAVIES evolution due to drift, not natural selection, and evolution due to drift can explain why some organismic traits, ones that are relatively simple in structure, take hold in a population. So Gould and Lewontin s point is partially sustained. But the Gould and Lewontin point does not hold with respect to complex traits. Or so it is claimed. For the probability that truly complex traits evolved via drift is simply too low to take seriously: The essential point is that no physical process other than natural selection can explain the evolution of an organ like the eye. The reason for this is that the structures that can do what the eye does are extremely low-probability arrangements of matter. (Pinker and Bloom 1992, p. 455) It is improbable that random drift could produce an arrangement of matter with the capacity to focus images, modulate light, and all the rest. So the argument by elimination given above is persuasive for complex traits on grounds of probability or so it is claimed but it is restricted to complex traits on grounds that there exist non-selectional scientific explanations in the case of simple traits. For the sake of argument, I suggest we grant that the evolution via drift of traits as complex as the human eye is unlikely. But in fact we should be sceptical of such claims. If Pinker and Bloom are claiming that it is subjectively improbable that the eye evolved via drift, then perhaps it is reasonable to agree, though it seems equally reasonable to suspend judgment. But if the claim is that it is objectively improbable, then we should have our doubts. For it is hard to imagine what prior probabilities Pinker and Bloom are employing and what evidence they have for such assumptions. Indeed if recent theories about the self-organizing powers of matter are even partly right, then Pinker and Bloom s claim, understood in terms of objective probabilities, may be false. 11 We should not be seduced by such sweeping conjectures for which we have so little evidence. Nevertheless, and for the sake of argument, let us grant that complex traits such as the human eye probably evolved via natural selection and not via random drift. 12 The reasoning here should be made more explicit. The assumption is that complex phenotypic traits are descended from relatively primitive traits possessed by distant ancestors. Consider again the human eye. The primitive trait may have been a simple photosensitive mechanism that responded to large changes in light. Over time various genetic mutations occurred and occasionally the phenotypic expression of such mutations was selectively efficacious. The new phenotypic trait thus proliferated in the population. At some future time, yet another efficacious mutation occurred, and so on. Each step involved the addition of some new feature or the alteration of some previous feature. In this way, what was originally a primitive photosensitive mechanism evolved into a complex eye. The odds of this kind of incremental and stepwise process occurring as a consequence of drift may be low indeed. Similar reasoning, it is claimed, applies to the human mind. Cosmides and Tooby are explicit in their endorsement of such reasoning:

9 THE METHODS OF EVOLUTIONARY PSYCHOLOGY 567 Evolutionary processes are the architect that assembled, detail by detail, our evolved psychological architecture. The distinctive characteristics of these processes are inscribed in the organizational specifics of these designs. Consequently, an understanding of the principles that govern evolution is an indispensable ally in the enterprise of understanding human nature and an invaluable tool in the disovery and mapping of the species-typical collection of information-processing mechanisms that together comprise the human mind. (Tooby and Cosmides 1992, p. 50) The minds of contemporary humans descended from relatively simple cognitive processes possessed by distant ancestors. Over time various mutations produced psychological capacities with behavioral expressions that were selectively efficacious. These took hold. A similar stepwise process gradually produced a complex set of capacities out of a few rather dumb capacities. The odds that drift could produce such complexity may be low indeed. The argument for the present empirical claim that the only scientific explanation for the existence of complex phenotypic traits is in terms of evolution by natural selection thus appears persuasive. 13 The claim concerning the functional mesh is that evolution by natural selection produced in our ancestors a significant degree of meshing a significant fittingness between organismic traits and environmental demands. The argument for this claim builds on the previous claim concerning the origins of complex phenotypic traits. As we have seen, evolution by natural selection is assumed to be the sole producer of complex traits, including our psychological capacities. Add to this the further claim that selection occurs only to the extent that organisms respond to enduring properties of the environment, and it appears reasonable to conclude that selectively successful organismic traits mesh with enduring features of the environment: [The] functional organization in the organism its set of adaptations is designed to exploit the enduring properties of the environment in which it evolved...and to solve the recurring problems posed by that environment. Adaptations evolve so that they mesh with the recurring structural features of the environment in such a way that reproduction is promoted in the organism or its kin. Like a key in a lock, adaptations and particular features of the world fit together tightly, to promote functional ends. (Tooby and Cosmides 1992, p. 69) There thus exists an evolved meshing an evolved fittingness between our various ancestral organismic capacities and the ancestral environmental forces that helped shaped them. Hence the evidence for the alleged functional mesh. I turn now to the central arguments of this paper Evolutionary Functions and How-Possibly Explanations I grant that there exists a functional mesh between ancestral organisms and ancestral environments, as evolutionary psychologists claim, and I grant that we can know that this is so on the strength of the arguments given in the previous section. That is, I grant that such a mesh exists and I grant that we can know of its existence on the grounds that only evolution by natural selection is capable of producing complexity in living things. 15 But I deny the much stronger and more specific claim that we can know of the existence of a functional mesh between ancestral

10 568 PAUL SHELDON DAVIES psychological capacities and ancestral environments; I deny we can know this, at any rate, on the grounds that evolution by natural selection is the sole producer of organismic complexity. 16 And this raises a serious problem for the methods of evolutionary psychology. I shall defend two rather simple claims. The first claim asserts that acceptable evidence for the existence of the more specific mesh between ancestral psychology and ancestral environments requires an adequate how-possibly explanation of the selective history of those organisms. But the second claim asserts that an adequate how-possibly explanation of our ancestral psychology of the adaptive functions that belong to our ancestral psychological capacities requires that we already know the correct computational theory, the correct psychological functions, of the relevant ancestral psychological capacities. If this is correct, then step (ii) in the methods expressed in (MEP) must be accomplished before step (i), in which case step (i) is idle. If both claims hold, then the appeal to the theory of evolution by natural selection in the discovery of the correct computational theory of our ancestral psychology is simply inert. Before defending these claims, however, I pause to explicate more fully the theory of adaptive functions or, as I shall call it, the theory of evolutionary functions. 17 To assert of any trait that it is a product of evolution by natural selection is to say one of two things. It may be to assert that the trait was selected for or it may be to assert that it was selected of. 18 Random drift aside, the processes involved in evolution by natural selection are not of a piece and, as we shall see, this opens up the possibility that there exists a natural selective explanation of our ancestral psychological capacities that does not entail the attribution of any evolutionary functions. An organismic trait possesses the evolutionary function of performing some specific task if and only if it was selected for the performance of that task. An organismic trait was selected for the performance of a task only if past instances of that kind of trait performed the task, where such performances increased the relative adaptedness of the organism, and where this increase in relative adaptedness explains why this kind of organism and this kind of trait exist in the population today. 19 Kettlewell s peppered moths provide an elegant illustration (see Kettlewell, 1973). During the last half of the nineteenth century, the peppered moth population in Manchester, England underwent dramatic evolutionary change. Soot from coal-burning darkened the lichen-covered trees upon which the moths sat. Prior to coalburning 99% of the moth population possessed light-colored wings, for light-coloration against light-colored lichen provided camouflage. But lightcolored lichen was deadly for dark-colored moths. Once soot darkened the trees, however, fortunes were reversed. By the end of the century, 99% of the population possessed dark-colored wings. So dark- and light-coloration provided heritable variation; dark coloration provided superior camouflage; this resulted in increased

11 THE METHODS OF EVOLUTIONARY PSYCHOLOGY 569 relative adaptedness; and this explains why dark-coloration exists today. The evolutionary function of dark-coloration, therefore, is to provide camouflage. An organismic trait was selected of if and only if past instances of the trait were maintained or caused to proliferate as a consequence of being suitably connected to traits that were selected for. Selection of is a consequence of selection for. Traits selected of are phenotypic expressions of genotypes linked to other genotypes, where the latter genotypes express phenotypic traits that were selected for. Consider, again, Kettlewell s moths. Dark coloration is the organismic property selected for, but such selection for resulted in the selection of various other organismic traits, including the genes that code for dark colored pigment, genes that code for wing tissue, and the rest. Two features of properties selected of are especially important. First, while properties selected for possess evolutionary functional properties, properties selected of do not. Properties selected of are the happy concomitants of properties selected for, but since they do not directly engage the demands of the environment they cannot acquire evolutionary functional properties. Second, while properties selected of do not possess evolutionary functions, their existence nevertheless is explicable in terms of evolution by natural selection. In particular, their existence is explicable in terms of the selective efficacy of the properties selected for with which they are connected. And this means that the existence of properties selected of is every bit as probable as the existence of properties selected for. I assume that functional properties comprise a proper subset of all the effects produced by any organismic trait; I assume, that is, that not every effect of any organismic trait is functional. The heart, for example, produces myriad causal effects, including the pumping of blood, the production of thumping noises, vibrations of the sternum, and so on, but not all such effects are functional. With respect to functional properties produced by natural selection, then, not all properties preserved by selection qualify as functional. In particular, only those that directly answer salient selective pressures those selected for qualify as evolutionary functions; other organismic properties preserved by selection those selected of do not qualify as evolutionary functions. It is important to note that we can explain the existence of all such effects those selected for and those selected of in terms of selection. This is a significant point: It is a mistake to assume that all selective explanations justify the attribution of evolutionary functions. They do not. A selective explanation of some organismic trait warrants the attribution of an evolutionary function only if the explanation appeals to an effect of the organismic trait that was selected for. If the explanation appeals instead to an effect that was merely selected of, no such attribution is warranted. I also assume that it is possible that not every organismic trait possesses an evolutionary function. One phenotypic trait, such as the human appendix, may be selected of because the genotypes that code for that trait are connected to other genotypes, where the latter code for phenotypes with effects that are selected for.

12 570 PAUL SHELDON DAVIES These latter effects may include, for example, the effects of the liver or of the kidneys. But, again, we explain the existence of all three traits in terms of natural selection. Livers and kidneys exist today because certain of their effects were selected for, or so we may assume, and the appendix exists today because it was selected of insofar as it is connected to traits such as livers and kidneys. Both explanations appeal to evolution by natural selection, but only one justifies the attribution of evolutionary functions. It may be objected that the theory of evolutionary functions endorsed here is not endorsed by evolutionary psychologists. That may be correct. Even so, my use of the theory of evolutionary functions is justified on four distinct grounds. First, this theory is the best developed and most widely accepted theory of natural selective functions within contemporary philosophy of biology. 20 Second, it is plausible that the theory of evolutionary functions captures the normative dimension that Cosmides and Tooby ascribe to functional properties (see Section I above). Third, any acceptable theory of functions must provide principled resources with which to justify the attribution of functions for tasks that are genuinely functional, like the heart s pumping blood, as well as the resources with which to justify the withholding of functions in the case of tasks that are genuinely non-functional, like the heart s thumping. And I submit that any acceptable theory of functions that appeals to evolution by natural selection will find such resources only within the distinction between effects that are selected for and effects that are selected of. Fourth, in the end it does not matter what theory of functions evolutionary psychologists employ, for the criticisms I develop below rest upon two rather minimal assumptions. I assume that, for any organismic trait, only some effects are functional and others are not; to assume otherwise renders the concept function vacuous. I further assume that it is biologically possible that some organismic traits have no functions at all; to assume otherwise flies in the face of apparent instances, such as the human appendix, and this places the attribution of functions on grounds that are insulated from disconfirmation. So my use of the theory of evolutionary functions is unobjectionable. I turn now to the central claims of this Section. The First Claim: The distinction between selection for and selection of has several important consequences. Perhaps the most important is that it opens up the possibility that some organismic traits, including ones that are component parts within a larger complex whole, may be products of selection and nevertheless not possess evolutionary functions. To see this, consider the following three points. (a) Recall that ex hypothesi complex traits such as minds evolve via selection in a stepwise process, acquiring relatively simple component capacities in a piecemeal fashion. Let us grant that this is so. To grant that selection produces complexity in this manner, however, is not to grant that every component of the complex whole possesses an evolutionary function. For some components may have been selected of, not selected for. That is, some components may exist today not as a consequence of their own past selective efficacy but rather as a by-product of the

13 THE METHODS OF EVOLUTIONARY PSYCHOLOGY 571 selective efficacy of other components. So even if there exists a functional mesh between ancestral organisms and ancestral environments, we cannot conclude that all of our psychological capacities are part of the mesh. We cannot conclude, in short, that all components of the ancestral mind acquired evolutionary functions. (b) But the point is stronger than this. To grant that selection produces complexity in a stepwise process is not to grant that even the majority of components possess evolutionary functions. For it may be that a critical minority of components were directly selected for certain of their causal effects, while the less critical majority were selected of as a consequence of being connected to the minority. It may be that the important selective work was accomplished by just a few, where that work sufficed to preserve the rest. So even if there exists a functional mesh between ancestral organisms and environments, we cannot conclude that most of the mind s components possess evolutionary functions and hence we cannot conclude that most ancestral psychological capacities are part of the functional mesh. This may appear unlikely, but in fact it is not. Consider again the peppered moth. The organismic property selected for is dark coloration; dark coloration was selected for its camouflaging effect and hence the evolutionary function of dark coloration is to camouflage the organism. But consider the developmental processes involved in the production of such camouflaging capacities: Presumably certain genes code for proteins that combine to form dark pigmentation; presumably these genes interact with the genes that code for the proteins that produce wing tissue; and presumably the orchestration of all these genes is achieved by certain regulator genes. The genetic processes that produce dark coloration in wings are complex much more complex than the passive trait of having dark coloration and yet these genetic processes are not selected for. Because of their causal relations to the trait dark coloration, where dark coloration is selected for, these genetic processes are selected of. But they are not themselves possessed of evolutionary functions. 21 Likewise it is possible that the majority of the mind s components were selected of without being selected for, in which case relatively few components would possess evolutionary functions. (iii) But the point is stronger still. To grant that selection produces complexity is not to grant that any of the components in the complex whole possess evolutionary functions. For it may be that all of the components within a specific complex whole were selected of as a consequence of their connections to other traits, ones not part of the complex whole, that were selected for. So long as at least some organismic traits were selected for, it is possible that all other traits, including other complex ones, were selected of. So even if there exists an organism/environment mesh, we cannot conclude that any of the mind s components possess evolutionary functions and hence we cannot conclude that any are part of the functional mesh. This third case is most likely to appear biologically implausible, but it is not out of the question. Consider the wonderful example suggested by Robert Nisbet concerning our capacity for boredom. 22 It is undeniable that humans today possess

14 572 PAUL SHELDON DAVIES great capacity for boredom. It is likewise undeniable that this is a psychological capacity of first importance. Advocates of evolutionary psychology, consistent with their methods, will offer conjectures concerning the evolutionary function of our capacity for boredom. But Nisbet speculates that boredom has no such function. He speculates that, at some point in the evolution of our species, distant ancestors were predominantly hunters; they were hunters and they were hunted. Organisms possessed of relatively sophisticated nervous systems, ones that provided relatively greater awareness of potential threats and relatively greater bursts of aggressive behavior, probably were favored by selection. More specificaly, the more sophisticated nervous systems probably were selected for long periods of awareness and sudden fits of aggression. Over several generations, they thus acquired the evolutionary function of making organisms aware and aggressive. But eventually the environment changed. Eventually our ancestors gave up the chase and took up the plow. Both the necessity to hunt and the incidence of being hunted were dramatically reduced. But the high-powered nervous system remained. This shift in environment is the origin of the capacity for boredom. Or, perhaps more precisely, this shift in environment is the origin of the manifestation of boredom, the capacity being latent all along. 23 If this is correct, then the existence of boredom today is an evolutionary accident. It has no evolutionary function whatsoever. Our sophisticated nervous system has an evolutionary function, but our capacity for boredom does not, for it was not selected for any of its effects. One may object that boredom may well have been selected for once it appeared in our agrarian ancestors. Maybe so. But this does not touch the general point, namely that changes in environment may result in the emergence of new but highly significant psychological capacities. Changes in environment may result in the emergence of psychological capacities that are, from the point of view of our evolutionary history, entirely accidental. The change from a non-industrial to an industrial environment may suffice to trigger the emergence of new psychological capacities; indeed, all sorts of changes, including some that are quite recent, may suffice. Of course Nisbet s speculations may be false, but they are nonetheless important, for they provide a realistic and plausible illustration of the way in which a rather significant psychological capacity may have been entirely selected of, not selected for. It is not unreasonable to wonder, moreover, just how many of our psychological capacities evolved in this manner. Clearly the burden is on the evolutionary psychologist to provide evidence for the claim that, for any psychological capacity, it is a product of selection for and not merely selection of. So the situation is this: It may be true, as advocates of evolutionary psychology insist, that there exists a functional mesh between ancestral organisms and ancestral environments. But this does not entail that ancestral psychological capacities are part of that mesh; this does not entail that all, or most, or any of our ances-

15 THE METHODS OF EVOLUTIONARY PSYCHOLOGY 573 tral psychological capacities possess evolutionary functions. The existence of an organism/environment functional mesh suffices to show that some traits or other belonging to our ancestors were selected for and thus acquired evolutionary functions. But it does not suffice to show which traits are involved in the functional mesh. The only way to save (MEP) is to show that the functional mesh does in fact include ancestral psychological capacities. Advocates of evolutionary psychology must show that at least some of the mind s component capacities are the product of selection for, not merely selection of, and thus possess evolutionary functions. But this requires argument. What is the evidence for the claim that some ancestral psychological capacities were selected for? Advocates of evolutionary psychology are wont to finesse this question. They simply appeal to the arguments rehearsed in Section II above. These assert that the complexity or efficiency with which a component capacity operates is evidence that it possesses an evolutionary function. 24 Since, at they claim, the probability of drift producing complexity is low, and since drift and selection are the only viable candidates, they conclude that complexity and efficiency in a phenotypic trait more or less entail selection for. 25 But as we have just seen, the inference is invalid. Complexity, efficiency, and the rest provide evidence only that some trait or other in the organism has been selected for; it does not suffice to indicate precisely which trait that is. So we may grant that the mind is complex, hence composed of interacting parts, hence efficient to some degree. We further may grant that ancestral organisms and ancestral environments comprise a functional mesh. But these claims alone are insufficient to establish that the mind s component capacities possess evolutionary functions. Advocates of evolutionary functions cannot demonstrate the existence of evolutionary functions on the basis of such observations; rather they must provide evidence concerning our natural selective past. And this is no easy task. I am not suggesting that all or most of our ancestral psychological capacities were in fact selected of; I am not embracing the claim that our minds are mostly or entirely the products of evolutionary accidents. I am making the modest methodological point that, since it is possible that some or all of our psychological capacities do not possess evolutionary functions, the burden lies with advocates of evolutionary psychology to show otherwise. Evolutionary psychologists are not entitled to take for granted that our ancestral psychological capacities are endowed with evolutionary functions; they must offer acceptable explanations for such historical conjectures. 26 And, again, this is no easy task. Indeed, to show of any trait that it is endowed with an evolutionary function requires an adequate how-possibly explanation, that is, an historical explanation of how it is possible that this particular kind of trait in this particular kind of organism was selected for. I will not explicate the structure of how-possibly explanations here, 27 but the important points are these: The goodness of an how-possible

16 574 PAUL SHELDON DAVIES explanation depends on the goodness of the information available concerning the actual selective processes out of which the relevant capacity emerged. The goodness depends on information concerning the specific capacity, the structure of the organism and its other capacities, the demands and resources of the selective environment, and the laws of nature operative during the relevant stretch of selective history. The less reliable our information concerning these elements, the less reliable the atribution of evolutionary furnctions. The Second Claim: Let us consider, then, how-possibly explanations of psychological capacities. The methodological aim of evolutionary psychology, recall, is to discover the correct computational theory of ancestral psychological capacities by discovering the evolutionary functions of such capacities. As we have seen, this means that the discovery of the correct computational theory requires an adequate how-possibly explanation of the relevant psychological capacities. The important point is that the goodness of an how-possibly explanation depends upon the availabilty of information concerning the organism s psychological capacities and the environments in which these capacities operate. In order to establish how it is possible that the specific capacity was selected for, we must begin with a specification of the organism s psychological constitution, in particular with a characterization of the specific capacity and a characterization of other psychological capacities that bear on the specific capacity under study. We must assume that this specification is more or less correct and we must show that, given the organism s psychological structure and the demands of its environment, it was this specific capacity that provided the selectively efficacious outputs and not other psychological capacities. If we cannot do this if we cannot show that it was this specific capacity that provided the selectively efficacious output and not others then we are not justified in attributing an evolutionary function to the capacity specified. The crucial upshot is that how-possibly explanations presuppose that we know in advance a good bit about the the psychological capacities we are trying to understand. Advocates of evolutionary psychologists, insofar as they endorse (MEP), claim that we discover the correct theory of our ancestral psychological capacities by appeal to the evolutionary functions of such capacities. But that is false; in fact, the very opposite is the truth. In order to justify the attribution of evolutionary functions, we must already know the psychological functions of the capacities possessed by our ancestors. If we do not, if we are unable to hold fixed the other capacities that were operative in our ancestors at the relevant time, then we cannot justify the claim that it was this capacity, as opposed to thoseothercapacities, that did the important selective work. So how-possibly explanations of the mind s component parts require the prior postulation of a computational theory of the mind s component capacities. Such explanations cannot, therefore, aid us in the discovery of the mind s capacities. At best, they can help confirm the quite different and relatively banal claim that our proposed computational theory is compatible with a natural selective explanation. 28