Adaptation versus Phylogeny: the Role of Animal Psychology in the Study of Human Behavior
نویسندگان
چکیده
Advocates of Darwinian approaches to the study of behavior are divided over what an evolutionary perspective is thought to entail. Some take "evolution-mindedness" to mean "phylogeny-mindedness," whereas others take it to mean "adaptationmindedness." Historically, comparative psychology began as the search for mental continuities between humans and other animals: aphylogenetic approach. Independently, ethologists and now behavioral ecologists have placed far more emphasis on the nichedifferentiated mental abilities unique to the species being investigated: an adaptive approach. We argue that the output of complex, dynamical systems can be dramatically changed by only minor changes in internal structure. Because selection acts on the consequences of behavior, the behavioral output of the psyche will be easily shaped by adaptive demands over evolutionary time, even though the modification of the neurophysiological substrate necessary to create such adaptive changes may be minor. Thus, adaptation-mindedness will be most illuminating in the study of cognition and behavior, whereas phylogeny-mindedness will be most illuminating in the study of their neurophysiological substrates. Similarly, a phylogenetic approach to cognition and behavior is likely to cause one to overlook our most interesting, complexly designed species-typical traits, whereas using animal psychology to exfoliate general principles of behavioral ecology represents our best hope of understanding humanity's many zoologically unique characteristics. Darwin, with the publication of the Origin of Species (1859) and the Descent of Man (1871), united the human and the animal worlds into a single system by proposing an explanation for species' characteristics, including their similarities and differences from each other, in terms of a the operation of intelligible natural causal processes. By tying all animals together in a single tree of descent, Darwin made the study of every species relevant to the study of every other species. Animals drawn from a different species are separated only by phylogenetic distance; character differences separating different phylogenetic groups were produced either by chance, or they were driven by niche-differentiating selection pressures. Address all correspondence to Leda Cosmides, Department of Psychology, Bldg. 420, Stanford University, Stanford, CA 94305, USA. @ 1989 Human Sciences Press 175 176 THE INTERNATIONAL JOURNAL OF COMPARATIVE PSYCHOLOGY This scientific account of the nature of living things embedded human characteristics in the world of natural cause and effect, and constituted a radical attack on the nearly universally accepted doctrine of total human singularity: the idea of humans as a special divine creation, or as outside of nature, or as subject to principles-supernatural or otherwise-totally alien to those operating in the rest of the world. Darwin's departure was so radical that the human sciences are still F trying to come to terms with it, and the debate about human singularity remains with us today. The guise in which this debate continually reemerges changes-from claims about reason, or intelligence, or language, or learning, or emergent social processes, or the superorganic nature of human culture-but the attachment to the idea of humans as subject to entirely unique principles is, to this day, the centerpiece of persisting, anti-Darwinian arguments (e.g., Durkheim, 1962; Kroeber, 1952; Sahlins, 1976). Implicit in the Darwinian revolution is the recognition that however interesting and endearing we humans might be to ourselves, we are simply one species out of an entire ocean of species; if humans are not the product of unique principles, if we are simply one causal outcome in a larger scientific landscape, then there is potentially a general and principled science that encompasses the entire animal world (Darwin, 1871; Williams, 1966; Staddon, 1988; Tooby & De Vore, 1987). There are three positions one might take on human singularity. The first is that humans are not unique, but rather are typical animals (e-g., Skinner, 1938,1953,1957; see also Macphail, 1987). Although this view is clearly true when humans and nonhuman animals are compared along certain dimensions, such as in terms of their biochemical and cellular processes, for many other dimensions such a view is difficult to sustain, given human agriculture, machine tools, VCRs, ping pong, nonsense verse, deconstructivist architecture, and so on. Much of manifest human behavior appears to be qualitatively different from that of other animal species. A second possible position derives from traditional religious and philosophical views, echoed in modern form throughout the social sciences (e.g., Descartes, 1977; Durkheim, 1962; Evans-Pritchard, 1954; L Geertz, 1973; Kroeber, 1952; Radcliffe-Brown, 1977; Sahlins, 1976): that humans are so singular, so incomparable, that they are either outside of nature, or at least a natural phenomenon sui generis, governed by their #I own special and incommensurate laws. More precisely, this position is that humans may (or may not) be legitimate objects of scientific study, but that the principles that created them or that govern them are unique, not derived from or applicable to any other species-that humans cannot be analyzed with conceptual tools other than those specially devised for understanding them. This, of course, means that that evolution is irrelevant, and that other animals are irrelevant, separated from humans by an unbridgeable gulf (Sahlins, 1976). JOHN TOOBY and LEDA COSMIDES 177 The third view is that humans have many unique, zoologically unprecedented properties that make us unlike any other species, but that this is not because humans are the product of unique principles particular to humans, but rather because we are the product of a unique combination of general evolutionary principles, which act across the field of animal life. Other forms of life also manifest zoologically strange features, such as the eusocial insects, with their separate castes linked to their bizarre genetic systems (Wilson, 1971). Yet, understanding them proved generally illuminating to our understanding of evolutionary principles, such as kin selection, that apply to all animals (Alexander, 1974; Hamilton, 1964; Williams & Williams, 1957; Williams, 1966; Wilson, 1975). By looking at each species as a unique combination of general evolutionary principles, it is possible to deduce what these natural causal principles are, and to see how, in each instance, they fit together in a unique, yet fully comprehensible way (Alexander, 1971, 1974; Tooby & DeVore, 1987). Darwin himself, along with his contemporaries, realized that the most controversial of his claims was that the evolutionary perspective applied with equal force to the psychological as well as the physiological (Darwin, 1873). Since Descartes, educated belief was quite willing to hold that the physical body was a machine, subject to physical law, and that animals were automatons, like the water-powered robots in the gardens of Louis XIY It was mental phenomena, believed to exist onlyin humans, such as reason, emotions, goal-seeking, language, and culture, that were separated off by religious belief and Cartesian dualism into the extraphysical, extranatural domain of the soul, the mental, the psychical (Descartes, 1977). It was this dualism that Darwin's much misunderstood concept of mental continuity was addressed to, and indeed was intended to refute: the dualistic claim that mental phenomena in humans represented a qualitatively different essence, a spiritual agency, constituting an abyss that evolutionary explanations could not bridge (Darwin, 1873). Darwin argued that mental faculties were explicable in "he same evolutionary terms that accounted for the origin of species and the acquisition of their physiological characteristics-a position that Alfred Russell Wallace, co-originator of the theory of natural selection, " was not willing to endorse: Wallace (1904) felt that human mental faculties required supernatural explanation. [Darwin's doctrine of mental continuity was subsequently overliteralized by early comparative psychologists into the idea that all animals existed on a linear continuum, with only quantitative differences in their capacities (see Hodos & Campbell, 1969, and Lockard, 1971, for a critical discussion of this issue; see Macphail, 1987, for a post-behaviorist defense of the doctrine of mental continuity)]. So Darwin's achievement was more than just the principled unification of the human, animal, and plant worlds: It was 178 THE INTERNATIONAL JOURNAL OF COMPARATIVE PSYCHOLOGY also the principled unification of the mental and physical worlds, joining the mental and physical characteristics of humans and other animals into the same system of causation, created by mechanistic evolutionary principles. This allowed a radical transformation of biology and psychology as sciences (Dawkins, 1976; 1986). Instead of being descriptive, particularistic sciences, fueled by unguided observation, the cataloguing of phenomena, and the inductive, atheoretical search for regularities, L biology and psychology could be seen as grounded in an elegant set of mechanistic evolutionary principles that provided a causal explanation of how each species acquired its distinctive characteristics-its design. Since Darwin's time, despite the appealing prospect of a powerful, general, evolutionary psychology, evolutionary approaches to psychology have waxed and waned in popularity several times, with, for example, the long behaviorist interregnum, motivated by the anti-Darwinian belief that learning and environmentalist influences somehow insulate behavior from evolutionary shaping and analysis (Skinner, 1953; Boakes, 1984). As evolutionary-oriented psychologists ourselves, we hope that finally, evolutionary psychology has arrived on the scene permanently, anchored this time by a deeper and more balanced understanding of the nature-nurture issue (Daly & Wilson, 1983; Lehrman, 1970; Lorenz, 1965; Marler and Hamilton, 1966; Mayr, 1974; Staddon, 1983; Syrnons, 1987; Tinbergen, 1951; for discussion, see Cosmides & Tooby, 1987; Tooby, 1985; Tooby & De Vore, 1987; Tooby & Cosmides, in press), and by more comprehensive and useful models of the evolutionary process (e.g., Hamilton, 1964; Williams, 1966; Dawkins, 1976, 1982). Unfortunately, however, there are fundamental differences in what an evolutionary perspective is thought to entail, generating substantial confusion. Two of the most important evolutionary principles accounting for the characteristics of animals are (1) common descent, and (2) adaptation driven by natural selection. Some take "evolution-mindedness" (Syrnons, 1987) to mean "phylogeny-mindedness," the search for phylogenetic continuities implied by the inheritance of homologous features from common ancestors. Others take evolution-mindedness to mean "adaptation-mindedness," the search for adaptive design, which usually entails the examination of niche-differentiated mental abilities unique to the species being investigated. Historically, comparative psychology began as the search for mental continuities between humans and other animals (Lockard, 1971; Hodos & Campbell, 1969): a phylogenetic approach that persists, in some measure, to the present (e.g., Boakes, 1984; Macphail, 1987). Independently, ethologists (subsequently joined by behavioral ecologists and many modern comparative psychologists) have placed far more emphasis on animal psychology as case studies in adaptive design (e.g., Alcock, 1989; Daly & Wilson, 1983; Dewsbury, 1984; Gould, 1982; Krebs & Davies, 1984; Staddon, 1988; Tinbergen, 1951; see discussion in Tooby & DeVore, 1987). The phylogenyJOHN TOOBY and LEDA COSMIDES I 179 minded tend to believe that the study of animal psychology provides observations that directly parallel human mental processes (Macphail, 1987), whereas the adaptation-minded tend to believe that the psychology of each species is distinct, and that direct leaps from one species to another are speculative and unfounded. One can see the interplay of these sentiments in the ape-language controversy: Half of the scientific audience cheers for the apes, hoping they can duplicate human linguistic feats, while the other half is confident that the apes' linguistic abilities will prove very limited. The phylogeny-minded form the apes' cheering section: They reason that if a human can learn a language, then our nearest relatives should be able to do so as well. The adaptation-minded are skeptics in the ape language controversy: They (correctly) see the acquisition of a language as a species-specific mental ability, requiring highly complex and specialized cognitive mechanisms that are not likely to be shared by other primates, who were not selected to participate in communication through linguistic behavior (Chomsky, 1975). Animals from different species are similar to each other in psychological architecture because of (I) common inheritance, (2) the same selection pressures operating on different species, or (3) both. Animals from different species differ in psychological architecture because of (1) independent descent, (2) the operation of different selection pressures on different species, or (3) random divergence. Both adaptive and phylogenetic components of the evolutionary approach havevalue, but their relative validity depends on exactly what level of psychological investigation they are applied to. To understand why this is so, it is necessary to deal with the issue of the complexity and domain-specificity of psychological mechanisms. As we and others have argued elsewhere (Cosmides & Tooby, 1987; Rozin, 1976; Syrnons, 1987), an evolutionaryperspective leads to the conclusion that although the psyche has some domain-general mechanisms, it must also include many domain-specific, function-specific mechanisms. This view draws support from artificial intelligence, whose history has largely been the history of discovering that information-processing procedures must be very complex indeed if they are to perform even very simple tasks (e.g., moving around a half a dozen blocks in a small area; see, e.g., Boden, 1977; Brown, 1987; Minsb, 1986; Cosmides,& Tooby, 1989). Work in cognitive science and artificial intelligence (AI) has shown that mechanisms capable of solving even supposedly simple real-world cognitive tasks must contain very complex "innate" prespecified procedures and/ or information, matched narrowly to the structural features of the domains within which they are designed to operate (Marr, 1982; Chomsky, 1975, 1980; Fodor, 1983). A1 programs are complex and function-specific because the world is itself complex in ways that are not logically analyzable or deducible without an enormous amount of a 180 THE INTERNATIONAL JOURNAL OF COMPARATIVE PSYCHOLOGY priori knowledge: In order to solve a task, you must already know a great deal about the nature of the circumstances in which the task is embedded. Of course, for "natural intelligence," as opposed to artificial intelligence, the origin of such necessary a priori knowledge and pro-
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