Evolutionary

Psychology

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1. What Is Evolutionary Psychology?

The term "evolutionary psychology" is sometimes used simply as "a shorthand for 'psychological theorizing informed by modern evolutionary theory'" (Daly & Wilson 1988, p. 7), a shorthand for "understanding the human mind/brain mechanisms in evolutionary perspective" (Buss 1999, p. 3). This portrays evolutionary psychology as a field of inquiry, which is so broad as to cover work ranging from studies of the optimality of foraging and birth spacing in hunter-gatherer societies to studies of encephalization (the progressive increase in brain size relative to body size in the human lineage) and the evolution of altruism and language. This broad range of work varies significantly in fundamental theoretical and methodological commitments, and it is united only by a very general commitment to the idea that human cognition, emotion, and behavior can be understood from an evolutionary perspective.

Many researchers in this field often deliberately resist the "evolutionary psychology" label, however, preferring to classify their work as, for example, human sociobiology, human ethology, human behavioral ecology, or evolutionary anthropology. The reason is that the term "evolutionary psychology" is increasingly being used to designate only work conducted within a specific set of theoretical and methodological commitments shared by a prominent and influential group of researchers (most notably the psychologists David M. Buss, Leda Cosmides, and Steven Pinker and the anthropologists Donald Symons and John Tooby). This group is united in the belief that adoption of an evolutionary perspective on human psychology immediately entails a number of very specific theoretical and methodological commitments. These commitments have been forcefully articulated in two important manifestos (Barkow, Cosmides, and Tooby's The Adapted Mind (1992) and Buss' "Evolutionary Psychology: A New Paradigm for Psychological Science" (1995)), which also reproduce what are considered exemplars of empirical research conducted by this group (for example, Buss' work on mate preferences, Cosmides' work on cheater detection, and Daly and Wilson's work on "discriminative parental solicitude"). Further, Buss compiled the group's fundamental commitments and exemplars in a textbook, Evolutionary Psychology: The New Science of the Mind (1999), which aims to be the source from which a growing number of future practitioners will be trained and to contribute "in some modest measure to the fulfillment of a scientific revolution that will provide the foundation for psychology in the new millennium" (p. xix). In this narrower sense, then, "evolutionary psychology" designates research within a Kuhnian paradigm (Kuhn 1996). And when researchers in the field of evolutionary psychology deliberately refer to their work as "human behavioral ecology," for example, they do so to distance themselves from this paradigm.

So as to clearly distinguish field from paradigm, in this Guided Tour and its Bibliography, I will refer to the field of inquiry as "evolutionary psychology" (in lower case) and the paradigm as "Evolutionary Psychology" (in upper case). In light of this distinction, I can clarify that what follows is not a Guided Tour of evolutionary psychology, since that field of inquiry is too amorphous to tour within the present forum. Instead, this is a Guided Tour of Evolutionary Psychology, the paradigm. In the next section, I will provide a brief overview of the fundamental theoretical commitments of Evolutionary Psychology and the reasons offered in support of these commitments. Subsequent sections will focus on criticism of a few theoretical tenets that are central to the paradigm. (Buller (in preparation) provides critical discussion of its methodological commitments and exemplars in addition.) Throughout, however, it must be borne in mind that critical comments are directed at Evolutionary Psychology the paradigm, and they are neither intended nor assumed to apply to all the work conducted with the field of inquiry that is sometimes called "evolutionary psychology."

2. Evolutionary Psychology, the Paradigm

The basic tenet of Evolutionary Psychology is that, just as evolution by natural selection has created morphological adaptations that are universal among humans, so it has created universal psychological adaptations. (An adaptation is a trait that has been fashioned by selection for its functional role in an organism). As Tooby and Cosmides say, "the fact that any given page out of Gray's Anatomy describes in precise anatomical detail individual humans from around the world demonstrates the pronounced monomorphism present in complex human physiological adaptations. Although we cannot directly 'see' psychological adaptations (except as described neuroanatomically), no less could be true of them" (1992, p. 38). The goal of Evolutionary Psychology, then, is to discover and describe the functioning of our psychological adaptations, which are the "proximate mechanisms" that cause our behavior (Cosmides & Tooby 1987).

Since the construction of complex adaptations is a very slow process of cumulative selection, typically requiring hundreds of thousands of years, Evolutionary Psychologists argue that our psychological adaptations cannot possibly be designed for modern life. Instead, they must be designed to solve the adaptive problems faced by our hunter-gatherer ancestors in the Pleistocene, the epoch spanning 1.8 million to 10,000 years ago (Tooby & Cosmides 1990b, 1992). In other words, "the evolved structure of the human mind is adapted to the way of life of Pleistocene hunter-gatherers" (Cosmides et al. 1992, p. 5). As a result, contemporary humans frequently behave non-adaptively (that is, in ways that don't promote their reproductive success), since our Pleistocene minds are not designed to respond adaptively to agricultural, industrial, or urban lifestyles (Symons 1989, 1990, 1992).

The adaptive problems faced by our Pleistocene ancestors ranged from acquiring mates and forming social alliances to avoiding predators and inedible plant matter (Buss 1995; Tooby & Cosmides 1992). Given the diverse natures of these many problems, Evolutionary Psychologists argue, a successful solution in one problem domain cannot transfer to another domain; so each adaptive problem would have selected for the evolution of its own dedicated problem-solving mechanism (Buss 1995; Cosmides & Tooby 1987, 1994; Symons 1987, 1992; Tooby & Cosmides 1992, 1995). As Symons says, "it is no more probable that some sort of general-purpose brain/mind mechanism could solve all the behavioral problems an organism faces (find food, choose a mate, select a habitat, etc.) than it is that some sort of general-purpose organ could perform all physiological functions (pump blood, digest food, nourish an embryo, etc.)" (1992, p. 142). Thus, Evolutionary Psychologists conclude, the human mind must be "organized into modules or mental organs, each with a specialized design that makes it an expert in one arena of interaction with the world. The modules' basic logic is specified by our genetic program. Their operation was shaped by natural selection to solve the problems of the hunting and gathering life led by our ancestors in most of our evolutionary history" (Pinker 1997a, p. 21).

For evolutionary psychologists, evolved modules have the following properties (Cosmides & Tooby 1994, 1997; Tooby & Cosmides 1992; compare this account with Fodor 1983).  (1)  They are domain specific, specialized to deal only with a restricted task domain. As such, their information-processing procedures are activated by, and sensitive to, only information about a particular aspect of the world, in much the way the ear is responsive only to specific vibratory frequencies.  (2)  They come equipped with substantial innate knowledge about their proprietary problem domains and a set of innate procedures specialized in employing that knowledge to solve problems in those domains.  (3)  They develop reliably and without formal instruction in every "normal" member of the species.

Given the enormous number of adaptive problems our Pleistocene ancestors faced, Tooby and Cosmides estimate that the human mind consists of "hundreds or thousands" of such evolved modules (1995, p. 1189). Thus inspired, Evolutionary Psychologists postulate evolved modules for incest avoidance, sexual attraction, mate choice, jealousy, mate retention, allocation of parental resources, kin relations, alliance formation, aggressive threat, danger avoidance, food preferences, habitat choice, and so on for all manner of complex cognitive and behavioral functions (Tooby & Cosmides 1992, p. 110).

Since evolved modules are adaptations, and since "selection usually tends to make complex adaptations universal," Evolutionary Psychologists argue that "humans must share a complex, species-typical and species-specific architecture of adaptations" (Tooby & Cosmides 1992, p. 38). Indeed, Tooby and Cosmides claim, "the psychic unity of humankind -- that is, a universal and uniform human nature -- is necessarily imposed to the extent and along those dimensions that our psychologies are collections of complex adaptations" (1992, p. 79; emphasis added). The modules that form this collection are "the psychological universals that constitute human nature" (Tooby & Cosmides 1990a, p. 19), and explanations of how these universal psychological mechanisms function will provide universal laws of psychology (Cosmides & Tooby 1987).

This idea of "the psychic unity of humankind" appears easily refuted by cultural diversity and individual differences. But Evolutionary Psychologists do not claim that there are universal human behaviors or that all humans share the same manifest attitudes and preferences. Rather, they claim that all humans share the same cognitive "Darwinian algorithms," which generate different behaviors and preferences in response to different developmental and occurrent inputs (Cosmides & Tooby 1987; Tooby & Cosmides 1990a, 1992). An Evolutionary Psychologist "observes variable manifest psychologies or behaviors between individuals and across cultures and views them as the product of a common, underlying evolved psychology operating under different circumstances....  [M]anifest expressions may differ between individuals when different environmental inputs are operated on by the same procedures to produce different manifest outputs" (Tooby & Cosmides 1992, p. 45). Consequently, Evolutionary Psychologists claim that "there is a universal human nature, but that this universality exists primarily at the level of evolved psychological mechanisms, not of expressed cultural behaviors" (Cosmides et al. 1992, p. 5; emphasis added).  A corollary of this claim is that "individual differences, including heritable individual differences, are unlikely to represent differences in the presence or absence of complex adaptive mechanisms" (Buss 1995, p. 11).

In summary, then, the fundamental theoretical tenets of Evolutionary Psychology are:

1.  The human mind consists of "hundreds or thousands" of evolved modules.
2.  These "hundreds or thousands" of modules are adaptations -- that is, they were designed by natural selection for their specific information-processing functions.
3.  These adaptations are designed to solve the problems of survival and reproduction that were faced by Pleistocene hunter-gatherer populations.
4.  Since selection typically makes complex adaptations universal in a species, evolved human psychology consists in "a single, universal panhuman design," which is human nature.
5.  Consequently, differences between individuals and across cultures are not produced by different psychological adaptations, but by a universal psychology responding to different developmental and occurrent circumstances.

The goal of Evolutionary Psychology, then, is to provide a complete account of human nature -- that is, a complete account of the modular psychological adaptations that comprise the human mind and of how those modules function under varying developmental, cultural, and occurrent circumstances.

3. Adaptationism

Stephen Jay Gould has accused Evolutionary Psychology of adaptationism, claiming that it "is the fatal flaw of Evolutionary Psychology in its current form" (1997, p. 57). In their famous critique of adaptationism, Gould and Lewontin (1979) characterize adaptationism as an attitude that "regards natural selection as so powerful and the constraints upon it so few that direct production of adaptation through its operation becomes the primary cause of nearly all organic form, function, and behaviour" (pp. 584-585). As a result, according to Gould and Lewontin, adaptationism views virtually every trait of every organism as an adaptation fashioned by selection for its role. (I will forgo a general discussion of adaptationism in this context and focus exclusively on Gould's argument against Evolutionary Psychology.)

According to Gould, Evolutionary Psychology's adaptationism resides in its claim that the mind consists of "hundreds or thousands" of modularized adaptations, since this greatly overestimates the extent of psychological adaptation. Gould argues that even beneficial psychological traits may not be adaptations, for "some useful characters did not arise by selection for their current roles" (Gould & Vrba 1982, p. 6), hence are not adaptations for those roles. Many useful traits are instead, he claims, exaptations, traits that "evolved for other usages (or for no function at all), and [were] later ‘coopted’ for their current role" (Gould & Vrba 1982, p. 6). As this definition entails, there are two kinds of exaptation. One kind is a co-opted adaptation, a trait that originated as an adaptation for one purpose, but then became used for some other adaptive purpose. The other kind is a spandrel, a trait that originally had no adaptive purpose whatsoever, having arisen merely as a developmental byproduct of an adaptation, but that became co-opted for some adaptive function.  (N.B.  A trait is adaptive if it enhances fitness in its bearers; it is an adaptation if it is possessed by organisms in a current population because they inherited it from ancestors in whom the trait enhanced fitness. In other words, a trait is adaptive if it has current utility; it is an adaptation if it had past utility, if it evolved because it was adaptive.)

Gould contends that "most of our mental properties and potentials may be spandrels" and not adaptations as Evolutionary Psychologists claim (1997, p. 52). According to Gould, the fundamental psychological adaptation in humans is a large brain, and virtually all other features of the mind are byproducts of a large brain. As Gould says, "I will accept the most orthodox of Darwinian positions -- that the human brain achieved its enlarged size and capacity by natural selection….  Large size is therefore an adaptation....  Natural selection built the brain; yet, by virtue of structural complexities so engendered, the same brain can perform a plethora of tasks that may later become central to culture, but that are spandrels rather than targets of the original natural selection" (1991, p. 57). It is absurd, Gould argues, to think that singing Wagner, reading, writing, religious belief, and consciousness of our own mortality were targets of selection (1997, p. 52; 1991, p. 57), so they must instead be mere byproducts of a large brain. Indeed, he claims, "for something so complex and replete with latent capacity as the human brain, spandrels must vastly outnumber original reasons, and exaptations of the brain must greatly exceed adaptations by orders of magnitude" (1991, p. 57).

There are several problems with Gould’s argument. First, Gould's examples of spandrels are red herrings, for Evolutionary Psychologists agree that such things as consciousness of our own mortality and religious belief -- not to mention singing Wagner -- are spandrels. The psychological phenomena that are adaptations, according to Evolutionary Psychologists, are "the major faculties of the mind" (Pinker 1997a, p. 174), the modularized psychological mechanisms that exhibit complex information-processing design and that are selectively responsive to information about adaptive problems our Pleistocene ancestors presumably faced. Since complex design is a standardly accepted hallmark of adaptation (Williams 1966), argue Evolutionary Psychologists, the very complexity of the "major faculties of the mind" makes them prima facie candidates for adaptation. But Gould does not even address -- let alone refute -- any Evolutionary Psychological hypotheses regarding such faculties.

Second, whether psychological exaptations outnumber adaptations "by orders of magnitude" depends on which phenomena one is counting. Gould’s examples -- such as singing Wagner, consciousness of mortality, and religious belief -- are of specific behaviors, mental acts, beliefs, attitudes, and preferences. Such phenomena are the outputs of psychological mechanisms, generated in response to the inputs of current experience and experiences during development. But Evolutionary Psychologists claim that our psychological adaptations are the mechanisms, or "major faculties of the mind," that generate such outputs, not the outputs generated by those mechanisms. To refute this claim, Gould would have to show that most of our psychological mechanisms are exaptations rather than adaptations. But Gould fails to show this, since his arguments appeal to the wrong class of examples.

Third, Gould argues that the fundamental psychological adaptation in humans is brain size. But there was surely no adaptive advantage to largeness of brain per se. As Pinker points out, the mere largeness of the human brain is purely detrimental. "If anything is a byproduct, it is the size of the human brain, which guzzles nutrients [18% of the body’s energy intake, although it is only 2% of the body’s weight], makes us vulnerable to blows and falls, compromises the biomechanical design of the woman’s pelvis, and makes childbirth dangerous. Bigness of brain is surely a byproduct of selection for more complex (and hence hardware-demanding) computational abilities, ones that allowed our ancestors to deal with tools, the natural world, and one another" (1997b, pp. 55-56). Thus, Gould’s claim about what is a psychological adaptation, which functions to separate adaptation from byproducts, gets things backwards: The capacities afforded by a larger brain are the adaptations, since they are what there was selection for, while the mere size of the brain is a byproduct of selection for those capacities.

At its most basic, Evolutionary Psychology can be seen simply as the attempt to articulate the selected-for capacities that were housed in an expanding brain. In this respect, then, Evolutionary Psychology seems unimpeachable. Of course, it doesn’t follow that each psychological mechanism postulated by Evolutionary Psychologists is in fact an adaptation or even that Evolutionary Psychologists have correctly identified any existing psychological mechanisms. There is plenty of room for Evolutionary Psychologists to be wrong about what kinds of psychological mechanism comprise the mind and to be wrong about the evolutionary history of a psychological mechanism they may have correctly identified. In fact, as we will see in the next section, Gould may be closer to the truth than Evolutionary Psychology (though for none of the reasons he gives): Our psychological adaptations are far fewer than Evolutionary Psychologists claim, and many of the "adaptations" postulated by Evolutionary Psychologists are indeed byproducts of more basic psychological adaptations. But, if Evolutionary Psychology is wrong in this way, it is not because it is benightedly searching for psychological adaptations, but because it has failed to correctly identify those adaptations.

4. Modularity and the Adapted Mind

As we have seen, Evolutionary Psychologists claim that the human mind contains "hundreds or thousands" of functionally specialized modules whose "basic logic is specified by our genetic program" (Pinker 1997a, p. 21). Over the course of human evolutionary history, according to Evolutionary Psychologists, modules "were cumulatively added because they 'reasoned' or 'processed information' in a way that enhanced the adaptive regulation of behavior" (Cosmides & Tooby 1997), and subsequently "their operation was shaped by natural selection" (Pinker 1997a, p. 21). Consequently, they conclude, "what is special about the human mind is not that it gave up 'instinct' in order to become flexible, but that it proliferated 'instincts' -- that is, content-specific problem-solving specializations" (Tooby & Cosmides 1992, p. 113).

Since each of our many modules was purportedly designed by selection to solve its own adaptive problem, Evolutionary Psychologists are committed to the claim that each module was shaped by selection independently of the others (though many modules may have evolved during the same time period). This in turn requires that, for each individual module, there were a large number of gene substitutions under selection that effected the addition and modification of the module. The genes that were involved in adding and modifying a module collectively constitute a gene complex that guides the development of the module (Tooby & Cosmides 1992, pp. 78-79). In Pinker's words, such a gene complex constitutes that part of the "genetic program" that "specifies" the "basic design" of the module (1997a, pp. 21 & 32).

This picture of the mind runs afoul of current knowledge of brain development (see Hardcastle and Buller (forthcoming), from which the following discussion is adapted). First, as Deacon (1997, p. 197) points out, if we possessed gene complexes that "specified" the designs of so many modules, there should be a positive correlation between brain complexity and genome size in the comparative record. But the comparative data show a relative constancy of genome size across enormous differences in brain complexity. Indeed, despite our vastly more complex brains, humans have the same number of genes (roughly 80,000) as the house mouse, Mus musculus. Moreover, it appears that most of the genetic "specification" for our brain concerns the more peripheral structures, rather than the cortical structures that implement our higher cognitive functions. As much as 50% of our genes may be concerned with building our brain, yet fully 4% of these (roughly 1,600) are concerned with building the sensory cells located inside our nose. Our genes do not seem to worry much about how to create most of our brain and are instead dedicated to making sure our sensory transducers are constructed properly.

Second, the brain structures that perform specialized functions develop through a process of diffuse proliferation of brain cells and connections followed by a "pruning" that shapes this diffuse connectivity into relatively specialized structures (Deacon 1997; Elman et al. 1996). That is, brain structures are the product of a process consisting of both "additive" events (the formation and migration of brain cells and the formation of neural connections) and "subtractive" events (the pruning of synapses through cell death and axonal retraction) (Elman et al. 1996). In this process, gene-directed protein synthesis is involved in the additive events that build the diffuse connectivity with which brain development begins. The subtractive events, however, are not under genetic control. Rather, the subtractive events occur through cell competition, whereby cells with the strongest patterns of innervation (primarily from sensory inputs) retain their connections and the others die. Thus, genes code for the proteins involved in the additive events during brain development, but the forms and functions of brain structures are then determined by brain cell/environment interaction. So the specialized brain structures we have are primarily environmentally induced, not "genetically specified."

We may, nonetheless, have been faced with recurrent adaptive problems throughout our evolutionary history, and our brains may have indeed recurrently produced information-processing solutions to these problems. But contrary to Evolutionary Psychologists' a priori argument, distinct "genetically specified" modules were not required to solve these recurrent adaptive problems.  Our brains hit upon a different solution: general plasticity that allows particular environmental demands to participate heavily in tailoring the responses to those very demands. This process can produce relatively stable brain structures that specialize primarily in particular information-processing tasks (that is, something like modules can emerge from this process), and these structures can be produced with some regularity across populations and down lineages. But, the extent to which modularized outcomes of human brain development have been regular throughout some of our evolutionary history is due to the fact that developmentally plastic human brains have encountered recurrent environmental demands throughout that history, not to "genetic specification" of the outcomes.

There are two morals to be drawn from this. First, the modularized outcomes of brain development have not been shaped by natural selection. For natural selection does not retain environmentally induced phenotypic characteristics of organisms; that would be Lamarckian evolution. Instead, natural selection retains only those genes that have fitness-enhancing effects on an organism's morphology. And, as we have seen, genes do not effect the pruning process that shapes brain structures. Consequently, the specialized structures in an adult human's brain are not the product of hundreds of thousands of years of cumulative selection for incremental, genetically-induced modifications to the human brain; they are instead the product of that individual's history of interaction with their environment.

Second, it is a mistake to seek adaptation among the products of brain development -- that is, among the relatively special-purpose brain structures that emerge during the course of brain development. Those products are highly plastic responses to environmental inputs. As Deacon (1997) argues, the human cognitive adaptation is, instead, the process that generates those special-purpose brain structures. That is, with the possible exception of our sensory transducers, it is not the contingently stable brain structures in an adult's brain that are adaptations; rather, the brain's very developmental plasticity is the adaptation, and the relatively stable structures are byproducts of that adaptation's functioning in a particular environment. (This is the sense in which Gould is closer to the truth than Evolutionary Psychologists.)

Evolutionary Psychologists frequently claim that their modular view of the mind straightforwardly follows from empirical results that appear to show that human reasoning is not "domain general" (or content independent), but "domain specific" (Cosmides & Tooby 1994). They explain these results with the hypothesis that each domain of reasoning must be subserved by its own "domain-specific mechanism," which is genetically specified and designed by selection. The alternative, according to Tooby and Cosmides, is the view that "all adult mental organization and content is ... cultural in derivation and substance" (1992, p. 115). But there is a middle ground, which is favored by the evidence about brain development.

Rather than consisting of a plethora of "genetically specified" modules, the "innate" structure of the brain consists in a comparatively small number of learning biases, which take the form of a heightened responsiveness to certain classes of stimuli (Karmiloff-Smith 1992; Elman et al. 1996). These learning biases increase the probability that interaction with the environment will eventually produce domain-specialized structures, but there is no isomorphism between the "innate" learning biases and the knowledge databases embodied in the eventually resulting structures. Rather, development proceeds by a process of gradually branching domain specificity (or problem specialization), and the initial learning biases pertain only to the first and most general domains in this process (Karmiloff-Smith 1992). For example, a relatively large chunk of an adult human brain is devoted to face recognition, but infants appear to preferentially attend to any stimulus consisting of three high-contrast blobs configured like the two eyes and the mouth of a face, and they show no preference for actual faces over blobs. There is a face-recognition learning bias, then, that takes the form of heightened responsiveness to three high-contrast blobs, but full-blown face recognition is the result of a gradual process of progressive specialization (Elman et al. 1996, pp. 115-118). There is no direct mapping from the very minimally specified "innate" learning bias to the complex knowledge structure embodied in a mature face-recognition "module." A brain that develops in this way will mimic one that possesses a plethora of "innate" modules, even though its "innate" structure is relatively minimal.

According to this alternative picture, human psychological adaptation does not consist in "hundreds or thousands" of "genetically specified" modules. Rather, the fundamental adaptation is the brain's developmental plasticity, which is capable of producing a wide variety of problem-specialized information-processing structures. Additional adaptations lie in the minimal learning biases instantiated in the early stages of brain development.

5. "Our Modern Skulls House a Stone Age Mind" (Cosmides & Tooby 1997)

As we have seen, Evolutionary Psychologists claim that each human psychological adaptation is adapted to Pleistocene conditions. Plotkin (1998) calls this claim "the thesis of ancient provenance," since it implies that our minds were fixed by "ancient" selection processes. I prefer to call it the "Flintstones theory of human nature."

Evolutionary Psychologists offer a single argument in support of this theory, which is a general theoretical argument concerning the evolution of complex adaptations. The 10,000 years since the end of the Pleistocene, they argue, "is only a small stretch in evolutionary terms, less than 1% of the two million years our ancestors spent as Pleistocene hunter-gatherers. For this reason, it is unlikely that new complex designs -- ones requiring the coordinated assembly of many novel, functionally integrated features -- could evolve in so few generations" (Cosmides et al. 1992, p. 5). In other words, there have only been 400 generations since the end of the Pleistocene, and that is too few, argue Evolutionary Psychologists, for genetic evolution significant enough to "assemble" complex psychological modules that are adapted to modern environments. Thus, they conclude, our psychological adaptations must have evolved, instead, during the Pleistocene, and the totality of these adaptations consequently "reflects completed rather than ongoing selection" (Tooby & Cosmides 1990b, pp. 380-381). Our psychological adaptations, then, must be adapted to the Pleistocene conditions under which they evolved.

But the argument commits a couple of simple fallacies. First, the issue is not whether "new complex designs" that require the "coordinated assembly" of many features could have emerged in the 10,000 years since the Pleistocene. For contemporary humans to differ in their evolved psychologies from their Pleistocene ancestors, it isn't necessary for selection to have built a new human mind from scratch in the last 400 generations. The issue is whether old complex designs, which evolved during the Pleistocene or even earlier, could have been significantly modified by selection in 400 generations. Since the argument doesn’t address this possibility, it fails to show that psychological adaptations must have remained adapted to Pleistocene conditions.

Second, the argument commits what Wilson (1994) calls "the 1% fallacy," in reference to the claim that the 1% of human evolutionary history since the Pleistocene is unimportant in comparison to the 99% spent as Pleistocene hunter-gatherers. As Wilson says, "it makes no sense to express evolutionary time as a proportion of the species history" (p. 226). It doesn’t matter whether a lineage spends only 1% of its evolutionary history in a new environment, Wilson argues; what matters is what kinds of change occur during that 1% of its evolutionary history. Thus, "rather than marvelling at the antiquity of our species, we should be asking what kinds of evolutionary change can be expected in 10, 100, or 1000 generations" (p. 226).

There is, in fact, ample evidence to think that the Flintstones theory of human nature greatly underestimates the kinds of evolutionary change that may have occurred since the end of the Pleistocene. In considering what kinds of evolutionary change may have occurred in the last 10,000 years, we need to distinguish two different questions. First, have the environments inhabited by human populations changed significantly since the Pleistocene? This question is important because, if the environment inhabited by a population remains unchanged, selection will favor the status quo in that population. On the other hand, if the environment changes, and with it the adaptive problems faced by a population, selection will pressure the population to adapt to the changing environment. Second, if the environment has changed, has there been sufficient time for an evolutionary response to the change? This question is important because, even if the environment has changed, it may be the case that the change has been too recent and too rapid to be tracked by an evolutionary response in human populations.

With respect to the first question, Wills (1998) demonstrates convincingly that it is undoubtedly true that the environments inhabited by (at least many) human populations have changed sufficiently since the Pleistocene to have created a selection pressure for evolution in some features of human psychology, and this is due largely to environmental changes produced by human activity. The agricultural and industrial revolutions, for example, changed environments in radical ways. Evolutionary Psychologists argue that, in spite of these changes, humans have still needed to live in groups and that the adaptive problems posed by human social life have remained largely unchanged from Pleistocene hunter-gatherer populations. But this is false. The agricultural and industrial revolutions precipitated fundamental changes in the social structures of human populations. Consider just two examples. First, while Pleistocene humans lived in groups of 50-150 individuals, post-agricultural humans have lived in increasingly larger groups, which has affected the challenges humans face when mating, forming alliances, or negotiating status hierarchies. Second, changing social structures have wrought radical changes in the kinds of task that must be performed to acquire the resources necessary for successful child rearing. Even if hunter-gatherers had evolved "Darwinian algorithms" that solved the problems of acquiring resources necessary for child rearing in savanna environments, such Darwinian algorithms would be useless in the world of Wage-Laborer Homo, since the tasks leading to acquisition of food and other resources have changed so radically. Such changes in human environments have surely altered the selection pressures on a variety of psychological mechanisms controlling interpersonal behavior. Consequently, there has undoubtedly been change in the selection pressures on human psychology sufficient to drive the evolution of human psychology.

But has there been sufficient time since the Pleistocene for an evolutionary response to these environmental changes? The question is not whether there has been enough time for human populations to evolve minds that are adapted to 21st century environments. Time lags are pervasive in evolution, and we can well expect human psychological adaptations to lag behind changes in the environments they inhabit. The question, instead, is whether there has been enough time for some evolution away from the putative psychological adaptations of our Pleistocene ancestors. There are two ways to answer this question, depending on whether we focus on genetic evolution or on phenotypic evolution.

Whether significant genetic evolution could have occurred in the 400 generations since the end of the Pleistocene depends on the degree of genetic variation that was present in human populations at the end of the Pleistocene. If a population is genetically diverse, then it can be highly responsive to a change in environmental conditions, since at least one of the existing genetic variants should be able to respond positively to the new conditions and begin to increase in frequency. In such a case, a population can evolve rapidly. On the other hand, if a population is genetically homogenous, it will be slow to respond to environmental change, since it must wait for mutation to introduce new positively responding variants.

Evolutionary Psychologists argue, as we have seen, that Homo sapiens emerged from the Pleistocene with a "universal and uniform" nature and with a universal genetic profile underlying this universal human nature (Tooby & Cosmides 1990a). Consequently, they obviously think that humans were genetically homogenous at the end of the Pleistocene (at least with respect to the genes that underlie adaptations), so that no significant genetic evolution can have occurred since. In the next section, however, we will see that there is no reason to think that current human populations are genetically homogenous with respect to the genes underlying adaptations.  And, if that is true of contemporary populations, there is no reason to think it was false of human populations as they emerged from the Pleistocene. Consequently, it is quite likely that there has been some genetic evolution in human populations since the Pleistocene. Of course, ultimately it is an empirical question as to whether there has been significant genetic evolution among humans since the Pleistocene. However that question gets answered, though, Evolutionary Psychologists cannot rule out such evolution on a priori grounds alone.

But, even if we grant Evolutionary Psychologists that there has been no significant genetic evolution in post-Pleistocene human populations, it doesn’t follow that the psychological mechanisms in contemporary humans strongly resemble those of their Pleistocene ancestors. For psychological traits -- including all psychological mechanisms -- are phenotypic traits, and there can be significant phenotypic evolution even in the absence of genetic evolution. The reason is that the frequencies of phenotypes in a population can change across generations simply as a function of transgenerational changes in developmental conditions. This is because genotypes produce phenotypes only in interaction with environments. So, the particular phenotype that will develop from some specific genotype depends on the developmental environment. Successive generations of corn plants, for example, can be progressively taller even if we control for genetic constancy, since these changes can be brought about simply by changes in fertilization. Thus, if the developmental environment of a population changes, later generations can possess different phenotypes than earlier generations, even in the absence of genetic evolution.

This kind of phenotypic evolution can be significant, and unlike genetic evolution it can occur very rapidly. In one recent experiment, populations of guppies living in high-predation environments were divided, with a part of each population left in its high-predation environment and the other part moved to a low-predation environment. The reproductive strategies of the transplanted guppies evolved significantly within a mere eighteen generations. The descendants of the transplanted guppies matured to a larger size and achieved reproductive viability at a later age than the non-transplanted guppies. More significantly, they produced fewer litters, with fewer and larger offspring in each litter, and they allocated less of their resources to reproduction during their early reproductive lives.

Thus, as Sterelny (1995) argues, the profound changes in human developmental environments since the Pleistocene could have resulted in the evolution of psychological phenotypes even in the absence of genetic evolution. That is, since the same genotype can produce different phenotypes in different environments, my genotypes could be identical to those of my Pleistocene ancestors, yet our psychological phenotypes could be different due strictly to differences in our developmental environments.  In that case, although my genotypes would be adapted to the Pleistocene, my psychological phenotypes would not be. Indeed, given the evidence I reviewed in the last section about how the development of brain structures is largely environmentally driven, many of the psychological mechanisms housed in the skulls of contemporary humans no doubt differ dramatically from those housed in the skulls of their Stone Age ancestors.

Of course, Evolutionary Psychologists admit that there may be significant differences between contemporary humans and their Pleistocene ancestors. But they claim that all those differences concern behaviors, attitudes, or preferences, and that these result from identical psychological mechanisms encountering different developmental inputs. The argument about phenotypic evolution, however, applies to all phenotypic traits -- not only to behavioral or attitudinal phenotypic traits, but to psychological mechanisms as well. Changed environments can affect the psychological mechanisms that develop in individuals just as easily as they can affect the behavioral outputs of fully developed psychological mechanisms. Thus, there is no justification for thinking that human psychological evolution stopped at the end of the Pleistocene, having produced a full battery of psychological adaptations, and has held steady ever since. With but perhaps the very rare exception, Fred and Wilma do not walk among us.

6. "Human Nature"

One of the most obvious things in the world is that people differ in their attitudes, preferences, and behavioral responses to similar situations.  This is true not only of individuals form different cultures, but of individuals within the same culture. Individual differences, in short, are ubiquitous. According to Evolutionary Psychologists, however, "variable manifest psychologies or behaviors between individuals and across cultures" are "the product of a common, underlying evolved psychology operating under different circumstances" (Tooby & Cosmides 1992, p. 45). This doctrine relies very heavily on the claim that our putatively universal psychological adaptations embody conditional rules that specify different manifest behaviors, preferences, and attitudes for each set of possible developmental or occurrent circumstances. For, only given this assumption is it possible to reconcile manifest variation with putative universality:  Variation exists only among the outputs of our psychological adaptations, and this variation is strictly a function of variation in the inputs to invariant psychological adaptations. Manifest psychological variation, according to Evolutionary Psychologists, is never a function of variation in the underlying psychological mechanisms themselves; indeed, according to Evolutionary Psychologists there is no variation among the psychological adaptations of "normal" human beings.

There are two arguments offered in support of this view. One is the commonsensical, largely rhetorical, appeal to Gray's Anatomy with which I opened section 2. I will return to this argument momentarily. The other argument is intended, and has been taken, to be a more serious and definitive theoretical argument. This argument is from Tooby and Cosmides (1990a), and it has been uncritically accepted and repeated by other Evolutionary Psychologists as definitive proof that adaptations must be species universals (Buss 1995; Pinker 1997a; Symons 1995), so that there can be no adaptive variation in human psychologies.

The argument is as follows (Tooby & Cosmides 1990a, pp. 43-48; cf. Tooby & Cosmides 1992, pp. 78-79):

1. Hundreds or thousands of genes are required to build each adaptation during the course of development.

2. If individuals of a sexually reproducing species differed in the genes required to build adaptations, sexual reproduction would break apart the gene complexes necessary to build adaptations, and no adaptation would be reliably reproduced across generations. (This is because sexual reproduction is a process in which an offspring's genetic makeup is determined by the random sampling of parental genes. So, for example, if one parent possessed all the genes necessary for an adaptation, while the other parent lacked some of them, the randomness of the sampling process would make it immensely improbable that all the genes necessary for the adaptation would be present in the offspring. As a result, adaptations would not be reliably reproduced across generations, and this in turn would mean that no adaptive trait would persist long enough to be modified by selection in the direction of greater functional effectiveness.)

3. Each adaptation must, therefore, be the product of genes shared by all humans and, hence, each adaptation itself must be a species universal (although we must recognize the minor exceptions of "abnormal" individuals).

4. Consequently, whatever genetic diversity exists in human populations must affect only non-adaptive traits (like hair color) or mere "quantitative variation" in adaptive traits (affecting the size of one's heart, for example, though not whether one has one).

On the basis of this argument, Evolutionary Psychologists conclude that there must be "a universal and uniform human nature" (Tooby & Cosmides 1992, p. 79).

Evolutionary Psychologists do, however, recognize one exception to this claim of species universality. Since mating and reproduction pose different problems for the two sexes, selection has designed certain sex-specific suites of complex adaptations for solving these problems. With respect to such phenomena as mate choice, then, human nature bifurcates along the fault line of sex, with each sex possessing its own unique set of adaptations, its own "nature." These adaptations, while not species universals, are nonetheless universal in each sex (again excepting "abnormal" individuals). So, male and female do constitute distinct adaptive morphs (Tooby & Cosmides 1990a, p. 40).

The argument, however, is multiply problematic. First, as Wilson (1994) and Griffiths (1997) point out, if the argument were sound, there would be no adaptive genetic variation (that is, no genetically based variation in adaptations) in any sexually reproducing species. In other words, if the argument were sound, no population of a sexually reproducing species would possess a balanced polymorphism, in which each adaptive morph is functionally specialized in a way that enables it to be as successful as its competing morphs. (A balanced polymorphism exists when selection maintains a mix of alternative phenotypes in a population. The alternative phenotypes are maintained at that relative frequency at which they enjoy equal fitness.) To simply illustrate a balanced polymorphism, consider male reproductive strategies among the marine isopod crustacean P. sculpta. Males of this species come in small, medium, and large, and these sizes perfectly correlate with distinct mating strategies. Large males secure and "guard" harems of females in the recesses of sponges, acquiring their copulations with the harem members. Small males are unable to compete with large males for the acquisition of a harem, so they acquire copulations by "sneaking" past inattentive large males and thereby gaining access to the females in the harem. Medium males morphologically resemble females, so they "mimic" the female courtship display with the large male; thinking he is acquiring another female for his harem, the large male allows the medium male entry to the harem, where the medium male then copulates with the females inside. These three morphs have equal reproductive success, so they are all equally adaptive, and the genes underlying these distinct adaptive morphs have been identified. So we have here a clear violation of the assumptions at play in Tooby and Cosmides' argument. And such examples abound.

Second, the argument mistakenly assumes that selection acts only on qualitative variation and that, as long as individuals are "qualitatively identical," quantitative differences are selectively irrelevant. This assumption is false, for selection frequently acts on, and serves to maintain, quantitative differences among individuals in a population. Indeed, sexual dimorphism, which Evolutionary Psychologists take to be a "qualitative" difference, is actually the result of selection acting on quantitative differences in gamete size. This form of selection, called disruptive selection, favored the two extremes of gamete size (favoring large gametes for the nutrients they could store and small ones for their motility in reaching the larger gametes), while selecting against medium-sized gametes. So quantitative variation in a population is not necessarily selectively irrelevant. Some observable quantitative variation may actually be acted on and maintained by selection; and, when it is, it is a kind of balanced polymorphism.

Third, the argument mistakenly assumes that, since adaptations require hundreds or thousands of genes for their development, if individuals differ in some adaptation, that difference must parallel a difference with respect to hundreds or thousands of genes. As we have seen, Evolutionary Psychologists consider male and female to be distinct adaptive morphs, yet sex difference is a product of a single gene difference, the SRY gene on the Y chromosome, which codes for testis-determining factor. Of course, SRY produces its effects only against a background of hundreds or thousands of genes shared by males and females; but the differences in adaptations result from a single gene difference against that background. As Wilson (1994) argues, there could be many other adaptational differences in humans that are likewise due to single gene differences.

Consequently, the argument utterly fails to show that there cannot be some balanced polymorphisms in human populations (that is, some variation that is maintained by selection). And this, in turn, means that the argument fails to show that selection must have created "a universal and uniform human nature." Once again, Evolutionary Psychologists fail in their attempt to substitute a priori argument for empirical investigation.

Evolutionary Psychology's other argument, however, is more of an appeal to common sense, and it thereby garners more intuitive credibility for Evolutionary Psychology's claim that there is a universal human nature, since it makes the denial of that claim seem quite literally incredible. This is the argument from Gray's Anatomy. As Tooby and Cosmides say, "everyone has two eyes, two hands, the same sets of organs, and so on" (1992, p. 78). There is no reason, Evolutionary Psychologists argue, to think that selection hasn't similarly built a "universal architecture" in the human mind, which constitutes the "human nature" that Evolutionary Psychology seeks to investigate. But there are problems even with this seemingly commonsensical argument.

First, as Wilson points out, "uniformity at the coarsest scale does not imply uniformity at finer scales" that are still selectively relevant (1994, p. 224). Every human may have a brain with two hemispheres, a cortex, an occipital lobe, and so on, but this doesn't imply universality of more micro-level psychological mechanisms. Since Evolutionary Psychologists claim that our universal psychological adaptations are modules, which are finer-grained brain structures, they need to demonstrate universality at this "finer scale." But the argument from Gray's Anatomy fails to do so.

Second, the "coarsest scale" to which Evolutionary Psychologists retreat in their argument from Gray's Anatomy is incommensurate with their definition of human nature as consisting of "qualities that define us as a unique species" (Buss 1999, p. 47). For the universals appealed to in these arguments (two hands, two eyes, a stomach, skin) characterize the whole primate order and sometimes the whole class of mammals and even all vertebrates. So the analogical appeal to this "coarsest scale" of uniformity supports no conclusion about universal psychological adaptations that are specifically human (which evolved during human evolutionary history) and that, hence, constitute human nature.

Moreover, third, the basic structural plan that typifies the "universal architecture" of a species -- and that, at ever coarser scales of description, typifies the body plan of an order, class, and subphylum -- consists primarily of features that have persisted down lineages and through speciations for tens to hundreds of millions of years. Thus, while selection probably played a role in designing the structural plan of humans, it did not design that structural plan during human history, but during the history of the common ancestor of humans and other primates or vertebrates. So we can't really infer anything about psychological adaptations, which purportedly resulted from selection during relatively recent human history, from the fact that all humans (except the "abnormal") have two eyes, two hands, one nose, and a mouth.

Of course, this doesn't mean that there are no psychological universals of the sort that might interest Evolutionary Psychologists (although I'm skeptical that there are any). It just means that there are no a priori considerations definitively showing that there are such universals awaiting discovery by Evolutionary Psychological investigation. It also means that, insofar as Evolutionary Psychology takes psychological adaptation as its object of inquiry, it must be prepared to investigate psychological variation just as studiously as any potential psychological universality. In other words, evolved "human nature" isn't constituted solely by psychological universals, but is at least partially constituted by adaptive variation.

This, however, prompts some questions. First, what if there are psychological universals? What should we make of them? As we saw in our discussion of the "Flintstones theory of human nature," Evolutionary Psychologists are mistaken in thinking that the totality of human psychological adaptations "reflects completed rather than ongoing selection" (Tooby & Cosmides 1990b, pp. 380-381). Selection is undoubtedly continuing to modify trait frequencies in human populations. In short, human populations are continuing to evolve. That means that any psychological universals we might happen to discover are temporally bound. They characterize human populations at a given moment in evolutionary history, and they are subject to change. Today's universals may be possessed by only a fraction of the species, or even extinguished, tomorrow. Thus, as Hull (1989) argues, it is a mistake to think, as Evolutionary Psychologists do, that any universals we might discover reveal to us the "nature" of our species, in any interesting sense of "nature."

Moreover, if there psychological universals, at least some of them will be the result of genetic drift, rather than natural selection (since some portion of all fixated traits are due to drift). Of course, Evolutionary Psychologists argue that drift-fixated traits are not typically incorporated into the "functional design" of the organism (Tooby & Cosmides 1990a). For this reason, they don't take drift-fixated psychological traits to be part of "human nature," and consequently such traits are not of explanatory interest to Evolutionary Psychologists. But why restrict the concept of "human nature" to adaptations, for drift-fixated traits are every bit as psychologically real as selection-fixated traits? Why conceive of "human nature" only as what has evolved by selection, rather than as the totality of psychological traits that have evolved by any evolutionary mechanism?

This question returns us to the issue of adaptationism, although not exactly in the way that Gould formulates it. Evolutionary Psychology truly is adaptationist in the sense that it takes adaptations to occupy some privileged place in our psychologies. But nothing in orthodox neo-Darwinian evolutionary theory justifies treating adaptations as somehow more "central" than drift-fixated traits, as somehow a part of the core "nature" of a species in a way that drift-fixated traits are not. To privilege adaptations in this way is to adopt what Godfrey-Smith (1999) calls a form of natural theology: It is to replace God with Natural Selection as the Creator, but to still maintain that the Creator's "intention" (as manifested in what is selected-for) represents the "nature" of a species, departure from which is "abnormal." But this particular way of wielding evolutionary theory is not intrinsic to evolutionary theory itself; it is an unjustified addition to evolutionary theory. Consequently, there is no evolutionary justification for the adaptation-centered concept of "human nature" employed by Evolutionary Psychology.

7. Conclusion

There can be little doubt that evolution has occurred and that Homo sapiens is among its products. There can also be little doubt that the evolutionary history of our lineage has left its marks on human psychology just as assuredly as it has left its marks on human morphology. The human mind, unquestionably, is the product of evolution. But what follows from this fact? In the foregoing, I have reviewed a paradigm calling itself "Evolutionary Psychology," which claims that, once we accept the fact that the human mind is the product of evolution, we can immediately infer a number of facts about the nature of the human mind. Previous sections have shown, however, that all of the theoretical commitments of this paradigm are either misguided or unsubstantiated. This does not imply, however, that the project of an evolutionary psychology (that is, qua field of inquiry) is bankrupt. It implies, rather, that the field of evolutionary psychology will progress toward a more adequate evolutionary perspective on human psychology only once the Evolutionary Psychology paradigm, which has become prominent in the field, has been replaced.

Acknowledgment
This work was supported by National Science Foundation grant SES9985820.

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