On the Universality of Human Nature and the Uniqueness of the

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On the Universality of Human Nature and the Uniqueness of the

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On the Universality of Human Nature
and the Uniqueness of the Individual:
The Role of Genetics and Adaptation
John Tooby and Leda Cosmides Center for Advanced Study m the Behavioral Sciences
ABSTRACT The concept of a universal human nature, based on a speciestypical collection of complex psychological adaptations, is defended as valid, despite the existence of substantial genetic variation that makes each human genetically and biochemically unique These apparently contradictory facts can be reconciled by considering that (a) complex adaptations necessarily require many genes to regulate their development, and (b) sexual recombination makes It improbable that all the necessary genes for a complex adaptation would be together at once in the same individual, if genes coding for complex adaptations vaned substantially between individuals Selection, interacting with sexual recombination, tends to impose relative uniformity at the functional level m complex adaptive designs, suggesting that most heritable psychological differences are not themselves hkely to be complex psychological adaptations Instead, they are mostly evolutionary by-products, such as concomitants of parasite-dnven selection for biochemical individuality An evolutionary approach to psychological vanation reconceptualizes traits as either the out-
We gratefully acknowledge David Buss for his detailed comments and many patient and helpful discussions of the issues addressed in this article Dan Weinberger for his assistance as our local "personologist' in residence, Don Brown, W D Hamilton, Don Symons, and two anonymous reviewers for their extensive and detailed comments on earher versions of this article, Jerry Barkow, Martin Daly, Roger Shepard, and Margo Wilson for assorted ideas, feedback, and inspiration, and Roger Shepard (and NSF Grant BNS 85-11685 to Roger Shepaid) and Irven DeVore for their ongoing support of our efforts We would also hke to thank Kathleen Much for her help with the manuscnpt, and the Gordon P Getty Trust and the Harry Frank Guggenheim Foundation for their financial support Correspondence should be addressed to John Tooby or Leda Cosmides, Center for Advanced Study m the Behavioral Sciences, 202 Jumpero Serra Boulevard, Stanford, CA 94305
Journal cf Personality 58 1, March 1990 Copynght © 1990 by Duke University Press CCC 0022-3506/90/$! 50


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put of species-typical, adaptively designed developmental and psychological mechanisms, or as the result of genetic noise creating perturbations in these mechanisms

Personality psychology has two distinct traditions the search for a universal human nature, and the search for an explanation of individual diflferences m psychological traits (Buss, 1984) These two traditions have developed m parallel, but cohabit m the same field uneasily because the conceptual relations between them are cloudy and often seem contradictory Paradoxically, theories of human nature make claims about a universal human psychology, whereas personality research into individual differences depends on the existence of stable, interesting diflferences between individuals, and correspondingly tends to ignore, deny, or minimize universals Of course, one half of the reconciliation between the two is a straightforward commonplace of psychological thinking A human nature composed of uniform psychological mechanisms may produce individual differences as a result of diflferent individual experiences It is the existence of genetic differences between individuals that poses problems It renders the study of the causation of individual diflferences difficult, and, more importantly, it calls into question the very idea of a universal human nature Indeed, some behavior geneticists are forceful about challenging the value of characterizing a shared human nature, given their estimation of the magnitude of genetic differences For this reason, they tend to focus on vanation rather than on universality "The questions that most often confront scientists studying human behavior are those dealing with differences among people And genetics, the study of vanation of organisms, is uniquely qualified to aid us in analyzing these individual diflferences" (Plomin, DeFries, & McCleam, 1980, p 11)
The tension between the two traditions m personality psychology has had Its direct analog in evolutionary biology (Buss, 1984) Theones of and claims about species-typical behavioral adaptations appear to conflict with the discovery, through molecular genetic techniques, of vast reservoirs of genetic vanabihty (Hubby & Lewontm, 1966, reviewed m Ayala, 1976, and Nevo, 1978, Lewontm & Hubby, 1966) Systematists find species to be clearly and recognizably charactenzable by species-specific, species-typical physical and behavioral traits, and yet on genetic grounds, each individual is a unique combmation of genes (with their associated traits), and vanes m tens of thousands of ways from Its conspecifics Is the concept of the psychic unity of humankind.

On Univeisality and Uniqueness


of a single, umversal human nature, insupportable in the light of what IS known about human and nonhuman genetics'' Can the uniqueness of the individual be reconciled with the claim of a universal human nature''
We believe that evolutionary biology provides the conceptual framework that allows this reconciliation Both the psychological universals that constitute human nature and the genetic diflferences that contribute to individual vanation are the product of the evolutionary process, and personality psychology must therefore be made consistent with the principles of evolutionary biology This means that every personality phenomenon is, from an evolutionary perspective, analyzable as either (a) an adaptation, (b) an incidental by-product of an adaptation, (c) the product of noise in the system, or (d) some combination of these Standards for recognizing these three varieties of evolutionary outcome will allow one to discover new, adaptively patterned personality traits and to place previous findings in evolutionary perspective In this article, we attempt to sketch out some of these standards In the process, we will argue that (a) some personality diflferences may be the expression of diflferent, environmentally triggered adaptive strategies, (b) diflferent adaptive personality strategies cannot, m principle, be coded for by suites of genes that diflfer from person to person, and (c) most heritable personality diflferences are not the expression of diflferent adaptive strategies They are either mutationally dnven genetic noise, or else an incidental by-product of an adaptation that has nothing to do with personahty per se—pathogen-driven selection for biochemical diversity (Tooby & Cosmides, 1988)

Evolutionary Foundations
An evolutionary perspective on nature and nurture
The environment as the product of evolution An evolutionary perspective IS not a form of "genetic determinism," if by that one means the idea that genes determine everything, immune from environmental infiuence Anyone with a biological education acknowledges that the phenotype is the result of the interaction between genes and environment, and all aspects of the phenotype are equally codetermined by this interaction Developmental programs (l e , the regulatory processes that control development) are directed by the genes, but they require and depend upon an entire range of properties of the environment bemg rehably and stably present in order to successfully produce


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a healthy individual If either the genes or the environment are sufficiently changed, the result will change Thus, as with all interactions, the product cannot be analyzed into separate genetically determined, as opposed to environmentally determined, components
However, because of the nature of the evolutionary process that creates this interaction, "genes" and "environment" exist in a highly structured relationship that is very different from popular conceptions of separate but parallel genetic and environmental "influences " Many social scientists have labored under the false impression that only certain things are under the "control" of the genes, that evolutionary approaches are relevant only to those traits under such "control," and that the greater the environmental influence or control, the less evolutionary analyses apply In place of evolutionary analyses of those things purportedly under genetic control, they conduct atheoretical or nonevolutionary explorations of those traits under (what they believe to be) "environmental" control This kind of erroneous thinking is associated with the idea that genes are "biological," whereas "the environment" IS nonbiological, the "social environment" is thought to be the opposite of "biological determination " But a close examination of how natural selection actually drives evolutionary processes leads to a very different view of how "genes" and the "environment" are related Evolution acts through genes, but it acts on the relationship between the genes and the environment The "environment" is as much a part of the process of evolutionary inheritance as are the "genes," and equally as "biological" and evolved No organism reacts to every aspect of the environment Instead, the developmental programs rely on and interact with only certain defined subsets of properties of the environment, while others are ignored For example, diflferent diets transform a female ant into a worker or a queen, but there is no diet that will transform her into a dog, and guitar music or religious exhortation does not affect her growth Over evolutionary time, genetic variation in developmental programs (with selective retention of advantageous variants) explores the properties of the environment, discovering those that are useful sources of information in the task of regulating development and behavior, and rendenng those features of the environment that are unreliable or disruptive irrelevant to development Across generations, this process of exploration of altemative gene-environment relations operates by varying developmental programs with respect to (a) what kinds of inputs from the environment they accept or are sensitive to, and (b) how they

On Universality and Uniqueness


shape phenotypic outcomes in response to these inputs "The environment" of an animal—m the sense of which features of the world it depends on or uses as inputs—is just as much the creation of the evolutionary process as the genes are Thus, the evolutionary process can be said to store information necessary for development in both the environment and the genes, in that it shapes the relationship of the two so that both are necessary participants in the ontogenetic construction of adaptations Both are "biologically determined," if such a phrase has any meaning

Environmentalism depends on nativism "The environment," per se, is powerless to act on the psyche of an animal, except in ways specified by the developmental programs and psychological mechanisms that already happen to exist m that animal at a given time These procedures take environmental information as input and generate behavior or psychological change as output The actual relationship between environment and behavior is created solely and entirely by the nature and design ofthe information-processing mechanisms that happen to exist m the animal, and m principle, information-processing mechanisms could be "designed" to create a causal relationship between any imaginable environmental mput and any imaginable behavioral output The smell of excrement may be repulsive to us, but it is attractive to dung flies Aside from a few gross effects, such as gravity, the relationship between the environment and the behavior of the organism is not a matter of physical necessity, but is decided by the structure of the organism's psychological mechanisms
The information-processing procedures that exist in an organism at a given time are either (a) genetically specified, that is, innate, or (b) the product of other, pnor procedures In the event they are the product of other, prior procedures, such prior procedures must themselves be either innate or the product of still other, even more antecedent procedures After ruhng out infinite regression as a tenable theory of the origins of psychological structure, one must necessarily conclude that the psyche of an organism at any point in time is the product of its innate procedures, plus the changes—including any constructed procedures and their eflfects—created by those mnate procedures operating on a sequence of environmental inputs Therefore, innate procedures must exist, are the necessary foundation of any full model of the psychology of any organism, and are always necessarily entailed by any envi-


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ronmentalist claim Environmentalist theories depend on pnor nativist theories, and therefore environmentalism and nativism are not opposed, but are instead interdependent doctnnes
Thus, valid environmentalism inescapably posits innately regulated psychological mechanisms Any environmentalist claim about the influence of a given part of the environment entails a claim about an innately specified relationship between the environment and the hypothesized psychological output Consider, for example, the claim that girls leam gender-appropriate behavior by watching their parents This entails the claims that (a) girls have innate mechanisms specialized for leaming gender-appropriate behavior (otherwise, why wouldn't a girl be just as likely to imitate her father''), (b) these mechanisms compute the frequency with which each parent performs vanous behaviors, and, for each behavior, compare the mother's tally to the father's, and (c) these mechanisms cause girls to imitate behaviors that their mothers perform more frequently than their fathers, and to avoid the behaviors that their fathers perform more frequently than their mothers Rather than escaping claims of innateness, this "socialization hypothesis" tacitly posits some rather sophisticated and specialized innate machinery linking informational input to behavioral output
Every coherent psychological theory has at its foundation innate mechanisms or procedures, either explicitly recognized or tacitly entailed To say such procedures are innate means that they are specified m the organism's genetic endowment, that is, in how genetically based programs regulate the mechanisms goveming development This genetically specified, innate foundation of the psyche is the product of the evolutionary process, and is the means through which the evolutionary process organizes the psychology of the animal over generations Evolutionary biology is relevant to psychology because it studies the evolutionary processes responsible for shaping the innate foundations of psychological mechanisms, just as it does for physiological mechanisms

Manifest variability and innate universals What is human nature"^ Genetics had enormous difficulty making progress as a science until geneticists drew the distinction between genotype and phenotype, that IS, between the inhented basis of a trait and its observable expression This distinction allowed them to move beyond the bewildenng complexity of surface charactenstics to an underlying level of clear pnnciples that explained the surface vanability We believe a similar

On Universality and Umqueness


distinction will be equally useful for an evolutionanly informed personahty psychology We will refer to this as the distinction between an individual's innate psychology and his or her manifest psychology and behavior If one believes in a universal human nature, as we do, one observes variable manifest psychologies, traits, or behaviors between individuals and across cultures, and views them as the product of a common, underlying evolved innate psychology, operating under diflferent circumstances (see, e g , Daly, Wilson, & Weghorst, 1982) The mapping between the innate and the manifest operates according to principles of expression that are specified in innate psychological mechanisms or in innate developmental programs that shape psychological characteristics, these expressions can diflfer between individuals when diflferent environmental inputs are operated on by the same procedures to produce diflferent manifest outputs (Cosmides & Tooby, 1987, Tooby & Cosmides, 1989) This set of umversal innate psychological mechanisms and developmental programs constitutes human nature Individual diflferences that arise from exposing the same human nature to diflferent environmental inputs relate the study of individual differences to human nature in a straightforward way Those researchers who are interested in applying an evolutionary perspective to individual differences can investigate the adaptive design of these universal mechanisms by seeing whether diflferent manifest outputs are adaptively tuned to their corresponding environmental input Does the algonthm which relates input to output show evidence of complex adaptive design''
Such a research program, however, would be obstructed if it were true that human nature is not everywhere the same if diflferent individuals had quahtatively diflferent innate psychological mechanisms and developmental programs, which reflected quahtatively significant genetic diflferences between humans It is certainly a well-established fact that humans and other similar species manifest enormous genetic diversity (Ayala, 1976, Nevo, 1978, Plomin, 1986, Plomin, DeFnes, & Loehlm, 1977, Plomin et al , 1980, Scan- & Kidd, 1983) How can this genetic diversity be reconciled with a universal human nature'' This question is central to personality psychology, and is the issue primarily addressed m this article We will argue that despite the existence of genetic diflferences, the hypothesis of diflferent human natures is incorrect By considenng evolutionary constraints on how adaptations must be implemented, and by considenng recent developments in evolutionary genetics, we conclude that the relationship of genetically caused individual diflferences to universal psychological mechanisms is circum-


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scribed Charactenstics in which individuals differ because of genetic differences are an unrepresentative subset of human phenotypic charactenstics, and are generally limited to quantitative vanation m the components of complex, highly articulated, species-typical psychological mechanisms Such genetically caused differences are almost entirely constrained vanation within an encompassing, universal, adaptively organized superstructure human nature

Evolution produces adaptive organization and a residue of nonadaptive disorder Reconceptualizing psychology from an evolutionary perspective requires the careful use of concepts developed in evolutionary biology, of which the most important is adaptation Evolutionary biology explains the characteristics of living processes primarily through relating their organization to adaptive requirements If evolution has anything to contnbute to personality psychology, it will be through investigating which personality phenomena are adaptations and which are not To address this issue, one needs clear standards for recognizing adaptations An adaptation is a charactenstic of the phenotype developmentally manufactured according to instructions contained in its genetic specification or basis, whose genetic basis became established and organized in the population because the charactenstic systematically interacted with stable features of the environment in a way that promoted the reproduction of the individual beanng the charactenstic, or the reproduction of the relatives of that individual (Dawkms, 1982, Hamilton, 1964, Williams, 1966) Adaptations are mechanisms or systems of properties "designed" by natural selection to solve the specific biological problems posed by the physical, ecological, and social environments encountered by the ancestors of a species during the course of Its evolution The evolutionary biologist's definition of adaptive function IS subtly but profoundly different from either common-sense notions of function or many psychologists' notions of function The promotion of the reproduction of the individual and/or his or her relatives IS a very different standard of functional operation from such intuitively reasonable standards as happiness, social harmony, success, welfare, well-being, adjustment, long life, health, goal realization, and self-actualization, although m many circumstances and at many levels of explanation they may correspond Nevertheless, in seeking an explanation for the organization of our innate (l e , evolved, genetically specified) psychological mechanisms and developmental programs, it IS the biological definition of function and adaptation that tracks the forces that have shaped us

On Universality and Uniqueness


To properly account for psychological phenomena in evolutionary terms, one must recognize that evolution produces both adaptations and nonadaptive aspects of the phenotype, and distinguish between them carefully (Williams, 1966) Although natural selection is the single major organizing process in evolution, promoting adaptive coordination between organism and environment, evolutionary outcomes are shaped, however weakly, by many other processes, many of which disrupt such coordination (e g , mutation, recombination, genetic hitchhiking, antagonistic pleiotropy, engineering constraints, antagonistic coevolution)
The outcomes from evolution break down into three basic categones (a) adaptations (often, though not always, complex and polygenically specified), (b) concomitants of adaptation, and (c) random effects Adaptations are the result of coordination brought about by selection as a feedback process, they are recognizable by "evidence of special design"—that IS, by a highly nonrandom coordination between properties of the phenotype and the environment, which mesh to promote fitness (genetic propagation) Concomitants of adaptation are those properties of the phenotype which do not contribute to adaptation per se, but which are tied to properties that are, and so are incorporated into the organism's design, they are incidental by-products of adaptation Bone happens to be white, but was selected not for its color but for its ngidity Such concomitant aspects will tend to be selectively neutral, m companson to the functional advantages conferred by the adaptive aspect of the concomitant system Similarly, there are an infinite number of personality traits one can define and measure, but evolutionanly analyzable order will tend to be found only in those causally related to adaptive function Finally, entropic effects of many types act to introduce disorder into the "design" of organisms They are recognizable by the lack of coordination between phenotype and environment that they produce, and by their vanability Examples of such entropic processes include mutation, environmental change, and rare circumstances
In analyzmg personality phenomena from an evolutionary perspective, adaptations will tend to be recognizable because of the functional coordination of psychological charactenstics or behavior Complex organization which systematically leads to adaptive outcomes constitutes evidence of "special design" (Dawkins, 1986, Symons, 1987, Williams, 1966) Moreover, complex architecture or articulation of parts per se suggests (though does not prove) that the properties were organized by natural selection, since random entropic effects are unlikely to construct complex systems of covanation by chance Uniformity with-


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out adaptive patterning or apparent functional significance (e g , all bones are white, all blood is red) suggests the characteristics in question are incidental concomitants of adaptation Finally, unstructured variation will tend to be the result of entropic processes, and will often be adaptively neutral Entropic processes will also cause maladaptation, either through disruption of developmental organization or through a mismatch between the organism and the environment
By applying these standards one can determine whether a personality trait IS the product of an adaptation, a concomitant of adaptation, or noise

Constraints on organic design
Many psychological adaptations will be complex Few would deny that humans successfully perform a wide array of tasks, including many that are functionally similar to what other animals do finding mates, having offspnng, helping relatives, seeing objects, identifying food, and so on Descnbed in terms of their goals, such activities can seem transparently simple Introspectively, we experience many of them (e g , seeing objects) as effortless But when one tries to discover sets of procedures that will actually implement such goals, their real complexity, mtncacy, and difficulty become oppressively clear (see, e g , Marr, 1982, on vision) The history of artificial intelligence has largely been the history of discovering how complex information-processing procedures must be if they are to perform even very simple tasks (e g , moving around half a dozen blocks m a small area) Work m cognitive science and artificial intelligence has shown that mechanisms capable of solving even supposedly simple real-world cognitive tasks must contain very complex "innate" prespecified procedures or information, matched narrowly to the structural features of the domains withm which they are designed to operate (Boden, 1977, Marr, 1982, Mmsky, 1986, on the "frame problem," see Brown, 1987, Fodor, 1983)
Expectations derived from evolutionary biology reinforce the conclusion that many psychological mechanisms will be complex and function-specific Our ancestors had to be able to solve a large number of different adaptive problems, and any attempt to specify procedurally how to solve such problems demonstrates that many of them, at least, are both lntncate and dependent for their solution upon mechanisms that differ m structure from one another For example, successful cooperation requires the coordinated operation of a surpnsmg number