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Copyright 1989 by the American Psychological Association, Inc 0735-7036/89/500.75 Journal of Comparative Psychology 1989. Vol. 103, No. 1.4-22 The Theory of Evolution: Of What Value to Psychology? Charles B. Crawford Simon Fraser University Burnaby, British Columbia, Canada Political considerations and the inadequacies of traditional evolutionary theory for explaining significant behaviors have discouraged psychologists from studying and using it in the past. However, the modern synthetic theory of evolution can enrich psychologists' repertoire of explanatory concepts by providing a critical perspective for analyzing human nature, helping them to integrate the costs and benefits of learning into their thinking, clarifying the origin and development of gender differences, and focusing attention on the interaction of humans with their environment. To illustrate these concepts, examples of each of these functions are presented. In addition, I describe the concept of adaptation and the methods for its study, with particular attention to a current study of the evolutionary significance of anorexia nervosa. Man with all his noble qualities, with sympathy. .. which extends not only to other men but to the humblest of living creatures, which has penetrated into the movements and constitution of the solar system—still bears in his bodily frame the indelible stamp of his lowly origin. (Darwin, 1871/1898, p. 634) Few psychologists have trouble with the argument that the theory of evolution is the explanation for the origin of life and ought to be taught in high school science classes. But if the phrase "bodily frame" is interpreted, as Darwin surely intended, to mean that man's behavior as well as his anatomy "still bears . . . the indelible stamp of his lowly origin," how many psychologists will then defend it? Controversy has raged around Edward O. Wilson, Richard Alexander, and other evolutionarily oriented researchers because they have been forcefully arguing that one cannot fully understand human nature until we explore its biological roots and how they affect human actions. Biologists have used ideas from the modern synthesis, which is an integration of ideas from natural selection and modern genetics, to help explain a wide variety of animal behaviors. Numerous anthropologists, sociologists, philosophers, psychologists, and geneticists have become fascinated by the application of these concepts to the study of human behavior. Its principles have been used in attempts to understand human helping behavior and altruism (Alexander, 1974, 1979; Axelrod & Hamilton, 1981; Chagnon & Irons, 1979; Krebs &Miller, 1985; Trivers, 1971), cooperation (Alexander, 1974, 1979; Trivers, 1971, 1985), incest avoidance (Shepher, 1983; van den Berghe, 1983), child abuse (Daly & Wilson, 1980), homicide (Daly & Wilson, 1982), rape (Shields & Shields, 1983; Thornhill & Thornhill, 1983), suicide and self-destructive behavior (de Catanzaro, 1980), war (Alcock, 1984; Shaw & Wong, 1987) male sexual jealousy (Daly, Wilson, & Weghorst, 1982), grandparental investment (Smith, 1982; Turke & Betzig, 1985), bequest behavior (Smith, Kish, & Crawford, The research in this article was supported in part by a Simon Fraser University Programs of Distinction Grant to Charles B. Crawford. Correspondence concerning this article should be addressed to Charles B. Crawford, Department of Psychology, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada. 1987), grief over the death of children (Littlefield & Rushton, 1986), and competition, sometimes involving war, rape, and resource allocation (Betzig, 1986). Hundreds of articles and dozens of books on evolution and human behavior have appeared during the last 10 years. Books by Wilson (1975) and Alexander (1979) provide much of the basic theory for applying the modern synthesis to human behavior. However, psychologists may find Symons's (1979) The Evolution of Human Sexuality the most stimulating introduction. Trivers's (1985) Social Evolution is the most up-to-date textbook. For a recent critical review, see Kitcher(1985). Psychological Resistance to Evolutionary Theory Although there are political reasons why psychologists have resisted incorporating ideas from the theory of evolution into their thinking, the primary reason they have not used it effectively is that until recently it did not provide a repertoire of useful constructs for formulating detailed explanations of human and animal behavior. Therefore, evolutionary theory was treated as a convenient framework for discussing adaptiveness rather than as a paradigm for developing testable hypotheses about behavior. During the last 2 decades, however, the modern synthesis has been enriched by concepts such as inclusive fitness, kin selection, and the evolution of life histories, which have made it more applicable to behavior. Patry (1983) states that "evolutionary theory is of limited value in formulating hypotheses about developmental processes because the notion that 'ontogeny recapitulates phylogeny' is no longer acceptable" (p. 1026). This quotation illustrates the fact that many psychologists still are not familiar with recent developments in evolutionary theory. The relevance of evolutionary thought to developmental psychology, for example, depends not on the long-discredited notion of ontogeny recapitulating phylogeny but on the theory of the evolution of life histories (Gadgil & Bossert, 1970; Hamilton, 1966; Stearns, 1976; Williams, 1957, 1966). This theory states that the developmental timetable of organisms (the allocation of resources among survival, growth, and reproduction at different ages) is a trait shaped by natural selection. Because it helps psychologists to understand variation in such traits as EVOLUTION, RELEVANCE, PSYCHOLOGY growth schedules, age at maturity, and birth spacing and their interactions with sources of mortality, life history theory may provide them with insights that will help to integrate the psychological and biological study of development. The aim of this article is not to explain in any detail the many other evolutionary concepts that may useful for psychologists, nor is it to provide a critical review of the application of the modern synthesis to the study of behavior. Its purpose is to ask in a more general and broad sense whether psychologists can benefit from an understanding of the modern synthetic theory of evolution. First, however, a brief review of evolutionary theory is necessary. The Theory of Evolution Adaptation and Natural Selection An adaptation is "an attribute that permits the possessor to accomplish those immediate objectives that it must achieve in order to survive and reproduce successfully" (Dunbar, 1982, p. 12). An anatomical structure, a physiological process, or a behavior pattern that contributes to an individual's ability to survive and reproduce in competition with other members of its species is said to be adaptive. Psychologists, accustomed to thinking of anatomical structures such as the teeth of lions or the hands of humans as adaptations, are less inclined to regard cub-killing behavior of male lions (Schaller, 1972) or love in humans (Barash, 1979) as adaptations that may have evolved through natural selection. A more detailed discussion of adaptations and their study concludes this article. Darwin (1859) explains natural selection as follows: Assumption 1 All species are capable of overproducing offspring. Assumption 2 The size of populations of individuals tends to remain relatively stable over time. Assumption 3 Resources for supporting individuals are limited. Inference 1 A struggle for existence among individuals ensues. Assumption 4 Individuals differ in their ability to survive and reproduce. Assumption 5 At least some individual variation in this ability is heritable. Inference 2 There is differential production or survival of offspring by genetically different members of the population, which is by definition natural selection. Inference 3: Through many generations evolution of traits that are more adaptive than others will occur through natural selection. In summary, some feature of the environment, the arrival of a new predator or a change in climate, poses a problem for organisms. A solution (e.g., longer legs or thicker fur) would be helpful. The above assumptions and inferences explain how natural selection provides the solution. Preexisting adaptations, called preadaptations, provide both stepping stones and limits to the solutions natural selection can provide (Gould, 1982). Hence, there are constraints on the "perfection" natural selection can provide. Assumptions 1, 2, and 3 do not require that nature be "red in tooth and claw." A variety of subtle, and not so subtle, strategies enable individuals to contribute differentially to the next generation of individuals. In "Geometry of the Selfish Herd," for example, Hamilton (1971) argued that animals in aggregations position themselves to minimize the relative risk of predation by placing other animals between themselves and potential predators. Sexually mature Florida scrub jays may propagate more of their alleles by deferring reproduction for up to 5 years while they assist their parents in rearing additional broods (Woolfenden & Fitzpatrick, 1984). Although genes are involved in all behavioral development, Assumptions 4 and 5 do not imply genetic preprogramming. Biologists call traits whose development in individuals is influenced by environmental factors facultative traits and the alleles involved in their development facultative alleles. For example, although male white crowned sparrows cannot learn the song of another species, they must hear an adult male of their own species sing while they are nestlings and must themselves sing while they are juveniles if they are to sing a full song as adults (Konishi, 1965). Nor do these two assumptions bar genotype-environment interactions: The biasing of development in different directions when different genotypes develop in different environments. For example, Cooper and Zubek (1958) reared Tryon's (1940) "maze bright" and "maze dull" rats in enriched and impoverished environments. The enriched environment had no effect on the bright rats, although it improved the performance of the dull rats. The impoverished environment had little effect on the dull rats but was very detrimental for the bright rats. Further research, within an evolutionary perspective, may show that ancestral populations of rats from which the two strains were developed contained alleles for both bright and dull behavior and that these alleles influenced fitness in different environments. Evolution and Population Biology This description of the evolutionary process, which does not involve genetic concepts other than genetic inheritance, may be called the phenotype view of evolution because it focuses on the anatomical structures, the physiological processes, and the behavior patterns that were the solutions to problems encountered in ancestral environments. Explaining evolution through changes in allele frequencies that are influenced by selection, migration, mutation, drift, and other population processes is the basis of a more quantitative genotype view. Although these approaches are complementary, researchers who use them often differ in their goals and methods. Practitioners of the phenotypic approach favor the comparative method for explaining the formation of adaptations through divergent evolution (the divergence of species from a common ancestor because of differing ecological pressures) and convergent evolution (the convergence of unrelated or distantly related species because of common ecological pressures). Because it requires species comparisons, the comparative method is difficult to use for studying the fine-grained behav- CHARLES B. CRAWFORD iors that are of interest to psychologists. Researchers interested in human behavior must, therefore, use additional methods for studying the evolutionary significance of behavior. Some of these methods are based on the assumption that the behavior of ancestral organisms evolved to increase their fitness. This assumption leads to the development of hypotheses and models that can be tested through experimentation, field studies, or computer simulation (Barash, 1982). Biologists, psychologists, philosophers of science, and anthropologists who may have learned their evolutionary theory in the context of comparative anatomy sometimes have trouble relating to this approach. Using Evolutionary Theory to Understand Behavior Human beings have discovered fire and cures for many diseases. Schools, churches, multinational corporations, and a host of other civilizing institutions have been devised. It is thus tempting to conclude that evolution by natural selection is no longer occurring in human populations, that evolved behavioral adaptations no longer influence our behavior, and therefore, that those who study human behavior have no need for the theory of evolution in their work. Although using the theory of evolution requires neither that evolution is now occurring nor that evolved behaviors currently contribute to fitness, its use does require that behavioral adaptations that evolved in ancestral populations can influence current behaviors. Darwin (1898) observed that behavior probably changes before the physiological structures that mediate it. An attempt at using tools, for example, may produce selection pressure for the evolution of more useful hands and a larger brain. Therefore, the behavior of individuals, which provides the data on which most current research and theorizing in psychology is based, must be of great importance in understanding evolution in general and the evolution of human nature in particular. Psychologists have an important role to play in understanding the evolution of man. It is a role that they have been reluctant to fill. How the Application of Evolutionary Theory Can Benefit Psychologists Broadens Our Understanding of the Causes of Behavior Most psychologists, biologists, and anthropologists are interested in proximate explanations, the immediate factors, such as internal physiology and environmental stimuli, that produce a particular response. Evolutionary biologists focus on ultimate explanations, the conditions of the biological, social, and physical environment that on an evolutionary time scale render certain traits adaptive and others nonadaptive (Mayr, 1961). For example, the proximate explanation of hibernation in ground squirrels is that current climatic conditions trigger physiological mechanisms that initiate hibernation. The ultimate explanation is that climate, predator pressure, food supply, and so forth over an evolutionary time scale make physiological mechanisms mediating hibernation adaptive. Considering both levels of explanation may deepen understanding of the development and significance of a behavior. Psychologists often confuse the use of the terms distal and ultimate. For psychologists, describing a stimulus as distal implies that it has a less direct connection with the behavior in question than some other stimulus that is more directly or proximally associated with the behavior. In a discussion of factors affecting a child's IQ for example, Scarr (1985) treated the mother's IQ as a distal variable and her positive discipline and control as proximate variables. But ultimate, in an evolutionary sense, implies interpreting an environmental factor in terms of its influence on evolving adaptations. Therefore, the mother's IQ is an ultimate cause only if it is interpreted as an environmental factor influencing adaptations that children are evolving. Sensation seeking: The "ultimate causes." A more subtle example is the explanation of the excess in male versus female mortality, due to disease, trauma and stress, accidents, and murder, that occurs in humans and many other animals. Given that excess male mortality is not attributable to the unguarded X chromosome (Trivers, 1985), the theory of sensation seeking (Zuckerman, Buchsbaum, & Murphy, 1980) may provide a proximate explanation. Measures of sensation seeking correlate with those of risk taking in a number of situations and are also related to a variety of behavioral variables, such as sexual experience, interest in new situations, experience with drugs, social dominance, sociability, playfulness, manic-depressive tendencies, and psychopathy. Men score higher than women on sensation seeking, and scores decline with age after age 16. Scores are also correlated with strength of initial orienting reflex, augmenting versus reducing of the average evoked brain potential, gonadal hormones, and the enzyme monoamine oxidase. Many of the correlates of sensation seeking have moderate to high heritabilities. Zuckerman et al. theorized that the bioamines, particularly the catecholamines, that may regulate the sensitivity of reward and activity centers in the limbic system play a central role in this behavior. Finally, depending on the level of analysis, gonadal hormones, catecholamines, or sensation seeking can be considered a proximate cause of the sex differences in mortality. But what are the ultimate causes of the excess in male versus female mortality? In most species, including humans, the minimal investment for a female to reproduce is considerably greater than that required for a male to reproduce. Therefore, males can benefit more than females by seeking additional matings. Trivers (1985) argued that natural selection favors traits that result in a reduction in survival prior to reproduction if they increase the reproductive success of those who do survive to mate. For example, diverting resources from growth of the immune system to risky behaviors of threat or aggression may increase the fitness of males if it reduces prereproductive male survival by one half but more than doubles the reproductive success of those who survive. The risky behaviors that led Zuckerman et al. (1980) to postulate a trait of sensation seeking may have contributed to male reproductive success in ancestral populations. EVOLUTION, RELEVANCE, PSYCHOLOGY The ultimate causes of the differential mortality are the factors that determine the extent to which males can benefit from seeking additional copulations. These are sex differences in (a) the cost of constructing a gamete (females usually invest much more in each gamete), (b) any additional postzygotic parental effort per offspring (females usually make the larger investment), (c) effort or resources (dowries and bride prices in humans and the nuptual gifts provided by some male insects) exchanged for a mating that limit the ability to compete for additional mates (males usually invest more; Thornhill & Alcock, 1983). From an evolutionary perspective these are the causes of the excess in male versus female mortality and the sex differences in sensation seeking. It is possible to construct a model for predicting species differences in differential mortality and sensation seeking. Males of species characterized by relatively low postzygotic male parental investment and relatively high costs of obtaining mates are predicted to exhibit risky anatomy, physiology, and behavior and to suffer higher mortality than females. Psychologists might call them sensation seekers. The model requires neither knowledge of the biochemistry or physiology of the species nor constructs such as emotions, motivations, and drives. Instead, it would require information on the differences between the sexes in investment in gametes, additional postzygotic parental investment, and resources exchanged for a mating. Value of ultimate explanations. Proximate explanations tell us how a behavior occurs; ultimate explanations tell us why it occurs. Of what value are ultimate explanations to psychologists interested in a single species? A consideration of ultimate causation may help scientists (a) choose independent variables for the development of within-species models and theories, (b) gain an understanding of the environmental factors that may alter a behavior, (c) determine which variables ought to be considered causes and which ought to be considered effects, and (d) develop explanations with greater generality. Consider these in the context of the example of differential mortality and sensation seeking. If evolutionary theory is to be used in constructing withinspecies theories of variability, researchers must assume that members of the species have evolved a repertoire of environmentally contingent behaviors for dealing with conditions in their environment. These behaviors may be either concurrently or developmentally contingent on environmental conditions. Subordinate males who have difficulty attracting mates because they lack social status and resources may increase their fitness by increasing the riskiness of their behavior to obtain these attributes. Males with a greater chance of forming long-term bonds with females because of their resources and social status may attain greater fitness by pursuing a less risky tactic involving higher parental investment. The former are expected to make higher sensation-seeking scores, whereas the latter are expected to make lower scores on these measures. The risky males are also expected to suffer greater mortality. A consideration of the ultimate causes of a behavior may help unravel the causal sequence of events between a stimulus and a response. For example, a consideration of the ultimate causes of sensation seeking suggests that it is a common element in behaviors that men performed in attempts to achieve the social status and resources that were necessary for obtaining mates in an ancestral human population. If this line of reasoning is followed, then gonadal hormones rather than the strength of initial orienting reflex, augmenting versus reducing of the average evolved brain potential, or monoamine oxidase are likely to be an important proximate biological cause of sensation seeking. The ultimate-cause analysis further suggests that training in social and technical skills is more likely to be effective than drug therapy in changing behaviors related to risk taking and sensation seeking. This suggestion is based on the assumption that humans have a single genetically organized life history with environmentally contingent behaviors for implementing it. Many psychologists prefer explanations that use proximate rather than distal variables (Scarr, 1985). Sensation seeking, for example, is apparently a concept derived from inspection and analysis of observed data. Although concepts developed in this manner have an intuitive and commonsense appeal and may function reasonably well as predictors within a particular context, they lack generality. If psychologists desire theories of behavior that are not situation and population specific, they must seek constructs with greater generality. In contrast to the natural sciences, which can call on theories such as classical mechanics, the theory of relativity, or the theory of evolution for guiding the search for explanations, psychology lacks a framework for guiding its search for more powerful explanatory theories of behavior. The earlier example illustrates how a consideration of both the proximate and ultimate causes of a behavior can lead to more general explanations of the behavior and a deeper understanding of it. Provides a Critical Perspective for Viewing Psychological Constructs The following discussions of acting for the good of the group and intrafamily conflict illustrate how an understanding of evolutionary theory can help psychologists take a more critical attitude toward the constructs they use. Acting for the good of the group. Though occasionally, in the territorial or rival fights, by some mishap a horn may penetrate an eye or a tooth an artery, we have never found that the aim of aggression was the extermination of fellow members of the species [italics added] concerned. (Lorenz, 1966, p. 47) Some degree of analytic-cognitive control over the fight-or-flight response must be necessary in social species where fighting and fleeing must be regulated for the good of the group [italics added]. The analysis of such emotion "sending abilities" and their implications for personality and social functioning in humans is just beginning (Buck, 1979). (Buck, 1985, p. 398) These authors assumed that individuals evolve to act for the good of group, as did Wynne-Edwards (1962) when he argued that selection acts on groups rather than on individuals. Models of group selection, although attractive to those attempting to understand the paradox of the existence of helping behavior in many species and the inability of individual fitness based evolutionary theory to explain its evolution, have usually foundered on the problem of cheating (Williams, 1966). CHARLES B. CRAWFORD Consider a group, made up of individuals who sacrifice some of their reproductive fitness for the good of other group members, competing with other groups. A group made up of such individuals may prosper. However, an individual group member possessing a mutation that programs it to accept, or enables it to learn to accept, helping behavior without reciprocating that help leaves more offspring than altruistic members of the group. A group or species made up of individuals acting for the common good is thus inevitably undermined by mutation from within and by immigration from without. Groups must appear and disappear at an unrealistically high rate to make group selection a viable evolutionary process (Maynard Smith, 1976). Although attempts to develop more sophisticated models of group selection are underway they do not yet provide a viable alternative to individual and kin selection (Barash, 1982; Wittenberger, 1981). The inadequacies of theories of group selection cast doubt on psychological theories that implicitly assume individuals behave for the good of the group. For example, many psychologists assume that empathy is a facilitator of altruism and an inhibitor of aggression (Zahn-Waxier, Cummings, & lannotta, 1986). The vicariously experienced painful consequences of distress in another individual, the argument goes, cause empathic distress that can be reduced by helping the distressed individual. The reduction in this stress provides the motivation for inhibiting aggression, behaving altruistically, and learning prosocial behaviors. Here the implicit assumption is that empathy is an enabling device for helping others at a cost to oneself, that is, that empathy is an enabling device for acting for the good of the group. Those with an evolutionary perspective are not surprised that there is little support for the prediction derived from this argument that empathy is positively correlated with prosocial behavior and negatively correlated with aggressive behavior (Eisenberg & Miller, 1987; Underwood & Moore, 1982). An evolutionary perspective, based on the assumption that individuals evolve to enhance their own fitness or the fitness of genetic kin, suggests that empathy is an enabling device for mediating a variety of behaviors ranging from altruism to avoiding blame and the manipulation of others in one's own interest or in the interest of others treated as kin by evolved kin-discrimination mechanisms. As such, empathy may be a less costly mechanism for attaining one's ends than aggression, and those high in empathy may be less likely to engage in aggressive behavior. Moreover, females are expected to be more empathic than males because physically aggressive behavior is more costly for them. Similarly, those high in empathy may be feared by those low in empathy and, therefore, receive aggression from them. This perspective also predicts that empathy leads to prosocial behavior when the individuals empathized with have characteristics that would result in their being treated as genetic kin in an ancestral population. Experiments on empathy, helping behavior, and aggression ought to be designed to take these possibilities into consideration. Within-family conflict. The romantic view of the family as a loving group of individuals cooperating to bring happiness to one another has led many to oppose abortion, divorce, and the division of children between parents after a marital breakup. A consideration of the evolutionary significance of the genetic relationships within the family, however, predicts conflict as well as cooperation within the family group (Trivers, 1974, 1985). Figure 1 provides the probabilities that parents, grandparents, and children possess an allele (say A or a) that is a copy of an allele possessed by an ancestor. These probabilities, known as genetic correlations, are computed by raising .5 (the probability that A or a is passed on when a gamete is formed) to the power of the number of arrows connecting two persons. For example, the genetic correlation between the focal person (B) and his or her niece or nephew (E) is .52 (or .25), because they are connected by two arrows. Trivers (1974) argued that the behavior of individuals, including parents, children, and grandparents, is shaped by natural selection to maximize the probability that copies of alleles they carry are replicated through their behavior. Let us consider this probability from the point of view of person B in Figure 1. The probability that B's alleles will be replicated through his or her offspring (D) is .5, whereas it is only .25 through B's niece or nephew (E). Therefore, natural selection will favor individuals in B's position who seek an unequal share of their parent's (A's) resources. Person A, however, is equally related to all his or her grandchildren and is selected to resist B's demands. The intensity of conflict varies as a function of a child's age. When the child is very young, the parents' fitness is maximized by investing in him or her, deferring the birth of additional future offspring. When the child is older and less dependent for survival on parental attention, parents can increase their fitness by investing in additional offspring. Conflict is most intense at intermediate ages, when the parents' fitness is increased more by investing in additional offspring but the child's fitness is increased more by continued parental investment. Sibling rivalry and parental efforts to suppress it as well as parent-offspring conflict over the amount and timing of Parent of B and C Grandparent of D and E Focal individual q^ 0.5 Offspring of B 0.5 Sibling of B 0.5 Niece or nephew of B Figure 1. Multiplying along the arrows connecting two individuals gives the probability that they will share an allele by common descent. (A grandparent, A, is equally related to all his or her children, B and C, and to all his or her grandchildren, D and E. However, the focal individual, B, shares an allele with his or her children, D, with probability .5 and with his or her nieces and nephews, E, with probability .25. Thus B is selected to desire an unequal share of A's resources.) EVOLUTION, RELEVANCE, PSYCHOLOGY investment are expected if ontogeny is viewed from an evolutionary perspective. Understanding and dealing with these problems may be easier once psychologists realize that conflict as well as helping behavior can be expected in families. For a discussion of human intrafamily conflict, see Daly and Wilson (1987). Enriches Our Repertoire of Explanatory Constructs The discussion of acting for the good of the group and intrafamily conflict illustrates how the theory of evolution can provide a critical perspective on the development of explanations. It also provides a repertoire of constructs that may be used in the development of explanations. Two of these are inclusive fitness and reproductive value. Inclusive fitness and helping behavior. Hamilton (1964) provided a putative resolution to the problem created by the widespread existence of helping behavior in animals and the inability of individual or group selection to explain it when he wrote The social behavior of a species evolves in such a way that in each distinct behavior evoking situation the individual will seem to value [italics added] his neighbours' fitness against his own according to the coefficients of relationship appropriate to that situation, (p. 19) Hamilton saw that biological altruism, self-destructive behavior performed for the benefit of others, can evolve, although it reduces the reproductive fitness of the donor of the help, if it aids genetic kin, some of whom must inherit the helping allele from a common ancestor of the donor. More specifically, biological altruism can evolve if the cost to the donor of a biologically altruistic act is less than the product of its benefit to the recipient and the genetic correlation (the probability they both have an allele that is a copy of a common ancestor's). Now consider, for example, the interactions between a donor, its full sibling, and its half sibling. The donor performs an altruistic act, reducing its own reproductive success from four to three offspring but increasing the reproductive success of the full sibling from three to five and that of the half sibling from two to six offspring. The cost to the donor is one offspring, but the benefit to the donor is 0.5(2) + 0.25(4) = 2 offspring. The donor's actions result in the equivalent of five offspring rather than the four direct descendants that would have been produced without the help directed to genetic kin. Alleles enabling individuals to perform nepotistic acts (helpful acts directed to genetic kin) can spread by natural selection. The expanded notion of fitness, known as inclusive fitness, can be defined as the sum of (a) an individual's direct fitness, its personal reproductive success, stripped of the effects of actions of genetic relatives and (b) the individual's indirect fitness, its influence on the reproductive success of genetic relatives, with the relatives weighted by the degree of genetic relation to the individual. Inclusive fitness is one of the most powerful concepts that evolutionary theory has to offer psychology for understanding helping and conflict behavior. It has helped explain phenomena as diverse as sterile castes in social instincts (Hamilton, 1964), coalition formation in pri- mates (Baker & Estep, 1985), and human bequest behavior (Smith et al., 1987) by focusing attention on the behavioral consequences of genetic kinship and mechanisms for discriminating kin from nonkin. Kin recognition. If social organisms have evolved the capacity to put their helping behavior where their alleles are, they must possess some proximate mechanisms for kin recognition, and these ought to be of interest to psychologists because they provide an important point of interface between psychology and evolutionary theory. Possible mechanisms for kin recognition include spatial distribution (treating individuals in a prescribed geographic area as kin), association (treating frequent associates as kin), phenotype matching (treating individuals similar to oneself or similar to those that one was raised with as kin), and recognition alleles (innately treating individuals with a particular genetic marker as kin). They have received attention from biologists (Holmes, 1986; Waldman, 1986) and psychologists (Rushton, Russell, & Wells, 1984, 1985). If small kinship groups were an important feature of human social structure during our evolution, then one or more of these kin discrimination mechanisms may have evolved and may still be influencing our behavior. Understanding the mechanisms involved may be critical if psychologists wish to change human behavior through social learning. For example, if spatial distribution or association evolved as mechanisms of human kin discrimination, then encouraging individuals of different ethnic groups to live together in the same geographic area may contribute to ethnic harmony. However, other kin discrimination mechanisms may complicate the process of achieving ethnic harmony. If treating persons similar to those one was raised with is a human kin discrimination mechanism, then ethnically integrating schools starting with preschools rather than high schools is more likely to lead to ethnic harmony. If treating persons similar to oneself as kin was an aspect of kin discrimination among our ancestors, then it may be necessary to encourage different ethnic groups to adopt similar accents, manners, customs, clothing, and so forth to increase ethnic harmony. Finally, if genetically produced signs evolved to discriminate among kin and nonkin among our ancestors, then producing ethnic harmony may, indeed, be difficult because social learning is not involved. Both theoretical and empirical work on the role of social learning in kin discrimination are needed if the concept of inclusive fitness is to be applied to human behavior. This work is proceeding. Hepper (1986) provides an up-to-date review of the biological literature on kin discrimination, and Porter (1987) reviews the little that has been done on human kin discrimination. Little work on the relation between inclusive fitness, kin recognition, and concepts from social psychology has appeared. It is a fruitful area for interdisciplinary research. Reproductive value and behavior. Fisher (1930) developed the concept of reproductive value, defined as the relative number of female offspring remaining to be born to a female of a given age, to quantify an individual's value in an evolutionary context. For example, the number of female offspring still to be born to 1,000 one-day-old females is less than number to be born to 1,000 one-year-old females because 10 CHARLES B. CRAWFORD some of the one-day-olds will die before reaching one year of age. Hence, the one-year-old females have greater reproductive value than the one-day-old females. Similarly, a group of 50-year-olds have less reproductive value than a group of 25year-olds because members of the latter group can expect to produce more offspring during the remainder of their lives. Thus, reproductive value rises during the early years of life, because of infant and juvenile mortality, peaks around the age of sexual maturity, and then begins a decline. Although usually computed for females, it can be computed for males if age-specific birth and death rates for males and the stable age structure of the male population are known. A variety of demographic variables influences reproductive value. For example, in a growing population a young female's reproductive value is devalued relative to an older female's value because the younger female's progeny make up a smaller proportion of the next generation then do the older female's. Similarity, if the size of the population is declining, the reproductive value of the younger female is relatively greater because her offspring will make up a greater proportion of the next generation. In humans the male and female reproductive value curves differ somewhat because the age-specific birth and death rates differ somewhat for men and women. Women, for example, both begin and cease producing offspring at an earlier age than men. Men die at a faster rate at all ages than do women. Figure 2 shows the female reproductive value curves for Keyfitz and Flieger's (1971) data for several industrialized and developing nations. Note the difference between the curves for Japanese and Mexican women for the year 1966. From an evolutionary perspective the same chronological age has a different meaning for each group. Even the curves for Japan in 1964 and 1966 differ. Reproductive value curves apparently reflect important ecological features of the populations they represent. Because individuals' reproductive value is a measure of their ability to propagate their alleles and because an essential assumption of the theory of evolution is that individuals evolve to behave in a way that maximizes the number of their alleles propagated, it follows that adaptations for assessing and responding to the reproductive value of oneself and others may have evolved. It is not adaptive, for example, for humans to be sexually attracted to members of the opposite sex who are of low reproductive value. It is well known that humans respond to individual differences in physical and psychological characteristics, such as clear skin, bright eyes, vitality, and social skill. If such characteristics are really being used as cues to reproductive value, then our emotional and cognitive reactions to them may be the product of natural selection, and reproductive value may be at least as important as age in understanding human behavior. However, most psychologists appear to be unaware of its power. Figure 3 shows the intensity of grief that undergraduates expect parents to experience at the death of daughters of different ages compared with reproductive values for the Province of British Columbia and for IKung Bushwomen plotted against age. The grief values were obtained by using Thurstone's (1927) law of comparative judgment. The reproductive values were computed from Canadian census data and from Howell's (1979) demographic data on the Dobe IKung. The linear component of the correlation of pooled female grief scores with reproductive values for British Columbia females is 0.65. The correlation of the same grief values Mexico 1966 Japan 1964 Japan 1966 Reproductive value 10 15 20 35 30 35 40 45 50 55 Age (years) Figure 2. Female reproductive value functions for several contemporary societies. (The number of female offspring that females of various ages can be expected to produce during the remainder of their lives is plotted against maternal age. Data from Keyfitz & Flieger, 1971.) EVOLUTION, RELEVANCE, PSYCHOLOGY 11 Grief for daughter Female reproductive value, British Columbia Scaled grief 0 7 _| and reproductive °-e ~ values Female reproductive value, Dobe IKung 10 15 20 25 30 35 40 45 50 Age (years) Figure 3. Intensity of grief (determined by Thurstone's, 1927, method of comparative judgment) for the death of children varying in age from 1 day to 50 years. (The reproductive values were computed from British Columbia, Canada, census data from 1981 and from data collected by Howell, 1979, for Dobe !Kung. The raters were 174 undergraduate university students.) with reproductive values for IKung Bushwomen is 0.95. Because the curves differ only for young children, it is tempting to speculate that the intensity of grief experienced at the death of a child reflects the reproductive schedule of our huntergatherer ancestors. Ideally, both reproductive value and inclusive fitness ought to be used when considering interactions between individuals. If they are to be used jointly, it is necessary to distinguish between direct and indirect reproductive value, just as it was necessary to distinguish between direct and indirect fitness when considering interactions between kin. Direct reproductive value is the number of offspring still to be born to a focal individual of a given age. Indirect reproductive value is the number of offspring still to be born to an individual of another age with a genetic correlation to the focal individual. A postmenopausal woman, for example, has zero direct reproductive value but may have considerable indirect reproductive value if she has interactions with an extended kin group. Now consider to which of three cousins, ages 1 week, 15 years, and 45 years, a childless donor ought to bequeath an estate to maximize the donor's fitness. Because the potential recipients are equally related to the donor, a consideration of kinship does not provide useful information. As the donor is childless, a consideration of the donor's direct reproductive value provides no useful information. If the persons were citizens of a developing nation with high infant mortality, a consideration of the indirect reproductive value of the donor (the direct reproductive values of the cousins; see Figure 2) suggests the estate ought to go to the 15-year-old. However, if the individuals were members of some highly developed country with low infant mortality, the same consideration suggests the estate should go to the 1week-old. Suppose the estate was to be left to either a 1-week-old grandchild, whose genetic correlation with the donor is .25, or a 15-year-old cousin, whose genetic correlation with the donor is .125. Now both the indirect reproductive value of the donor and the degree of kinship of the donor and beneficiaries must be considered. On the basis of joint considerations of both reproductive value and kinship, the person in the developing country may be expected to favor the 15-yearold cousin, whereas the one from the developed country may be expected to favor the 1-week-old grandchild. Finally, a relatively young childless donor with high direct reproductive value might respond differently from an elderly childless individual with zero direct reproductive value. Note that all these computations assume humans have evolved psychological mechanisms for assessing kinship and reproductive value. In Kreb's (1970) review of the literature on altruism, he found benefactor effects for sex, age, ordinal position, social class, and nationality and recipient effects for friendship status, in-group affiliation, and social class. These results are not surprising to anyone with a knowledge of evolutionary theory. The use of inclusive fitness, mechanisms of kin discrimination, and reproductive value may be of considerable use to psychologists in the development of more general theories of altruism and helping and conflict behaviors. Moreover, although it may place some limits on the way cognitive and social learning theories are formulated and used, it does not preclude them as proximate explanations. Focuses Attention on the Costs and Benefits of Learning Although the benefits of the flexibility and adaptability made possible by learning are well known, its costs are less 12 CHARLES B. CRAWFORD often appreciated. Learning requires delicate neurological structures, which are energetically costly to develop and maintain. An animal may learn fitness-reducing information, as when a Norway rat or a human learns to avoid an edible food because on first contact it was tainted or associated with a tainted food. Humans and other animals may exploit learning by providing false information for others to learn (Alexander, 1979). Given the costs and benefits associated with learning, it is unlikely that any species, including Homo sapiens, has completely general learning abilities. From the premise that a complex environment offers many opportunities for learning fitness-reducing responses, Lumsden and Wilson (1981, 1983) have argued that a species without genetic constraints on the learning of culture must evolve toward having such constraints. Moreover, the speed of the evolution is a function of the complexity of the environment. Thus natural selection will equip the mind with specific rules and principles for learning about the world in an advantageous way. Lumsden and Wilson have argued that brother-sister incest avoidance, the learning of color vocabularies, infant preferences from object shapes and arrangements, the development of facial recognition, fear of strangers, mother-infant bonding, and the acquisition of the phobias are the result of problems encountered by our Pleistocene ancestors. Every graduate student in psychology knows and admires Garcia's (Garcia, Hankins, & Rusiniak, 1974) work on the learning of biologically relevant associations, yet it has had little impact on theory and practice in psychology. Although the concept of genotype-environment interaction is taught to introductory psychology students, they are also taught, sometimes in the same course, that if men and women are to achieve equally as adults they must be treated in the same way as children. An evolutionary perspective can help psychologists integrate an understanding of the costs and benefits of learning and genotype-environment interactions into their thinking. Cosmides and Tooby (1987) recently took up the challenge of developing a framework for evolutionary psychology, a discipline concerned with exploring the naturally selected design features of the mechanisms that control behavior. Arguing that cognitive processes are the most essential proximate mechanisms, they define Darwinian algorithms (Cosmides, 1985) as innate specialized learning mechanisms that organize experience into adaptively meaningful schema or frames. When activated by appropriate environmental and proprioceptive information, these mechanisms focus attention, organize perception and memory, and call up specialized knowledge required for domain-appropriate inferences, judgments, and choices. They suggest that aggressive threat, mate choice, sexual behavior, pair bonding, parent-offspring conflict, kin recognition, friendship, disease avoidance, resource distribution, and social contracts are only a few of the domains of human activity regulated through Darwinian algorithms that evolved in response to ancestral problems. Cosmides and Tooby (1987) argued that ancestral environments selected not for behavior but for mechanisms producing behavior, just as these environments selected mechanisms for detoxifying the blood or extracting oxygen from the atmosphere rather than the actions of these organs. These mechanisms were selected because they contributed to average fitness in ancestral populations. In the study of the naturally selected design of these mechanisms, we can use the theory of evolution to help understand the human mind. However, they go on to argue that it is at the cognitive rather than the physiological level where psychological explanations must be sought. Adaptive behavior is predicated on adaptive thought, and adaptive thought can only be understood at the level of information processing. Although the information processing mechanisms must be instantiated in neural hardware, a knowledge of that hardware tells us little about how ancestral animals solved their problems and how the evolved information processing mechanisms function today. To test this theory Cosmides (1985) studied content effects on the Wason selection test, an experimental paradigm for studying whether people reason in terms of the contentindependent principles of formal logic (Wason, 1968). If the content of a logical problem affects reasoning, then a content effect is said to have occurred. She argued that in the context of social contracts, reasoning ability may reflect the action of Darwinian algorithms that evolved in our ancestors for dealing with associates who cheated on social contracts by accepting the benefits of the contract but not paying its costs. She carried out a number of experiments that compared predictions from Darwinian algorithms with those of availability theory, the theory that familiarity with the content of the statements used in testing reasoning affects errors in reasoning. She varied content (familiar vs. unfamiliar) and instructions (prescriptive vs. descriptive). The results strongly supported her evolutionary perspective in that the errors in logic were predicted much more accurately on the assumption of Darwinian algorithms than on the assumption of availability. Cosmides and Tooby's (1987) focus on the invariance of cognitive mechanisms rather than on the invariance of behavior has several benefits for those wishing to use an evolutionary perspective in their work. It focuses attention on the ancestral conditions under which mechanisms mediating behavior evolved and makes it clear that behavior in ancestral populations had only to be adaptive, on average, for the evolution of mediating mechanisms to occur. Hence, those using the evolutionary perspective need not assume that every action must contribute to fitness or that all behaviors are currently adaptive. This realization makes it possible to use the theory of evolution for studying a wide variety of normal and abnormal behaviors. The focus on proximate cognitive mechanisms alleviates the necessity of assuming that behavior is under stimulus control and thereby brings the evolutionary approach to behavior into the mainstream of current psychological thinking. Mental processes, if they are clearly defined as Darwinian algorithms, can become objects for evolutionary analysis. Just as hands are not constraints on eating, Darwinian algorithms are not constraints on learning. Both evolved in ancestral populations for enabling individuals to deal efficiently with conditions in their environment. Hence, from an evolutionary perspective it is incorrect to speak of constraints on learning. Moreover, Cosmides and Tooby's (1987) arguments lead to the expectation that the mind comprises numerous, specific, complex mechanisms rather than simple, EVOLUTION, RELEVANCE, PSYCHOLOGY general processes of association and symbol manipulation— a view shared by Symons (1987) and Fodor (1983). Psychologists have been leery of sociobiologists and evolutionary ecologists because of their concentration on ultimate or "why" questions to the exclusion of proximate or "how" questions. Cosmides and Tooby's (1987) work is ground breaking because it shows us how to use the evolutionary approach in the study of proximate questions without becoming involved in the details of physiology and neurochemistry. In the past psychologists using the theory of evolution in their work have tended to concentrate their attention on emotion (e.g., Plutchik, 1980). Cosmides and Tooby (1987) expanded the use of evolutionary theory in psychology by bringing it to bear on the fine structure of the mind. Helps Us Understand Sex Differences There are at least two sources of selection pressure that interact and complement each other in producing evolved sexual dimorphism. The production and rearing of offspring may have resulted in some specialization of roles for ancestral males and females, with resulting selection for sexual dimorphism in physical body size, behavior, and brain development (Kimura, 1987; Tooby & DeVore, 1987). The other evolutionary explanation for sexual dimorphism is Darwin's theory of sexual selection. Sexual selection and sexual dimorphism. Sexual selection refers to differences in the ability of individuals with different genotypes to acquire matings. If, for example, tall and short males have similar abilities to survive, but tall males obtain more matings, then sexual selection is said to be acting on the males. Darwin (1871/1898) argued that it involves two processes that are usually referred to as intrasexual selection and epigamic selection. In intrasexual selection the competition between males for access to females produces selection pressure for increased physical size, organs of threat, and aggressiveness in males and differences between males and females on these traits. Male elephant seals, for example, compete vigorously among themselves for access to females. They are about 60% larger than females and about four times as heavy. A few males may have as many as 100 females in their harems; most never get near a female during their entire lives (LeBoeuf, 1974). Epigamic selection is the result of female choice generating competition between males resulting in the elaboration of traits that females desire. The tail of the peacock provides the classic example. But why do males compete for females, and do females ever compete for males? In the earlier discussion of sensation seeking, I wrote that males can usually benefit more than females from additional copulations because a male's minimal investment required to reproduce is usually less than a female's. Therefore, females are a limiting resource for males, and males can benefit by competing among themselves for access to that resource. Males are usually, but not always, the sexually selected sex. Exceptions that prove the rule are provided by organisms with sex role reversal such as seahorses, certain frogs, and several species of marsh and shorebirds (jacanas and phalaropes). In these species, although males make a small invest- 13 ment in sperm, their relative minimal parental investment is large because they take sole responsibility for parental care (Barash, 1982). As a result, the females have evolved to be larger or more colorful than the males and to compete among themselves for access to males. Sexual dimorphism in anatomy, physiology, behavior, and especially variance in reproductive success are evidence that that sexual selection has acted on a species. In humans sexual dimorphism exists in size (men are larger), in physical aggressiveness (men are more physically aggressive), life span (women live longer), mortality (men have greater mortality rates at all ages), date of maturity (women mature earlier), and minimal parental investment (women make the greater minimal parental investment). Throughout history most individual men have mated monogamously. However, in a sample of 849 societies, 83% are either usually or occasionally polygynous, 16% are monogamous, and 0.5% are polyandrous (Daly & Wilson, 1983). Even in monogamous societies such as the Hutterites, variance of men's reproductive success exceeds that of women (Crawford, 1984). This evidence indicates that humans are a moderately sexually selected species. Psychologists have studied sex differences in behavior for a century, but they have shown little interest in their evolutionary origin. For example, since January 1986, the Psychological Bulletin has published five major articles on gender differences in behavior: human motor activity level (Eaton & Enns, 1986), masculinity, femininity, androgyny, and cognitive performance (Signorella & Jamison, 1986), gender and helping behavior (Eagly & Crowley, 1986), gender and aggressive behavior (Eagly & Steffen, 1986), and sex differences in unipolar depression (Nolen-Hoeksema, 1987). Although the authors occasionally referred to proximate biological factors as possible causes of gender differences, not one mentioned their possible evolutionary significance! Since the time of Darwin, the theory of sexual selection and observed sexual dimorphisms in anatomy and behavior have led many to conclude that males are at the cutting edge of evolutionary change and that females are passive and noncompetitive. However, the nature and extent of femalefemale competition is now becoming a topic of major importance to evolutionary ecologists, primatologists, and anthropologists (Hrdy, 1981;Wasser, 1983). Equality of the Sexes. Although evolutionary theory tells us why many species exhibit sexual dimorphism in physique and behavior, it also tells us that males and females are equal in a biological sense. Fisher (1930) proved that if individuals in a species have one parent of each sex, it then follows that the amount of parental investment put into males and females must be the same at the time offspring become independent of their parents. (See example in Table 1.) Suppose that it costs two units of maternal time and energy to rear a daughter to independence and one unit to rear a son to the same point in development and that a mother has 100 units of parental care available for rearing offspring. If she invests in an equal number of sons and daughters, her 100 units of parental care produce 33.3 (33.3 Daughters X 2 Care Units) daughters and 33.3 (33.3 Sons x 1 Care Unit) sons for a total 66.6 offspring. As each grandchild must have one parent of each sex, the 33.3 sons and 33.3 daughters give her, on average, the same number of grandchildren. Our hypo- 14 CHARLES B. CRAWFORD Table 1 Number of Offspring Produced by 100 Units of Parental Investment for 1:1 and 1:2 Sex Ratios Investment strategy Produce equal sex ratio Males Females Total Produce sex ratio inverse to ratio of unit costs Males Unit cost Number produced Expenditure 2 1 33.33 33.33 66.66 33.33 66.66 100 Females 25 50 50 50 Total 75 100 thetical female may expect 66.6 X 66.6 or 4,435.56 grandchildren. Now consider another female in the same population who puts two units of investment into each of 25 daughters and one unit into each of 50 sons. The same 100 units of parental care now bring 25 daughters and 50 sons to independence. Each of her offspring, on average, will give her the same number of grandchildren. Such a "wise" female can expect 75 x 75 or 5,625 grandchildren. Thus natural selection produces an equilibrium sex ratio inversely proportional to the investment in males and females: the Cost of Males x the Proportion of Males = the Cost of Females x the Proportion of Females. If the sex ratio of a species is 50:50 at the time of offspring independence, then the cost of bringing males and females to independence must be the same. If at conception the amount of investment put into males equals the amount put into females, if the sex ratio of the species is 1:1, and if offspring competence is a function of parental investment, then at the time of independence from the parents, the average competence of males and females must be the same. In this context competence refers to any trait enhancing inclusive fitness, such as the ability to acquire a mate, to breed successfully, to rear offspring, to detect predators, to acquire food, to learn a skill such as hunting, gathering, or growing vegetables, to recognize kin, to detect liars, to cooperate with others, and so forth. The fact that males and females are biologically equal in the sense that they receive equal investment does not imply that they are biologically identical, that patterns of investment in them are identical, or that similar patterns of investment produce similar competencies. Males, for example, may require more food, whereas females may require more protection from predators. It may be easier to teach males to be physically aggressive and females to be good caregivers. The theory of sexual selection discussed earlier suggests some ways in which evolved differences in male and female competencies may come about. Finally, suppose some external agency provided additional investment to only one sex, say, for example, males. If this investment occurred before offspring independence, males would be less expensive, and natural selection would favor the production of more of them. Similarly, if the cost of females were lowered, more would eventually be produced. Can mechanisms evolve for facultatively adjusting investment in male and female offspring, and hence their competence, as a function of the environmental fluctuations that temporarily alter the costs of producing males and females? This question is considered in the next section. Sex allocation. Suppose that an organism desired many grandchildren and that it did not care whether they were males or females, should it then invest more in its male or its female offspring? This is the question posed by sex allocation theory, which when applied to mammals, describes how an individual can maximize its fitness by partitioning its investment between its male and female offspring (Charnov, 1982). Fisher's (1930) theory that natural selection adjusts the sex ratio to produce equal investment in males and females requires a key assumption that parents do not have information on the likely reproductive success of their offspring. Suppose the females of a hypothetical species leave approximately the same number of offspring (low variance in female reproductive success), but that some males leave many offspring because their physical condition or social status enables them to compete successfully for many females, whereas other males leave few offspring because they lack these attributes (high variance in male reproductive success). Now, further suppose that offspring inherit some of their mother's health or social status. Given these suppositions, a mother in poor health or of low social status can improve her fitness by investing in daughters at the expense of sons, because her weak or subordinate sons are likely to give her fewer grandchildren than her weak or subordinate daughters. Similarly, a healthy or dominant mother can increase her fitness by investing in sons at the expense of daughters, because her healthy or dominant sons are likely to give her more grandchildren than her strong or dominant daughters. Therefore, an adaptation that uses information about one's own physical condition or social status to allocate investment in male and female progeny is expected in species in which the variance of reproductive success is greater for one sex than for the other (Trivers & Willard, 1973). Because variance in male reproductive success exceeds that of females in many animal species, including humans, adaptations for sex allocation may exist. The sex ratio of progeny is usually used as the measure of allocated parental investment. Trivers and Willard (1973) cited evidence supporting their theory from a variety of species, including pigs, sheep, mink, seals, deer, and humans. A striking piece of evidence supporting Trivers and Willard is McClure's (1981) finding that when lactating wood rats are put on a food-restricted diet, they selectively eliminate the males from their litters. Females can improve their fitness by beginning allocation of resources to male and female offspring as early in development as possible. Recently Wright, Crawford, and Anderson (1988) reported evidence that suggested that dietary stress can induce mice to alter the sex ratio of their progeny during gestation. Evidence that Trivers and Willard's (1973) theory applies to humans is beginning to accumulate. Tentative evidence EVOLUTION, RELEVANCE, PSYCHOLOGY suggests that parental ability to invest in offspring may influence the sex of human offspring (Clutton-Brock & Albon, 1982). Dickemann (1979), Voland (1984, 1988), and Boone (1986) have found evidence that supports Alexander's (1974) suggestion that in highly stratified societies male-biased sex ratios are produced among high-status parents by female infanticide, abuse, and neglect. Dickemann, van den Berghe and Mesher (1980), Smith et al. (1987), and Boone (1986) have provided evidence that suggests individuals consider their own social status when bequeathing wealth to their male and female kin. Betzig (1986) provides strong support for the theory of Trivers and Willard from a large cross-cultural sample. A recent study by Betzig and Turke (1986) may be of particular interest to psychologists, who are less interested in the sex ratio than in patterns of differential investment that may be involved in the development of sex differences in behavior. They studied parent-child associations on a small Pacific atoll south of Guam and found that although the sex ratio did not differ among high- and low-status parents, higher status parents spent more time with sons, and low-status parents spent more time with daughters. These results, which are in agreement with the findings of the studies of bequest behavior already mentioned, ought to be of considerable interest to anyone concerned with the development of sex differences in personality, abilities, and gender roles. Focuses Attention on Ecology Evolutionary theories of social behavior have considered behavior to be like other biophysical structures [italics added] that have evolved in natural history. One upshot of this reification— viewing behavior as structure—is that behavioral propensities are assumed to be conserved, unchanged, across generations. Since the biological substrates for, say, aggression and altruism, have evolved over hundreds or thousands of generations, it has been assumed that they are inevitable [italics added] in the ontogeny of each person. (Zahn-Waxier et al., 1986, p. 82) This quotation, from a recent book on the biological and social origins of altruism and aggression, is one of many illustrating that psychologists, as well as other scientists and scholars, often confuse the ideas and methods of classic ethology with those of sociobiology, behavioral biology, and evolutionary ecology. The study carried out by the classical ethologists of behaviors and biophysical structures that are "conserved, unchanged, across generations" is fundamental for our understanding of life through natural selection. However, it is focused on the evolution of between-species differences and similarities in behavior at the expense of within-species behavioral variation. The fact that a trait has evolutionary significance does not imply that it cannot be affected by environmental conditions, that it must have high heritability, or that it must appear the same in all individuals. Evolutionary theory focuses attention on the interaction of organisms with their physical environment and the actions and products of other organisms living in it. For example, when animals engage in what appears to be destructive behavior, such as infanticide, they may do it because it contributes to their lifetime reproductive success, 15 given the ecological circumstances under which they are living (Hausfater & Hrdy, 1984). Let us briefly examine some of the concepts currently being used by ecologists to help explain how biological adaptations enable organisms to deal with varying conditions in their physical and social environment. A life history strategy is a genetically organized program for allocating resources to survival, growth and reproduction throughout an organism's life. Tactics are decision rules and methods for achieving the life history (Gross, 1982). Within a population strategies are in competition with each other; tactics are the way they compete. The members of a population may have one or more strategies each with its own tactics for competing with other strategies. Tactics may be either concurrently or developmentally contingent on environmental circumstances. Some examples may help clarify these ideas. Male scorpion flies have three mating tactics: They may obtain a dead insect, present it to a female, and copulate with her as she eats it; they may generate a salivary mass, present it to the female and copulate with her as she eats this nuptial gift; or if they cannot obtain a dead insect or generate the salivary mass, they may attempt a forced copulation. Thornhill (1980) has shown that all three tactics are available to adult males and that success in male-male competition determines the tactic used. Even dominant, successful males become vigorous forced copulators if they cannot successfully compete for a nuptial gift. The mating behavior of male scorpion flies is thus an example of a single mating strategy with concurrently contingent tactics because the tactics used depend on current conditions in the environment. Figure 4 illustrates this adaptation for adjusting male behavior to the level of male-male competition. It is drawn on the assumption that all males have alleles for all behaviors and the behavior exhibited is contingent on conditions in the environment. A zero heritability is expected for the behavioral differences because they are completely determined by environmental differences. However, a nonzero heritability may be found in a standard behavior genetic study if, for example, body size is heritable and is correlated with the tactics used. Tactics may also be contingent on the environment during development. Male gorillas apparently succeed to breeding rights within their natal troop or seek matings elsewhere Environment Mating tactic (Male-male competition) Dead insect + courtship Low Medium ,, High Proteinaceous mass + courtship h = 0.0 Attempted forced copulation Figure 4. Mating tactics in scorpion flies (Panorpa sp.). (The level of male-male competition determines the reproductive tactic used. Heritability of the tactics is zero because all males have alleles for all tactics.) CHARLES B. CRAWFORD 16 depending on their relationship with the dominant male during infancy and adolescence (Harcourt & Stewart, 1981). Males with a close social relationship with the dominant male remain in their natal group, whereas males without such a relationship leave to compete for mates elsewhere. The tactics of gorillas are developmentally contingent because they depend on environmental conditions present during development. It is not known whether gorillas using different tactics differ genetically. However, it seems likely that the tactics are enabling devices for a single genetically organized reproductive strategy because the heritability of traits closely related to reproduction is usually low (Falconer, 1960; Mousseau & Roff, 1987). Male bluegill sunfish have two genetically different life histories, parental and cuckolder, each with its set of tactics for competing with the other life history (Gross, 1982). The parental males grow slowly, mature later than cuckolders, build nests, court females, and provide paternal care once the eggs are fertilized. The smaller cuckolders grow rapidly and never provide parental care. When they are small, they attempt to sneak copulations by darting in and releasing sperm just before the parental male releases his sperm. When they are larger, sneaking copulations becomes difficult, so they mimic females and attempt to release sperm before the parental male can release his sperm. Figure 5 diagrams the genetic and environmental effects for the bluegill sunfish mating system. Note the difference between the parental and cuckolder strategies has a nonzero heritability, but the difference between the sneaker and mimic tactics has zero heritability because it is the size of the individual that determines the tactic used, not its alleles. Draper and Harpending (1982) have applied this type of thinking to the study of the effects of father absence on human development. They argued that the type of family environment experienced by a child before the age of 5 affects the reproductive tactics pursued in later life. The child with low father involvement is being prepared for life in a society where men frequently compete for access to a number of women and do not form enduring bonds or provide much investment in their offspring. The child with high father involvement, on the other hand, is developing attributes enabling it to maxi- Genetic strategy Behavioral tactics Nest + courtship f care Parental h 2 > 0.0 Cuckolder ^S. Sneaker » Female mimic » h = 0.0 Figure 5. Mating strategies and tactics in bluegill sunfish (Lepomis macrochirus). (The difference between life history strategies is heritable. The difference between tactics used to implement the life history is determined by environmental conditions.) mize its reproductive success in a society where men form long-lasting relationships with one woman and provide a high level of investment in their offspring. From this perspective the effect of father absence is an example of a developmentally contingent tactic. Recall that Darwinian algorithms (Cosmides, 1985) are innate, specialized learning mechanisms that organize experience into adaptively meaningful schema or frames. When activated by appropriate environmental and proprioceptive information, they focus attention, organize perception and memory, and call up specialized knowledge required for domain-appropriate inferences, judgments, and choices. In the context of life history theory, they are mechanisms that mediate environmentally contingent tactics by processing information. The three mating tactics of male scorpion flies, for example, can be seen as the result of a Darwinian algorithm for processing information about the male's relation to other males in its environment and producing an appropriate action in response to that information. Similarity, the higher verbal abilities of boys reared in father-absent homes (Draper & Harpending, 1982) can be understood as the reflection of algorithms that develop in the context of children's experience and contribute to the success of a particular adult reproductive tactic. Heritability and genetic involvement in development. It is sometimes assumed that if identical twins who are reared in different environments develop different abilities or personalities, then genes are not important in the development of the traits in question. A consideration of the mating behavior of identical triplet scorpion flies reared in three environments differing markedly in the level of male-male competition illustrates why this belief is incorrect. Males in the low malemale competition environment exhibit the dead-insect tactic. Males in the moderate competition environment exhibit the proteinaceous-mass tactic. Males in the high male-male competition environment may forgo courtship and attempt to force a copulation. The forced copulation tactic is not seen in an environment with low or moderate male-male competition. Although the heritability of the behavioral differences is zero, genes control the development of the mechanism for adjusting the behavior to environmental conditions. From an evolutionary perspective the primary issue is not the heritability of a trait but its correlation with inclusive fitness in the environment in which it occurs or, if the environment has changed, the environment in which it evolved. An ecological perspective suggests that psychologists who wish to use the theory of evolution in their work ought to initially focus their attention on the development of environmentally contingent tactics. Progress may be most rapid if they concentrate on behaviors (a) that have low heritability, (b) that are closely related to reproductive function, and (c) for which sensitivity to environmental conditions may have contributed to inclusive fitness in an ancestral population. The operation of contingent environmental tactics and the algorithms mediating them can be seen most clearly when there is no genetic variation, and hence zero heritability, for the behavior in question. Behaviors closely related to reproductive function are likely to be energetically expensive for the organism; therefore adaptations making them contingent EVOLUTION, RELEVANCE, PSYCHOLOGY on environmental conditions are likely to have evolved. The final requirement is necessary because the environment of some species, such as humans, has changed considerably in recent evolutionary history, so it is necessary to consider the ancestral environment in which the behavior in question evolved when studying contingent tactics. Evolutionary significance of behavior. Psychologists may gain valuable insights by studying the evolutionary significance of behaviors, the conditions in the ancestral environment of a species that may have rendered certain traits adaptive and others nonadaptive, and the effects on current behavior of mechanisms that evolved in response to these ancestral conditions. Early ethologists, such as Konrad Lorenz, believed that identifying behavioral homologies, similarities between two species explained by their descent from a common ancestor, was essential for understanding the evolutionary significance of behaviors. The wings of birds and bats, for example, are said to be homologous as forelegs because their evolution can be traced to forelegs in a common reptile ancestor. Although the evolutionary origin of a behavior must always be considered, understanding the evolutionary significance of a behavior does not depend on tracing its phylogenetic origin, nor does it depend on discovering analogies, similarities in two or more species, such as the wings of birds and insects, that are due to their response to similar ecological pressures rather than their common ancestory. Although Thornhill and Thornhill (1987) in an evolutionary analysis of rape in scorpionflies and humans suggest some similarities in the evolution of this behavior in the two species, they do not claim that human rape is analogous to scorpionfly rape or that understanding scorpionfly rape helps explain human rape. They do argue that the same theory, the theory of evolution, can help understand the behavior of both species. Understanding the evolutionary significance of a behavior requires distinguishing functions from beneficial effects (Williams, 1966). Beneficial effect refers to an attribute that currently contributes to fitness but that did not necessarily contribute to fitness in an ancestral population. Function refers to an effect that was the basis of differential reproduction in an ancestral population. A knowledge of an adaptation's function tells us about the solution that natural selection designed for dealing with a problem that our ancestors encountered in their environment. Although an adaptation may currently have many beneficial effects, it must have had at least one function. Although writing is a beneficial effect of the anatomy of the human hand, it is not a function of the hand because differential reproduction with respect to writing did not occur when the human hand was evolving its present form. Consider a hypothetical example designed to illustrate the importance of this distinction. Under current environmental conditions, grief at the death of an offspring contributes to current lifetime inclusive fitness through three beneficial effects: (a) eliciting help and support for the griever from close associates, (b) concentrating the care and attention of the griever on the offspring of close relatives, and (c) causing sexual arousal, which increases the likelihood of replacing the lost offspring (David, 1972). Now further suppose that elicit- 17 ing help from close associates was the only effect contributing to fitness in the ancestral population and that the other two effects are due to differences between the ancestral and current environments. On the assumption that the emotion of grief evolved to elicit help from close associates rather than because of the other two putative effects, providing support similar to that received from ancestral close associates is likely to be a better therapy than focusing attention on relative's children or producing sexual arousal in the griever. This example illustrates that an understanding of an adaptation for dealing with environmental conditions encountered by our ancestors can help us to understand how current environmental conditions affect current behavior. Studying the evolutionary significance of a human behavior is not simple. Looking for analogies or homologies is difficult because other species of Homo are extinct. Evaluating the fitness consequences of current behavior is not a reliable guide because of the massive changes in human environments during the last 30,000 years. Moreover, these traditional evolutionary methods do not provide the fine-grained data that interest psychologists. The predictive method, which uses a variety of contemporary data to test predictions derived from evolutionary theory (Barash, 1982), seems to hold the most promise for studying the evolutionary significance of human behaviors because it does not require tracing the history of the behavior in the fossil record, comparing human behavior with that of other species, or assuming that the behavior in question is currently adaptive. When it is applied to human behavior, I believe it involves at least three processes: (a) developing quantitative models to explore the strongest prediction that any hypothesis makes about a putative behavioral adaptation—that the hypothesized behavior actually could have increased fitness under ancestral ecological conditions; (b) developing predictions about current behavior from two or more competing hypotheses and using a variety of data to test them; and (c) studying putative evolved, proximate psychological mechanisms associated with the behavior. Consider how these processes can be used in studying Surbey's (1987) suggestion that anorexia nervosa might be the result of an ancestral adaptation for suppressing reproduction when environmental conditions are not conducive to the survival of offspring. Anorexia nervosa is a syndrome characterized by a relentless pursuit of a thin body shape and an exaggerated dread of fat and weight gain. The vast majority of cases are found in adolescent females, and emaciation and amenorrhea are essential features. Body image distortions, hypothalamic and pituitary dysfunction, sociocultural factors, and family patterns are associated with the disorder. For discussion of this syndrome, see Garfinkel and Garner (1982). I consider four of the many hypotheses for explaining this disorder (see Table 2). Two are competing evolutionary explanations, and two are nonevolutionary explanations. The example, taken from our research, is part of a more elaborate research design that compares predictions made from three evolutionary and seven nonevolutionary hypotheses. First, consider the evolutionary hypotheses. Imagine an ancestral population of protohumans, assumed for the sake of argument to be living in small groups characterized by hierarchical CHARLES B. CRAWFORD 18 Table 2 Evolutionary and Nonevolutionary Predictions About Anorexia Nervosa Predictions Socioeconomic status (SES) of family Incidence positively correlated with SES of family because of pressure for high-status girls not to marry low-status boys. Age of onset Peaks in incidence around puberty and at times of high female competition (e.g., entering a new school or job). Sex of anorexic Female. Inclusive fitness" Anorexia increases the resources available to close relatives, improving their survival and hence the inclusive fitness of the anorexic. Incidence negatively correlated with SES because low-status families have restricted resources. Incidence increased in those of low reproductive value (i.e., in the very young and the very old). Male or female; boys will have a greater tendency to become anorexic, because girls are a better bet for parents in difficult situations (Trivers & Willard, 1973). Developmental psychobiologyb Anorexia results from fear of attaining adult weight and sexual function. No relation, unless age at puberty is related to SES. Onset from puberty to age when other systems mature enough so that adult body shape is no longer feared. Female. Socioculturalb Anorexia is an attempt to conform to the cultural ideal that thin women are beautiful. No relation, unless concern for appearance is related to SES. Hypothesis Female-Female Competition" Anorexia serves to delay reproduction in situations involving severe female-female competition. Onset around puberty. Inci- Female or homosexual male. dence in later years depends on concern for beauty of women in different social situations; for example, the incidence of anorexia should decrease among older married women but may remain high among those still seeking husbands. " Evolutionary hypothesis that requires assumptions about behavioral adaptations in ancestral populations, ' Nonevolutionary hypothesis that requires no assumptions about behavioral adaptations in ancestral populations. social organization of both males and females. High-status males might offer their offspring better alleles and better resources. The female hierarchy could control access to resources needed for successful child rearing and high-status males. In such groups adolescent females lacking social and physical skills might have trouble dealing with (a) sexual attention from peripheral males, and (b) female-female competition for access to resources required for child rearing. The reversal of pubertal changes (achieved through moderate weight loss in lean girls) could eliminate male attention and reduce the intensity of female-female competition, giving these females time to develop social and physical skills. Such adaptive reproductive suppression has been documented in many species by Wasser and Barash (1983). Adaptations are often not ideally engineered but instead are improvised by natural selection from an assortment of preexisting anatomical structures, physiological processes, and behavior patterns (Gould, 1982). Thus, the anorexia nervosa syndrome could be the expression in exaggerated form, altered because of changed culture and ecology, of an adaptation for dealing with female-female competition or inappropriate male attention. The body image distortions, desire for thin physique, and hypothalmic and pituitary dysfunctions frequently causing extreme weight loss in our culture may be remnants of the preadaptations that natural selection put together to produce more moderate weight loss in protohuman adolescent females. The high levels of female-female competition and inappropriate male attention characteristic of some urban settings, such as the large city high school, may result in a malfunctioning of the adaptation that was fine-tuned to the environment of the ancestral population, producing the psychiatric anorexia nervosa syndrome. Viewing this syndrome from the perspective of ultimate causation not only enables us to understand the multiple proximate causal factors associated with its onset but also suggests that the training of social and physical skills, rather than behavior modification or drug therapy, ought to be the treatment of choice. The first phase of our work involves developing quantitative models for the evolutionary hypotheses to explore their consequences for fitness in an ancestral population. An extension of Wasser and Barash's (1983) reproductive suppression model to situations of high female competition and early menarche shows that even pathological anorexia nervosa, with its high fitness costs, might result in a net increase in fitness if it delayed first reproduction of hunter-gatherer women when the probability of reproductive success was low but expected to improve (Anderson & Crawford, 1988). From the second phase, Table 2 shows some of the many predictions made from the assumption that anorexia nervosia is related to an adaptation for dealing with excessive female- EVOLUTION, RELEVANCE, PSYCHOLOGY female competition (Crawford & Anderson, 1988). In addition, Table 2 includes predictions from an alternative evolutionary hypothesis that food refusal could be a form of altruism directed to close relatives, increasing the inclusive fitness for the fasting person. Note that predictions from the evolutionary hypotheses do not require interspecies comparisons, the tracing of evolutionary history, or the assumption that anorexia nervosa is adaptive in modern societies. Table 2 also includes two nonevolutionary hypotheses from the psychological literature. The sociocultural hypothesis (Garfinkel & Garner, 1982) states that anorexic behavior is a consequence of the desire for a thin body form because of cultural pressure. The developmental psychobiology hypothesis (Crisp, 1967) states that anorexia nervosa is caused by the girl's fear of attaining adult weight and sexual function. Consider the pairwise comparisons of the predictions about socioeconomic status (SES) of the family, age of onset, and sex of anorexics in Table 2. Because the two evolutionary hypotheses, female-female competition and altruism, lead to different predictions for all three variables, data can be obtained to exclude one of them. Note that these three variables allow a clear decision between the inclusive fitness hypothesis and both of the nonevolutionary hypotheses. However, given the three variables, it is not possible to distinguish between the female-female competition evolutionary hypothesis and either of the nonevolutionary hypotheses, as they make different predictions for only one variable, SES of the family. Similarity, it is not possible to distinguish between the two nonevolutionary hypotheses because they do not make sharply different predictions about the SES of the family and age and sex of anorexics. Adding variables and making additional predictions are necessary to distinguish between these hypotheses. For example, the two nonevolutionary hypotheses make different predictions about prognosis. The developmental psychobiology hypothesis predicts recovery once intellectual and social maturity have occurred, whereas the sociocultural hypothesis predicts that recovery does not occur until the girl no longer perceives the pressure to conform to current standards of beauty. Finally, an evolutionary explanation can be strengthened by showing that proximate psychological mechanisms operate in a way that does not violate the principle of adaptive design. For example, if anorexia nervosa is a consequence of an ancestral adaptation for reproductive suppression, it ought to be possible to make predictions about preferences for different types of food, given that the function of the weight loss is to stop reproduction while still maintaining the energy for continued physical and psychological development. It may also be possible to use the methods devised by Cosmides (1985) for studying the structure of the cognitive processes in anorexic patients. This example can be used to make a number of points. First, it reveals how a consideration of the ultimate causes of a behavior can enhance our understanding of the proximate factors responsible for its development. Second, it illustrates the wide variety of data that can be used to test hypotheses derived from evolutionary theory. Third, it helps clarify the role that the social environment can play in producing a 19 behavior disorder. Fourth, it emphasizes that determining whether a behavior is an adaptation or merely a beneficial effect of an adaptation is a difficult task. Fifth, it illustrates how hypothesis testing, ecological modeling, and exploring the naturally selected features of cognitive processes can contribute to the understanding of a current behavior. Finally, it shows how a consideration of the evolutionary significance of a behavior can give us valuable insights into its origin and development and hence into environmental interventions that might alter the behavior. This example ought not to be taken to imply that all human pathologies can be traced to putative adaptations in ancestral populations. Any environmental change has the potential of disrupting the biological and social functioning of an organism and reducing the level of adaptedness. If the environment change is small or preadaptations leading to a new adaptation are available, natural selection may in time enable the organism to adjust to the new environment. However, if the change is large or relevant preadaptations are not available, the new environment may produce great stress for the organism and may even result in extinction of the species. During the last 30,000 years, human inventions ranging from agriculture to television have irreversibly changed human culture. Each of the changes may lead to stress and pathology. It will be possible to trace some of these pathologies to the overloading and distortion of ancestral adaptations. However, there will be others that do not have an evolutionary history. For example, the change from a hunter-gatherer to an agricultural way of life may have resulted in changes in behavior that reduced the level of exercise. If the level of physical activity in ancestral humans was involved in some aspect of metabolism, its reduction in modern humans may result in a slight biochemical dysfunction that may lead to a physical or behavioral pathology. Such a pathology does not have an evolutionary history in the strict sense because it cannot be traced to an adaptation for dealing with ancestral environmental conditions. Conclusions Psychologists must pay attention to the theory of evolution for a number of reasons. It can help them to eliminate outmoded concepts from their thinking and can provide a rich source of ideas for developing explanations for behaviors that cannot readily be explained with current psychological theories. 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