Evolutionary Biology: a Basic Science for Psychiatry?

Review article

Evolutionary biology: a basic science for psychiatry?

McGuire MT, Marks I, Nesse RM, Troisi A. Evolutionary biology: a M. T. McGuire’, I, Marks’, R. M. Nesse3, basic science for psychiatry? A. Troisi4 Acta Psychiatr Scand 1992: 86: 89-96. ’ Department of Psychiatry-Biobehavioral Sciences, School of Medicine, University of California at Los Angeles, USA, Institute of Psychiatry, DeCrispigny Park, London, United Kingdom, Department of Psychiatry, School of Medicine, University of Michigan, Ann Arbor, Michigan, USA, Clinica Psychiatrica, Universita di Roma, Italy

Evolutionary biology has much to offer psychiatry. It distinguishes between Key words: evolutionary biology: psfchiatric ultimate and proximate explanations of behavior and addresses the theory; psychiatric research functional significance of behavior. Subtheories, frequently voiced Michael T. McGuire, Department of misconceptions, specific applications, testable hypotheses and limitations Psychiatry-Biobehavioral Sciences, School of of evolutionary theory are reviewed. An evolutionary perspective is likely to Medicine, University of California at Los Angeles, improve understanding of psychopathology, refocus some clinical 760 Westwood Plaza, Los Angeles, CA research, influence treatment and help integrate seemingly unrelated 90024, USA findings and theoretical explanations. Accepted for publication February 29, 1992

ory can be found in Williams (l), Wilson (2) and Introduction Alexander (3). Recent advances in evolutionary biology have implications for psychiatric theory, research and clin- Overview of the evolutionary approach to psychiatry ical practice. Evolutionary theory introduces a broad Behavior and much needed deductive framework; it facilitates the functional analysis of behavior; it identifies im- Consider behaviors such as acquiring a mate, sexual portant differences between ultimate causes and intercourse, having offspring, parent-offspring bond- proximate mechanisms; it promotes a reassessment ing, stranger anxiety, infant and juvenile play, sibling of current views about etiology and pathogenesis; rivalry, competition over resources, preferential in- and it alters treatment strategies and options. Evo- vestment of resources in kin, reciprocal exchanges lutionists acknowledge that much human behavior is among nonkin, competition for social status, recog- a product of personal and cultural experience, but nition and rejection of cheaters, deception of others argue that mind and culture themselves are products and self-deception. Natural selection has shaped of evolution and are better understood when ana- predispositions (strong biases) to engage in such be- lyzed within the evolutionary framework. haviors. Predispositions range from strong (such as Our assessment of the contributions that an evo- withdrawal from a painful stimulus) to weak (such lutionary point of view can make to psychiatry be- as preference for particular styles of music). gins with an analysis of its theoretical contributions The strengths of predispositions vary across indi- followed by a discussion of common misconcep- viduals. Experience and learning, which is also an tions. Possible applications to psychiatry are then evolved capacity, modify predispositions. Environ- evaluated. Finally, the limitations of the theory are mental options and constraints interact with evolved reviewed. and prioritized behavior strategies to affect the prob- Only selected parts of evolutionary theory are dis- ability of immediate and long-term behavior. Psy- cussed. More comprehensive discussion of the the- chological mechanisms (such as cognitive and psy-

89 McGuire et al. chodynamic ones) filter, modify, organize and focus responsible for that predisposition. Kin-selection information that influences behavior. Behaviors to theory explains how altruistic behavior among kin which most persons are not predisposed (such as facilitates the replication of genes shared by kin. The playing the piano) can be mastered, but usually after theory predicts, for example: that parents will invest considerable effort. Nonetheless, the general behav- more in their orspring than vice versa; that such ior profiles and patterns of human behavior are set investment is greater with higher parental assess- by the species’ genome and, within limits, unfold in ment of the offspring’s reproductive potential; and predictable ways. that stepchildren are more likely to be abused that The above and other behaviors are of special in- biological offspring. These predictions are well sup- terest to psychiatry for several reasons: 1) mental ported by evidence (17-19). disorders are associated with detectable abnormal- ities in the expression of such behaviors, such as Reciprocal altruism. Although kin selection explains reduced frequency of reciprocation (4-6); 2) an ad- investment in kin, unrelated individuals also help equate understanding of some mental disorders (such each other (20). Such reciprocity exchanges are not as phobias, depression and somatoform disorders) confined to humans. Vampire bats, for example, requires an appreciation of the functional signifi- share their blood meals after returning to their roost cance of such behaviors and of the evolved mecha- at dawn (21). Such exchanges even occur between nisms that regulate them (7, 8); and 3) treatment species (22); alarm calls by one species alert mem- strategies may change (for example greater empha- bers of other species, and cleaner fish remove par- sis on functional outcomes and increased attention asites from inside the mouths and gills of larger fish to environmental variables) with increasing appre- who protect cleaners from predation (23). ciation of the evolutionary perspective. Reciprocal altruism theory predicts: 1) A will help Natural selection favors brain mechanisms that B (nonkin) if, in A’s view, the probability of recip- result in behaviors conferring a reproductive advan- rocation by B times the probable benefit of this re- tage. Generally, such behaviors are neither atypical ciprocation equals or exceeds the initial cost to A nor pathological. With few exceptions, however, (reciprocal behavior is distinguished for mutualism, mental disorders are atypical, pathological and mal- where both parties gain simultaneously); 2) failure to adaptive. Evolutionary theory provides a framework reciprocate evokes moralistic aggression; 3) nonre- for analyzing adaptive behavior, the mechanisms ciprocation will rise at the end of a period of asso- that regulate it and the circumstances interfering with ciation (24); and 4) nonreciprocators will have fewer its normal expression (9). Psychopathology can be allies. These predictions are also supported by avail- illuminated by insights into its normal adaptive pre- able evidence (25,26). cursors. Most phobias, for example, are exaggerated Evolutionary theory also explains why such emo- normal fears (10, 11); minor adaptive rituals such tions as pride, happiness, guilt and moralistic anger as perfectionism are precursors for obsessive-com- and such behaviors as guilt induction, intimidation pulsive disorder (10); normal mood swings can be and social ostracism have evolved. They facilitate viewed as precursors for manic-depressive illness reciprocal relationships (20, 25, 27, 28). Much of (12); and normal sadness and social withdrawal may development and learning deals with refining recip- be precursors to clinical depression (13-15). rocal behaviors, such as how to successfully manage reciprocity exchanges without being exploited or re- Subt heories jected - battles among children often focus on who owes what to whom (29). Deviations from expected Since the early 1960s, a number of testable subthe- patterns of early kin and non-kin social relationships ories have been developed to explain specific behav- may contribute to disordered reciprocity relation- iors. Examples include: inclusive fitness or kin se- ships later in life (30). lection, reciprocal altruism, parent-offspring conflict, and cognitive/feeling mechanisms. Parent-ofspring conyict and sibling rivilary. Although cooperation between parents, offspring and siblings Inclusive fitness. Preferential investment in kin is is promoted by the genes they share in common, prominent in humans, as it is in other species. The conflicts can be explained by the fact that many of tendency is to invest more in close than in distant their genes are not shared (31). Parental fitness (the kin, and for parents to invest more in offspring than number .of parental genes replicated among off- vice versa. Hamilton (16) postulated that certain spring) is enhanced by the reduction of investment kin-related behavior is selected because it favors in- in maturing offspring in order to sooner reproduce dividuals who share genes. A behavior costly to the again. Existing offspring try to counter this reduction self may nevertheless evolve if it increases the sur- and maximize their own fitness by delaying the birth vival and reproduction of kin who share the genes of new siblings and by extracting as much from par-

90 Evolutionary psychobiology ents as they can. The tendency in many species for such mechanisms may help to explain some anxiety offspring to object to weaning supports this view. disorders. In other disorders, deficits in mechanisms The conflict has a natural end-point when existing for reciprocity negotiation seem probable. Such def- offspring’s inclusive fitness is increased more by hav- icits may be a product of proximate mechanisms ing a sibling with many shared genes than by con- leading to social isolation and associated chronic tinuing to try to extract more resources from parents depression. (3 1). An important aspect of evolved cognitive/feeling Sibling rivalry has a parallel explanation and may mechanisms is the distinction between symptoms arise in the same way. Siblings aid each other be- and disorders. Some mental signs and symptoms, cause they share genes, but simultaneously compete such as anxiety, sadness, irritability and withdrawal for parental resources. may be normal and adaptive when not severe. Like other aversive states, such as pain and nausea, they Other conflicts. Similar evolutionary-based explana- may point to circumstances liable to decrease indi- tions account for male-female differences and as- vidual fitness, but not necessarily be manifestations pects of marital relationships. Males, for example, of disorders (7, 39). Interfering with their expression are more promiscuous, take more physical risks, and thus may be disadvantageous for the patient, just as engage in more antisocial behavior (2,32,33). Spou- blocking coughing or vomiting reflexes can be dan- sal relationships combine reciprocity, kin selection gerous for patients with pulmonary or gastrointesti- and the management of conflict between kin. Spous- nal infection. (Such behaviors are distinguishable es’ mutual dependence for their reproductive success from signs and symptoms that are more direct con- leads to common interests and reduces conflict (34), sequences of mental disorders, such as hiding from while conflict results from pressures on the partner others because of persecutory delusions.) to provide more care for offspring (3 l), to raise off- The evolutionary arguments discussed thus far are spring in specific ways, to help nongenetic relatives reasonable, intuitively acceptable and supported by or to reciprocate help more completely. research findings in some respects. Several points relevant to psychiatry emerge. First, many behaviors Cognitivelfeeling mechanisms. Specific ment a1 capac- are strongly predisposed, such as sibling rivalry, ities appear to have evolved to serve specific func- moralistic aggression and familial rejection of non- tions, such as the ability to spot cheaters (35-38). maturing relatives. Second, some features of psy- Similar points apply to assessing the value of differ- chopathology reflect the prepotent and preparedness ent types of reciprocation, the reproductive potential of responses. Third, features of disorders may reflect of offspring and the adaptive value of different be- the normal operation of an adaptive response. De- haviors. pression following the loss of a loved one or a de- The cognitive, physiological and behavioral pre- cline in social status is not only expected from an dispositions that characterize fear offer selective ad- evolutionary perspective (13), but also may be tem- vantages in dangerous situations (10). Behavior that porarily adaptive in evoking social support from kin is phylogenetically predisposed is not necessarily in- and friends (40). Similarly, jealousy, anger, decep- evitable, however. Indeed, it can, on occasion, be tion, self-deception and anxiety may be temporarily changed dramatically (10). For example, the fear of adaptive. Fourth, evolutionary interpretations are blood injury is strongly familial, develops in child- compatible with proximate mechanism explanations, hood and is characterized by a vasovagal response such as postulated physiological changes associated and syncope. Yet it can be permanently eliminated with depressive disorders, inheritance of disorder with a few hours of systematic exposure therapy. vulnerability (pedigree studies) or compromised The same applies to strongly predisposed fears of learning following aversive early experiences. Evo- snakes or other animals. (Similarly, even innate de- lutionary theory offers existing psychiatric theories a fensive responses in molluscs, mantids and bii-ds can context in which interrelationships between different be eliminated by habituation analogous to exposure findings and explanations emerge, and ultimate ev- therapy.) The capacity for change is a built-in, se- olutionary and proximate mechanism explanations lected trait that adjusts the behavioral preferences are clarified. characteristic of the species. A better understanding of such evolved mecha- Misconceptions about evolutionary theory nisms is likely to deepen our insights into psycho- Evolutionary theory applies mainly to other animals pathology. For instance, stranger anxiety, separation anxiety and phobias of heights, animals, storms and This view appears to assume that evolutionary bi- leaving one’s home range appear to be prepotent ology consists primarily of reconstructing a species’ stimuli to which pre-prepared fear is readily con- phylogeny. Evolutionists argue instead that even if nected ( 10). Over-responsiveness and/or rigidity of humans had no close phylogenetic relatives, evolu-

91 McCuire et al. tionary theory (or a similar theory) would be neces- Evolutionists do no ask, “Is it learned or innate?”, sary to help explain human behavior. Mechanisms but rather, “How easily can this particular response of adaptation operate primarily at the individual level be learned?”. A child’s readily learning to speak, for and shape behavior accordingly (functional analy- example, reflects built-in design features based on sis). Viewing humans in an evolutionary context adaptive cognitive, sensory, and behavioral algo- leads to predictions that fit many otherwise hard to rithms. Playing the piano, however, is difficult to explain facts (such as prepotency for many behav- learn - it has not been selected for. iors, step-parent abuse, sibling rivalry and parent- offspring conflict). Moreover, the theory leads to un- Culture is an alternative to evolutionary explanations expected connections that are unlikely to be made without its guidance. Examples include: the possible It is a mistake to see biology and culture (a set of adaptiveness of some antisocial behavior (such as behavioral rules) as sharp opposites (48-50). Cul- cheating as an adaptive strategy) (4 1); abandonment ture is not an alternative explanation to evolutionary of kin who are poor reproductive bets; child abuse theory. Rather, the ability to absorb culture is an by step-parents; male and female sexual jealousy evolved trait. Behavioral predispositions shape cul- and possessiveness (42); and agoraphobia as an ex- ture and are shaped through it, leading, for example, traterritorial fear associated with past threats when to stable patterns of sexuality, pair-bonding, child- venturing outside the home range (10, 43, 44). rearing, nonkin reciprocation and social status rela- Any species-characteristic behavior has a distri- tionships. The forms of these behaviors vary across bution limited in width by natural selection. Individ- societies, but all societies have stable patterns and uals vary in capacities to empathize, reciprocate, values associated with strongly predisposed behav- develop new goal-achievement strategies, compete ior. Amplification of biological predispositions is il- and maintain physiological and/or psychological ho- lustrated by avoidance of inbreeding (incest taboos), meostasis (45). Human variations in these behaviors which is observed across cultures and among all may have led to successful adaptation in many dif- studied primates (5 1). Similarly, deviant behavior ferent environments. Seeming facets of disorders, (such as excessive risk-taking, impulsive behavior both symptoms and personality, may thus be con- and non-reciprocation) that jeopardizes others’ sequences of a range of variation rather than mani- health and resource control evoke rejection of the festations of psychopathology. In evolutionary terms deviant (25). a trait can be regarded as biologically normal if it enhances adaptive capacities. Severe psychopathol- Evolutionary ideas are not relevant to treatment ogy nearly always reduces fitness (46). Compared to normal people, those with mental disorders help kin Advances in basic knowledge are more likely to less and are less helped by kin; they more often fail lead to better formulations of treatment-relevant to reciprocate help from nonkin and have fewer problems than to immediate changes in treatment friends; and they have reduced resource access and procedures. Optimal treatment and prevention pre- control (McGuire, submitted). supposes detailed knowledge about etiology and Evolutionary theory provides psychiatry with some pathogenesis. Pediatric hospital procedures im- explanatory tools regarding individual differences: proved following Bowlby’s evolutionary-based stud- 1) a framework that specifies the range of adaptive ies of attachment behavior. Appreciating the high species-characteristic behavior patterns against probability of child abuse among stepchildren is which the behavior of individuals can be compared; likely to increase the recognition of the need for 2) a general theory and subtheories to interpret the vigorous preventive measures. Therapists who un- adaptive significance of behavior; and 3) a frame- derstand the principles of reciprocal altruism and work to explain cross-person differences in attributes the possible functions of self-deception and repres- (trait distribution). sion (52, 53) are more likely to enhance the rehabil- itation of social skills. And attention to person- Evolutionary biology ignores individual learning environment interactions is likely to optimize the selection of therapeutic recommendations: for exam- The human capacity to learn is vastly greater than ple, teaching patients to select social environments that of other species. The view that evolutionary that are responsive to their needs. biology ignores individual learning seems to arise from the incorrect assumption that if something has Evolutionary hypothesis can not be tested been learned then it has no evolutionary roots. How- ever, the capacity to learn has been selected, like Evolutionary hypotheses are eminently testable. For every other trait (47). Each species, including Homo example, Daly et al. (42) provide extensive support sapiens, learns certain things more easily than others. for the prediction that, in most societies, sexual jeal-

92 Evolutionary psychobiology ousy should be more extreme in males than in fe- males. Essock-Wale & McGuire (18) confirm that, Existing theories in other disciplines explain findings at least as well within kin networks, resources flow from older to as evolutionary theory younger persons. Buss (54, 55) has shown that the Evolutionists accept this view for certain features of frequency of divorce increases in infertile marriages. current theories. Evolutionary theory does not re- There is mounting evidence for the prepotency and place other psychiatric theories. Rather, it 1) offers preparedness of fear (lo). Scenarios of past evolu- an additional perspective, 2) helps explain hereto- tionary events cannot be proven, but the number of fore overlooked and understudied features of disor- possible scenarios can be limited, because given sce- ders and 3) provides an integrating framework. By narios predict certain outcomes, some of which can adding ultimate causation to the current focus on be tested. For example, the hypothesis that domain- proximate factors (such as genes, physiology, learn- specific cognitive and feeling mechanisms mediate ing and psychodynamics), the theory identifies bi- reciprocal exchange implies that basic exchange rules ases and constraints of behavior and physiological will be comparable across cultures, and that precur- systems. Proximate psychiatric theories explain sors will be present in our close phylogenetic rela- many of the mechanisms of disorders, such as the tives (35). biochemical explanations of depression, reduced serotonin turnover among chronic abusive criminals Natural selection should eliminate genes for mental disorders (57) and petty criminality as the substrates on which evolution acts (58-60). Evolution can help to ex- If most mental disorders are maladaptive, then why plain why these mechanisms exist at all. do they persist? Evolutionists have advanced several The degree to which psychiatry is likely to incor- explanations for the persistence of mental disorders. porate an evolutionary perspective will depend on First, atypical or deviant traits tend only to reduce, specific applications that inform our understanding not eliminate, the number of offspring. Thus, once and improve treatment. We now turn to the subject a disorder appears in a population, it may remain of applications. for a considerable time. Reduced reproductive success is seen in obsessive-compulsive disorder, for Applications example, but success is only marginally lower than Criteria for distinguishing normal from abnormal behavior among normals (10). Second, many disorders (such as late-onset depression, Huntington’s disease and The current criteria for defining mental disorders Alzheimer’s disease) mostly appear after peak re- include: 1) association with a somatic lesion (ana- production periods, so that inclusive fitness is less tomical to molecular); 2) statistical deviances from affected. normality (such as excessive self-centeredness, learn- A third possibility is heterosis, in which even if it ing disabilities and bipolar illness); and 3) subjective is maladaptive to have the entire set of genes that is distress. Some authors have suggested “biological needed to produce a given disorder, having some of disadvantage” as a more appropriate criterion for them (short of the number necessary for the full identifying mental disorders (61, 62). This view ac- disorder) may be advantageous in certain environ- cords with an evolutionary approach. A disorder ments. Possessing all the genes for sickle cell anemia must impose some form of fitness disadvantage. Sta- (homozygous state) leads to an early death, but car- tistical deviance does not directly equate with dis- rying only some of them (heterozygous state) en- orders - mental retardation is a disorder but genius hances resistance to malaria. Manic-depressive psy- is not. Moreover, it is difficult to know whether a chosis and obsessive-compulsive disorder are condition is pathological without considering the en- maladaptive, but cyclothymic and meticulous per- vironment in which it occurs. Depression may be sonalities may provide an adaptive advantage in cer- adaptive in a supportive social environment, just as tain environments. (A variation of this idea has been schizoid personality may be adaptive in isolated suggested for the persistence of senescence (44, 56): areas. Insofar as our present-day environment dif- the very genes that lead to senility in old age may be fers from past environments in which current traits those that confer some benefit in youth.) were adaptive, past adaptive behaviors may become Fourth, recessive traits may be hard for natural maladaptive (such as the prepotency of animal fears selection to eliminate if they only partially compro- being more fitting for our ancestors). Subjective suf- mise adaptation, and many forms of mental illness fering has a high probability of association with mal- are so infrequent that selection may be slow. Finally, adaptive situations, whether caused by external cir- some behaviors that are disorders in one culture cumstances or by brain disorders. Feelings of might not be disorders in other cultures. Dyslexia, pleasure and pain probably evolved to tell us whether some forms of antisocial behavior and certain un- prevailing circumstances are adaptive (27). Yet, even stable personality variants are examples. emotional suffering may be useful, just as pain, nau-

93 McGuire et al. sea and fatigue are useful, although all indicate the or from the failure of mechanisms that inform one presence of some danger. that a danger has passed. Conversely, we may come The preceding points imply that both the features to recognize new disorders characterized by defects of disorders and the disorders themselves should be in capacities for anxiety, normal sadness and jeal- evaluated in the context of evolution. Further, fea- ousy. A clear implication is that appropriate treat- tures of disorders may be adaptive or result as a ment depends on both the mechanism involved and consequence of ultimate causation. These possibili- how it is awry (66). ties result in alternative etiological explanations of disorders, raise questions about classification and imply different approaches to treatment. Functional capacities and disorder classification Functional deficits are considered in Axis V in DSM- Pathogenesis 111-R, but they are accorded far less importance in the day-to-day practice than are Axes I and 11. The Evolutionary theory focuses attention on distinctions concept of function and functional deficits is not the between etiology (cause) from pathogenesis (mech- province of evolutionary theory. However, evolu- anism). Showing that depression is associated with tionary theory provides a conceptual framework in alterations in norepinephrine or serotonin neuro- which to organize and interpret functions and, in an transmission (mechanism) does not reveal its cause evolutionary-derived classification, an assessment of (etiology). Biochemical alteration may be a common functional capacities would be a pivotal taxonomic end product reflecting an environmental or genetic category. Persons with high anxiety, moderate de- etiology, or both. Serotonin neurotransmission is al- pression, self-deception, mild bipolar illness and, in tered by changes in social status, and testosterone some instances, schizophrenia, often function above and cortisol levels rise in response to a variety of average. The same is true for some people who have social events (25,63). Such changes, in turn, alter socially unattractive traits, such as extreme self- the probability of behavior (such as responsiveness centeredness and extreme needs to dominate and to others and vigilance), how information is pro- control others. For some disorders, decreased func- cessed and response to drugs (64). Certain social tional capacity is the primary finding, as is the case interactions are as essential for physiological and with antisocial personality and malingering. psychological homeostasis among adults as they are Assessment of functioning may also help explain among infants. Persons vulnerable to mental disor- within-disorder variance among current classifica- ders tend to have reduced capacities to manage their tion categories. Generalized anxiety disorder with social environments to produce homeostasis and a minimal functional consequences may be classified greater probability of developing a disorder (45). separately from generalized anxiety disorder with Such evolutionary-oriented studies focus on major functional consequences. A focus on func- behavior-physiology interactions and provide data tional capacities would also alter treatment strate- for the empirical foundation of a more comprehen- gies and choices. A number of drugs that effectively sive model of mental disorders (65). reduce symptom intensity also reduce functional capacities (such as decreased motivation and ability to concentrate). Evolved psychological mechanisms As mentioned, natural selection is likely to have fa- Integrating current theories vored the development of cognitive/feeling mechanisms preadapted to particular situations. Examples Advocates of one school of psychiatry frequently include: respond immediately to perceived external disregard findings developed by other schools. Bio- danger; avoid or retaliate against those who inflict logical psychiatrists are seldom interested in the pain; assess others’ ability to reciprocate helping; findings of psychoanalysts or social psychiatrists, detect cheaters; and be vigilant towards actual and and some behaviorists ignore physiological aspects potential competitors. The characterization of such of mental disorders. Can such parochialism be mechanisms is likely to be a major focus of research changed? during the next decade. This research should lead to Evolutionary theory can encompass the major a description of mental disorders in terms such as conceptual frameworks of psychiatry, help integrate underactive and overactive mechanisms (for exam- relevant findings, appraise new ones and introduce ple, over-reading or under-reading environmental additional possible explanations. Biological psychi- dangers) as well as in the specification of such re- atry focuses on genetic-physiological predispositions sponses to behavior, physiological states and func- and proximate mechanisms contributing to disor- tional consequences: for example, chronic anxiety ders, social psychiatry on environmental contribu- results from an overactive response to danger and/ tions, behavioral psychiatry on atypical behavior and

94 Evolutionary psychobiology psychoanalysis on thoughts and feelings. Each of opmental experiences in the shaping of individuals, these focuses may best explain certain abnormal the importance of specific events and the time-frames features, although in differing degrees within and in which they optimally occur remains largely an across disorders. The addition of ultimate causation empirical issue. Extreme upbringing conditions (such (such as predispositions to behave in certain species- as the total social deprivation of infants) have clear characteristic ways), functional assessment, the developmental effects, as do instances of the absence possible adaptive function of certain features of dis- of essential neurochemicals for normal development orders, trait distribution and the physiological con- (such as Tay-Sacks disease) or chromosomal aber- sequences of person-environment interactions en- rations (such as Down’s syndrome). However, much riches the database of psychiatry and allows us to of our knowledge is based on atypical situations that address what is intuitively obvious, the concept of do not themselves directly inform our understanding multiple bases for disorders. of the complex process of development, its timing, Evolutionary theory also offers psychiatry a de- and the impact of conditions generally accepted as ductive framework and a set of testable hypotheses. being within the normal range. For example: multiple family placements of young children during critical periods of development is Conclusion predicted to lead to both suboptimal refinement or learning of basic social skills and to the expression A strong case can be made for evolutionary biology of certain predisposed traits that otherwise might deepening our perspective by clarifying the concepts not be expressed; diminution of within-disorder and explanatory power of ultimate to proximate ex- adaptive traits because of drug treatment would be planations and by offering psychiatry an integrating predicted to result in noncompliance with treatment paradigm. The theory puts humans back into an recommendations; and relinquishment of behavior ecological context and raises questions about the that is offensive to others will occur only if alterna- normal operation of adaptive mechanisms, particu- tive behaviors are associated with desired benefits. larly those elicited by unusual circumstances. Per- haps surprisingly, its understanding and recognition Limitations of evolutionary theory of variation results in a particularly humanistic ori- entation to people’s problems. A number of questions and unresolved issues remain. The limits of evolutionary explanations have Acknowledgement not been fully explored, particularly where 2 or more The authors wish to thank the late Robert J. Stoller for his helpful tendencies conflict (su:h as reciprocate by helping comments on an earlier draft. another vs invest in kin). For some behaviors, evolutionary explanations may prove to be the most References parsimonious: for example, excessive risk-taking, antisocial behavior and depression following loss. 1. WILLIAMSGC. Adaption and natural selection. Princeton: Nevertheless, proximate events (such as the critical Princeton University Press, 1966. 2. WILSONM, DALYM. Competitiveness, risk taking, and vi- physiological details of a drug overdose) associated olence: the young male syndrome. 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