Evolutionary Developmental Psychology: A New Tool ...

Paper Human Development 2003;46:259–281 DOI: 10.1159/000071935

Evolutionary Developmental Psychology: A New Tool for Better Understanding Human Ontogeny Carlos Hernández Blasi a

David F. Bjorklund b

a Universitat Jaume I, Castellón, Spain; Fla., USA

b

Florida Atlantic University, Boca Raton,

Key Words Developmental psychology ` Evolutionary developmental psychology ` Evolutionary psychology ` Evolutionary theory ` Human ontogeny

Abstract Evolutionary developmental psychology (EDP) is described and contrasted with previous (e.g., sociobiology) and other contemporary (e.g., mainstream evolutionary psychology) approaches to applying evolutionary theory to human behavior. We argue that understanding the ‘whys’ of development will help us acquire a better understanding of the ‘hows’ and ‘whats’ of development, and that in addressing the ‘whys’ an EDP perspective has the potential to provide a fuller understanding of human ontogeny. To this end, we propose five ways of applying EDP to contemporary issues of psychological development. These include (1) classifying developmental features according to their evolutionary or functional status, (2) proposing hypotheses and microtheories to explore the function of developmental traits, (3) collecting data from different sources to test developmental evolutionary hypotheses, (4) describing the phylogenetic and sociocultural history of human developmental features and (5) designing ‘evolutionary experiments’. We argue that an EDP approach should not be seen as replacing other, more proximal, explanations of development, but rather that an evolutionary perspective should be incorporated in all accounts of human ontogeny. Copyright © 2003 S. Karger AG, Basel

The publication of Origin of Species by Charles Darwin in 1859 initiated the beginning of a scientific, cultural and social revolution – one that caused science and society to view the origin of humankind and its relationships with other living

Ó2003 S. Karger AG, Basel 0018–716X/03/0465–0259$19.50/0

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Carlos Hernández Blasi Departamento de Psicología, Universitat Jaume I ES–12080 Castellón (Spain) Tel. +34 964 729 728, Fax +34 964 729 349 E-Mail [email protected]

beings differently. Not surprisingly, Darwin’s ideas have often been compared with those of Copernicus because of their deep impact, not just in science, but in culture and human understanding. As is the nature of science, ideas also change and evolve, and the core of evolutionary thinking since the middle of the 20th century (the so-called New or Modern Synthesis) is actually a mixture of Darwinian ideas about natural selection and contemporary genetic theory, as initially developed by people such as Dobzhansky [1937], Mayr [1942] and Simpson [1944]. Yet, the principal mechanism of evolutionary change proposed by Darwin remains alive, namely natural selection, which involves four straightforward principles: (1) there are more members of a species born in each generation than will survive (i.e., superfecundity); (2) there is variation in physical and behavioral characteristics among individuals within species; (3) this variation is heritable, and (4) characteristics that result in an individual surviving and reproducing tend to increase in frequency in the population, whereas characteristics of nonsurvivors decrease. Evidence from a broad range of disciplines, including biogeography, paleontology, comparative anatomy, embryology and molecular biology [Ayala, 1999; Weiner, 1995; US National Academy of Sciences, 1984] has provided consistent support for Darwin’s contention, making the ‘fact’ of evolution undeniable, with natural selection being the most potent (though likely not the sole) mechanism responsible for changes in species over time [Gould, 2002; Mayr, 1982]. Psychological science as a whole – and particularly developmental psychology – has not been immune from the findings and theories related to biological evolution. In fact, the origin of developmental psychology as a discipline has often been linked with the evolutionary thinking of the scientific community during the latter half of the 19th century [Delval, 1982, 1988, 1994; Cairns, 1983; Dixon and Lerner, 1992]. However, others have questioned the importance of such a link [Bradley, 1989; Charlesworth, 1992] or proposed that developmental psychologists have misunderstood Darwin’s message, resulting in what Morss [1990] called the ‘Darwinian myth’, or what Lickliter and Berry [1990] referred to as the ‘phylogeny fallacy’. Moreover, the main theoretical streams that have led psychological thinking over the last century (behaviorism and cognitive psychology) have focused on levels of analyses that did not make completely necessary reference to either biology or evolution [Hernández Blasi, 2000], and this includes mainstream developmental psychology. To date, after almost 150 years since Darwin’s ideas were published, the influence of evolutionary ideas on developmental psychological theory and research has been weak (although there are some exceptions, for instance, attachment theory, see Bowlby [1969]). Evolutionary thinking has remained within our discipline more as a vague theoretical reference than as an active pool of ideas to serve as a framework for doing research on development. As Charlesworth noted in 1992, ‘the revolutionary implications for studying human behavior from an evolutionary point of view are just starting to be realized’ (p. 9), and we believe that developmental psychology should partake fully in 21st century zeitgeist that recognizes phylogenetic influences on human thought and behavior. Although few contemporary psychologists, developmental or otherwise, deny the fact of human evolution, there has been (and continues to be) a long-standing resistance in the social and behavioral sciences to the application of evolutionary theory to explain human behavior [Brown, 1991; Geertz, 1973; see Tooby and Cosmides, 1992, for a review]. This stems, in part, from a belief that humans possess a 260

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unique set of mental faculties that provides them a degree of intentional control over their behavior, freeing them from the constraints of biology and obviating a need for an understanding of Homo sapiens’ phylogenetic history in order to understand its behavior. Relatedly, evolutionary psychology and other biologically based approaches to human behavior smack of genetic determinism and serve, in the minds of some, to minimize the role of culture in influencing behavior. According to Tooby and Cosmides [1992], the standard social science model, which proposed that humans possess no ‘nature’ but are solely the products of their culture, dominated thinking in the social sciences through the 20th century and made no room for evolutionary thought in explaining human behavior. Tooby and Cosmides [1992] discussed at great lengths many of the shortcomings of the standard social science model, and research over the past several decades has clearly shown that humans are not born as blank slates but have biologically influenced constraints that affect their perception, cognition and social and emotional behavior [Bjorklund and Pellegrini, 2002; Gelman and Williams, 1998]. Moreover, contemporary evolutionary psychological theory emphasizes that there is substantial plasticity in development, making any claims of genetic determinism incorrect (see further discussion below). Finally, an explosion of research in comparative psychology has revealed both many similarities and differences in the social and cognitive abilities of H. sapiens and our great ape relatives [Byrne, 1995; Tomasello and Call, 1997; Parker and McKinney, 1999; Suddenhof and Whiten, 2001], supporting the existence of a phylogenetic continuity of mental function. Although such discoveries and insights do not require that contemporary human functioning and development be examined from an evolutionary perspective, we believe that such advances make the time ripe for such an analysis. What Is Evolutionary Developmental Psychology?

What today is called evolutionary psychology (EP) is an outgrowth of earlier movements in sociobiology, ethology and behavioral ecology that applies the basic principles of neo-Darwinian evolution to explain contemporary human behavior. According to EP, the human mind has been prepared by natural selection, operating over geological time, for life in a human group. As such, according to Buss [1999], EP should be especially committed to answering four key questions: (a) Why is the mind designed the way it is? (b) How is the mind designed – what are its mechanisms? (c) What are the functions of these mechanisms, and how are they organized? (d) How does environmental input interact with these mechanisms to produce behavior? Within this framework, evolutionary developmental psychology (EDP) can be viewed as a more specific theoretical perspective that has been broadly defined as ‘the application of the basic principles of Darwinian evolution, particularly natural selection, to explain contemporary human development. It involves the study of the genetic and environmental mechanisms that underlie the universal development of social and cognitive competencies and the evolved epigenetic (geneenvironment interactions) processes that adapt these competencies to local conditions; it assumes that not only are behaviors and cognitions that characterize adults the product of selection pressures operating over the course of evolution, but so are characteristics of children’s behaviors and minds’ [Bjorklund and Pellegrini, 2002, Evolutionary Developmental Psychology


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p. 4]. From this point of view, EDP can be considered simultaneously as (1) a general psychobiological framework to understand development, (2) a source of experimental hypotheses and questions about development and (3) a perspective to produce and test interpretations about developmental outcomes from a phylogenetic point of view. The basic principles assumed by EDP can be summarized as follows [Bjorklund and Pellegrini, 2000, 2002]. (a) The extended juvenile period of H. sapiens was favored by the need (and the time necessary) to master an increasingly complex social-technological environment. This extended youth had implications for the evolution and development of the brain as well as for psychological development. (b) Evolution favors ontogenetic diversity (variation), because humans live in a wide range of environments, and this requires flexible cognitive and behavioral systems to survive. (c) Concerning the effects of natural selection on ontogeny, EDP holds that (1) many aspects of childhood serve to prepare the way for adulthood and were selected over the course of evolution – we refer to these as deferred adaptations; many sex differences in social and cognitive abilities are good examples of deferred adaptations1; (2) there have been different selection pressures on organisms at different times in ontogeny, and some characteristics of infants and children were selected in evolution to serve an adaptive function at that time in development and not to prepare them for later adulthood – we refer to these as ontogenetic adaptations [Oppenheim, 1981]; (3) there are aspects of childhood that did not evolve to solve any recurring problem and have not been shaped by natural selection but are a consequence of being associated with deferred or ontogenetic adaptations (ontogenetic by-products) or are simply random effects attributed to mutations, changes in the environment or aberrations of development (ontogenetic noise). This could be the case, for example, of the belly button (by-product) and the shape of one’s belly button (noise), respectively [Buss et al., 1998]. (d) Many, but not all, evolved psychological mechanisms are proposed to be domain specific in nature [Fodor, 1983]. Consistent with contemporary thinking in much of cognitive science [Pinker, 1997], evolutionary psychologists [Buss, 1995; Tooby and Cosmides, 1992] have emphasized that humans (and other animals) evolved specific cognitive abilities to deal with recurring problems faced by our ancestors (e.g., obtaining food, acquiring a mate). Language abilities are often used as examples of domain-specific mechanisms. From this perspective, humans did not evolve general, all-purpose problemsolving faculties that can be applied across domains to any and all circumstances. We concur with the significance of evolved domain-specific abilities, but an EDP approach argues that domain-general mechanisms, such as speed of processing and working memory, also underwent selective pressure over the course of human evo-

1 By using the term ‘deferred adaptation’, we do not mean to suggest any teleological implications, as if adaptations in childhood anitcipate adult needs. Rather, such adaptations likely function throughout life, adapting juveniles to their niche in childhood but also to the lives they will likely lead as adults. This is most apt to occur, we argue, when environmental conditions/pressures of the juvenile and adult period are similar, or even continuous (as social group composition would likely be in small groups of hunters and gatherers).

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lution [Geary and Huffman, 2002]. This is consistent with what we believe is the majority view among researchers in cognitive psychology, cognitive development and intelligence, that human cognition is multifaceted, consisting of both relatively domain-general and domain-specific abilities [Bjorklund, 2000; Fodor, 2000; Sternberg, 1997]. (e) Behaviors, cognitions and physical features that arise and change from infancy to old age emerge from an interaction of evolved mechanisms and the environment. As such, developmental patterns are not conceived as genetically predetermined (regardless of whether they are universal or not) but as a result of an evolved epigenetic process that adapts human competencies to local conditions, as described, for example, by the developmental systems approach [Gottlieb, 2000; Oyama, 2000]. (f) Because the living conditions in which our species evolved (the environment of evolutionary adaptedness) are far different from the information age environments in which humans now live, many of our species’ evolved behavioral and cognitive adaptations are not well suited to modern life and may actually be maladaptive. As a whole, we see EP, and particularly EDP, as an overarching theory or perspective that can and should be applied to all realms of psychology and its ontogeny. From our view, all accounts of human behavior should include the question ‘Does this make sense from an evolutionary perspective?’ Developmental science (as other disciplines concerned with historical processes) has traditionally assumed that knowing the past helps in understanding the present and in predicting the future. This is no less true, we argue, for phylogeny. Knowing the evolutionary history of a species can help explain present and future behavior, including the amelioration of some ‘problem’ behaviors (e.g., child abuse, male-on-male violence, rape, reading/math disabilities) [Bjorklund and Bering, 2002; Bjorklund and Pellegrini, 2000; Geary and Bjorklund, 2000]. We argue that EDP can be a useful tool to achieve such purposes. However, at the same time, we propose that an EDP approach should not be seen as replacing other theoretical perspectives that attempt to account for the proximal causes of behavior. Rather, evolutionary developmental accounts should be seen as adjuncts to other theoretical accounts (and vice versa) [Bjorklund, 1997]. EDP uses the extant knowledge of how development and evolution proceed and integrates it into a single, hopefully coherent, perspective. This perspective is based on a number of other well-developed theories, both in ontogeny and in phylogeny, is subject to falsification and to modification as new information/ideas are introduced and can generate/test specific lower-level theories and hypotheses. In some cases, we will surely oversimplify both development and evolution, and perhaps in other cases make the processes more complicated than they actually are. But we strongly believe that the application of evolutionary ideas to the study of human development, with a critical evaluation of such research and theory, will advance our science immeasurably and may serve as a common ground for developmental researchers studying disparate topics.

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What Is New in EDP in Comparison with Evolutionary Approaches to Psychology?

EP is an outgrowth of earlier movements in sociobiology, ethology and behavioral ecology. Perhaps the earliest influential work that can be said to be from an explicitly evolutionary psychological perspective was anthropologist Donald Symons’s 1979 book The Evolution of Human Sexuality. Other research articles and books through the 1980s that helped define the field include those of Cosmides and Tooby [1987], Daly and Wilson [1988], Buss [1989] and, especially, because of its contribution to the acceptance of EP as a serious discipline, the volume The Adapted Mind edited by Barkow et al. [1992]. EDP has less of a history and likely does not yet have the recognition as a separate field of inquiry. However, as we mentioned previously, evolutionary thinking is not new to developmental psychology but can be found in the writings of many of its pioneers (e.g., James Mark Baldwin, G. Stanley Hall) and of the theoretical giants that followed them, including Vygotsky, Piaget and Werner [Morss, 1990]. For example, evolutionary theory played a central role in Werner’s theory of comparative mental development [Werner, 1948; Langer, 1970], in which he adopted a form of recapitulation theory (i.e., ontogeny recapitulates phylogeny) that he applied to both ontogenetic and phylogenetic aspects of mental development. Although the evolutionary thinking of many of these theorists continues to influence research today [Langer, 1998], evolutionary theory took a back seat, at best, in developmental psychology throughout most of the latter part of the 20th century, although there were some additional exceptions. Harold Fishbein published the first textbook from an evolutionary developmental perspective in 1976, titled Evolution, Development and Children’s Learning, although it was ahead of its time and was not influential. The earliest collection of papers organized around an explicitly evolutionary developmental psychological perspective was Kevin MacDonald’s [1988] edited volume Sociobiological Perspectives on Human Development. And the first paper to achieve a wide audience in this perspective was likely ‘Childhood experience, interpersonal development, and reproductive strategy: An evolutionary theory of socialization’ by Belsky et al. published in Child Development in 1991. Finally, a number of papers and books published in the late 1990s and 2000s seemed to more clearly and broadly identify the field [Bjorklund, 1997; Bjorklund and Pellegrini, 2000, 2002; Geary, 1995, 1998, 1999; Geary and Bjorklund, 2000; Hrdy, 1999; Keller, 2000; MacDonald, 1997; Surbey, 1998]. However, the key question to our minds is: What is new in EDP in comparison with earlier attempts to link evolutionary ideas and behavior with contemporary perspectives of EP? Certainly some disciplines emanating from biology, such as ethology, sociobiology and primatology, have applied an evolutionary analysis to behavior during the last century (see, e.g., Hernández Blasi [1998] for a brief review of the former two), as have others, such as comparative psychology and anthropology. Yet, it is our opinion that those disciplines are different from an EDP approach with respect to some core issues. Most ethological and sociobiological approaches, although explicitly adopting evolutionary theory, place an emphasis on ‘instinctive’ behavior, giving little credence to the role of the environment in shaping the expression of inherited dispositions. (Of course, notable exceptions can be found, such as in the work of ethologist 264



Robert Hinde [1974] and in the field of cognitive ethology, see, e.g., Bekoff et al. [2001].) Sociobiology also overemphasized reproductive fitness when applying concepts to humans. Comparative and animal psychology, although often based in evolutionary theory, do not usually have their primary concern with human functioning and development. Obviously, comparative research, particularly involving our primate cousins, can provide insights into what the common ancestor of modern humans and extant great apes may have been like and, as such, may provide clues to the roots of some aspects of human thought, motivations, emotions and social functioning. In fact, we believe that contemporary comparative research has greatly enriched our understanding of the human condition [Parker and McKinney, 1999; Suddenhof and Whiten, 2001]. However, this is only one source of information that is important to an EDP perspective; EDP theories must also be supported by and tested with data from humans of various ages. The two issues that most distinguish EDP from the more general perspective of mainstream EP are the significance of the environment in influencing behavior and the role of evolution on the development of the human mind/brain. Regarding the role of the environment, we note that, although evolutionary psychologists argue for the significance of the environment in influencing behavior and for the fact that evolutionary explication does not imply genetic determinism [Buss, 1995; Tooby and Cosmides, 1992], they rarely present well-developed models to support their claims. In contrast, EDP provides such models, assuming an explicitly geneenvironment perspective consistent with the developmental systems approach articulated by Gottlieb and his associates [Gottlieb, 1997, 1998, 2000; Gottlieb et al., 1998; Lickliter, 1996; Oyama, 2000]. This approach emphasizes the concept of epigenesis, the emergence of new structures and functions during the course of development, through a bidirectional relation between all levels of biological and experiential factors, beginning with the level of DNA and progressing through the influence of one’s culture. From such an EDP perspective, functioning at one level of organization influences functioning at adjacent levels, and therefore there should be substantial plasticity in development. Regarding evolution, evolved mechanisms can be thought of as genetically coded ‘messages’ that, following epigenetic rules, interact with the environment to produce behavior. This approach also assumes that humans (or any animal) inherit not only a species-typical genome but also a species-typical environment. Such environments begin before birth, with mammals, for example, ‘inheriting’ a womb and later a lactating mother, and ducks ‘inheriting’ an egg and brood mates. This is expressed in Lickliter’s [1996] statement that ‘… the organism-environment relationship is one that is structured on both sides. That is, it is a relation between a structured organism and a structured environment. The organism inherits not only its genetic complement, but also the structured organization of the environment into which it is born’ (pp. 90–91). To the extent that an organism grows up under conditions similar to that in which its species evolved, development should follow a species-typical pattern. But if this species-typical environment is modified, then species-typical patterns – often (and erroneously) attributed only to a species-typical genome – can become substantially modified [Gottlieb, 2002]. Concerning development, EP has focused primarily on the first two of the causes of behavior as described by the ethologist Tinbergen [1963] (see also Parker [2001]): What is the immediate benefit to the organism? (i.e., what function does Evolutionary Developmental Psychology


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the behavior have?), and what are the immediate causes? (i.e., what are the environmental causes and the psychological mechanisms responsible for the pattern?). EP essentially ignores the other two causes to which Tinbergen pointed: How does this behavior develop within the species, and how did it evolve across species? In general, despite its focus on the evolved adaptive value of behavior and thought, EP has generally paid little attention to the actual phylogeny of behavior and even less to its ontogeny. Evolutionary developmental psychology takes the last two of Tinbergen’s questions seriously, placing them in an explicitly evolutionary perspective and interpreting them in terms of psychological mechanisms. Thus, EDP updates in part Tinbergen’s ‘four questions’ and applies them in the context of modern evolutionary psychological and developmental theory. To sum up, conceptually speaking, EDP can be considered a new evolutionary approach to behavior because: (1) it focuses primarily on human behavior (especially when compared with primatology, comparative psychology and ethology); (2) it focuses mostly on developmental issues (especially when compared with human ethology and EP), and (3) it is less reductionist and determinist as an approach (especially when compared with sociobiology, EP and some more ‘instinctual’ ethological perspectives, such as that of Konrad Lorenz). Furthermore, EDP also assumes, in comparison with other evolutionary approaches, the need to propose concrete functional interpretations about current developmental data, particularly asking how such behavior might prepare the child (or might have prepared the child in the environment of evolutionary adaptedness) for life as an adult – what we have called here deferred adaptations – and what the potential adaptive function might be (or might have been in the environment of evolutionary adaptedness) for the child at this time in development – ontogenetic adaptations. An EDP approach also argues for the testing of those interpretations through a methodological convergence using different sources of validation (many of them provided by earlier evolutionary perspectives, e.g., naturalistic observations or cross-cultural assessments), assessing the phylogenetic past (e.g., through paleontological and/or comparative data) and testing predictions about future behavior/developmental outcomes, according to what could be called ‘evolutionary experiments’ with human children, nonhuman primates and other animals. How to Apply an EDP Perspective to Further the Understanding of Human Development

One of the most frequent questions that arises when presenting a new theoretical approach within every field of psychology is: ‘How can this perspective help me to understand better my current research field, and how will it affect the way I conduct research?’ We do not have a recipe to recommend, but we do have some suggestions and examples for a research agenda that we elaborate in the remainder of this article. All of our suggestions are linked to the two defining aims of an EDP perspective: (1) proposing concrete functional interpretations about current developmental data and (2) testing these interpretations empirically.

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Classifying Developmental Features according to Their Evolutionary or Functional Status Above we distinguished three classes of evolutionary products: deferred adaptations, ontogenetic adaptations and ontogenetic by-products [Buss et al., 1998]. (Other products of evolution have also been proposed, specifically exaptations: ‘features that now enhance fitness, but were not built by natural selection for their current role’ [Gould, 1991, p. 47]. For our purposes here, we classify exaptations as forms of adaptations.) One important challenge for EDP is to elaborate a catalogue of different types of behaviors and cognitions characteristic of humans from birth to adulthood in different cultures and to classify them according to their adaptive status. So, for example, it has been suggested that some gender differences in styles of play may be a type of deferred adaptation [Pellegrini and Bjorklund, in press; Pellegrini and Smith, 1998], whereas infant reflexes and early imitation can be considered as types of ontogenetic adaptations [Bjorklund, 1987]. Consider the first case – gender differences in play styles as an example of a deferred adaptation. Some have suggested that males from a variety of mammalian species, including humans, gain valuable experience that will be useful to them in later life through rough-and-tumble play [Symons, 1978; Biben, 1998; Pellegrini and Smith, 1998]. In fact, such vigorous play is more frequent in males than in females, and this sort of play can be useful later in life in competitive interactions of all sorts, but particularly in their encounters with other males relating to dominance and eventual mating choices. In contrast, girls engage in more play parenting (i.e., doll play) than boys, a sex difference that is even found in some nonhuman primates [Pryce, 1995]. In fact, females in nearly every mammal species, including humans, devote considerably more time in childcare than do males, and the greater interest in play parenting by girls has been interpreted by some as reflecting preparation for their likely adult role [Biben, 1998; Geary, 1998]. Consider the second case – early reflexes and imitation as an example of an ontogenetic adaption. Infant reflexes, such as the sucking reflex, have often been considered good examples of ontogenetic adaptations because they serve an important and specific function after birth but later disappear [Oppenheim, 1981]. Something similar may happen with imitation of facial gestures by newborns [Meltzoff and Moore, 1977], an infant behavior that decreases to chance levels by about 2 months of life [Abravanel and Sigafoos, 1984; Jacobson, 1979]. In this case, Bjorklund [1987] hypothesized that the primary function of neonatal imitation is facilitating mother-infant social interaction at a time when infants cannot easily direct their gaze and control their head movements in response to social stimulation. In this way, he suggested that early imitation appears to have a specific function (i.e., maintaining infant-mother interaction) that is presumably different from the function that it serves in the older infant and child (i.e., acquiring behavior via observation). Support for this position comes from a study in which aspects of motherinfant social interaction at 3 months were related to the degree of neonatal imitation so that infants who displayed higher levels of facial imitation shortly after birth showed greater levels of social interaction with their mothers 3 months later [Heimann, 1989].



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Proposing Hypotheses and Microtheories to Explore the Function of Developmental Traits The ‘theory of evolution’ is a grand theory, indeed, and will not be confirmed or disconfirmed by any single set of experiments. Yet, there are mid-level theories within an evolutionary perspective, such as Trivers’ [1972] parental investment theory, which can be used to generate testable hypotheses within an EDP perspective. The principal idea behind the parental investment theory is that males and females invest differently in the time and resources they devote to offspring. In the vast majority of mammals, humans included, the females’ investment in offspring is substantially greater than that of males. With internal fertilization and postnatal lactation, postcopulatory investment in females is obligatory, whereas in males it is not. Depending on the life history of the species and ecological conditions, male investment in his offspring may be required to insure his genetic continuity, but it is invariably less than that of the female. As a result, males and females have evolved different psychologies relating to the attaining and maintaining of mates and to the care of offspring. For a slowly developing species such as humans, male investment in his offspring enhances the likelihood that his progeny will live to reproduce so that human males devote more time and resources to their offspring (and their offspring’s mother) than the males of most other mammals. Females, in turn, select males as mates not only for their obvious genetic characteristics (e.g., is he healthy?, a trait desirable for her offspring), but also for the likelihood he will devote resources to her and her children. Parental investment theory has been used frequently to explain and generate hypotheses about human sex differences, primarily in adulthood, but also in childhood [Bjorklund and Shackelford, 1999; Buss and Schmidt, 1993; Geary, 1998; Hrdy, 1999]. For example, because of the greater potential investment of copulation for females than for males (9 months of pregnancy and several years when the female is the principal source of food through nursing), women are more selective in assenting to sex than are males [Oliver and Hyde, 1993]. It may have been to the ancient (and perhaps contemporary) females’ advantage, then, to inhibit their expression of interest in males, providing greater time for the female to ‘check out’ a potential mate. Also, because females are the primary caregiver to often aversive infants, it may have been to their advantage to have enhanced inhibitory abilities (e.g., withholding aggressive responses) when it comes to dealing with infants. Following this reasoning, Bjorklund and Kipp [2001] noted that human females of all ages show greater inhibitory abilities than males in the social domain (e.g., delay of gratification, ability to control emotional expression), although no sex difference was found in cognitive inhibition, a finding consistent with their hypothesis generated from the parental investment theory. (We will provide another example of research programs based on the parental investment theory later in the article.) In contrast to a top-down approach of using evolutionary theory to explain or discover phenomena, researchers may take a bottom-up approach and begin with interesting or important phenomena and search for an evolutionary explanation of them. One must take such an approach cautiously, however, or the end result could be a ‘just so story’, with little justification for one’s particular evolutionary expla268



nation over other explanations, evolutionary or otherwise. What is important in such cases is that hypotheses be generated that are testable. One instance of such a bottom-up approach turned into a testable research agenda from an EDP perspective is the case of infants’ cries. Such cries have caught the attention of parents and researchers alike for many years (millions in the case of parents). An evolutionary hypothesis, consistent with the theorizing of Bowlby [1969], is that infants who had cries that resulted in attaining and maintaining their parents’ attention were more likely to survive than infants with less demanding cries. Although this is an attractive and intuitive hypothesis, it requires a more specific (and testable) theory concerning how distress cries may have initially elicited parental care and came eventually to be selected in infants. For example, researchers have proposed that mothers use the pitch and frequency of infant cries as an indicator of infant health [Furlow, 1996; Mann, 1992]. Infants with hearty cries are advertising that they have substantial energy and can afford to expend some of that energy on vigorous crying. Consistent with this is evidence from the Masai, a pastoralist society from Kenya, that, of the 6 infants (of a total of 13) who died during a period of famine, 5 had been classified as ‘easy’, with few crying bouts [DeVries, 1984]. Examining hypotheses of the origins of infants’ crying in the laboratory, Thompson and his colleagues have proposed that infants’ cries mimic respiratory emergency and are thus a form of deception infants use to gain attention and nurturing from parents [Thompson et al., 1998, 1996; Soltys et al., 2000]. They provided evidence of this, observing (as predicted) that deviations in infants’ pitch-to-cryrate ratio deviated more from normal the longer infants were separated over a 3min period [Soltys et al., 2000]. However, counter to their predictions, adult judges rated the infants as becoming less, rather than more, distressed over the course of the 3-min session, complicating the interpretation of the findings. Although research on the functions and evolution of infants’ cries is far from definitive, this research reveals that evolution-based hypotheses can provide light to their purpose and origins, and they are testable. Collecting Data from Different Sources to Test Developmental Evolutionary Hypotheses Most evolutionary explanations cannot be tested directly. One cannot repeat the ancient past or conduct longitudinal studies of sufficient length to detect species level, evolutionary changes (at least not in humans). But this does not make evolutionary hypotheses untestable. What is required is convergent data from a variety of sources. Evidence for the functional value of pregnancy sickness provides one such example. Sickness is common during the early months of pregnancy, occurring in the majority of women around the world [Tierson et al., 1985]. Symptoms include nausea, vomiting and food aversions, making it understandable why it is considered an illness. However, Profet [1992] hypothesized that pregnancy sickness is an adaptation, with women who have an aversion to foods containing toxins being more likely to have healthy babies. Several lines of evidence converge to make the hypothesis a sound one. Evolutionary Developmental Psychology


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First, pregnancy sickness corresponds to the time when an unborn child is most susceptible to the effects of teratogens. It typically begins between 2 and 4 weeks after conception, when organ development is most rapid, and usually wanes by 12 weeks, when the fetus is less susceptible to the adverse effects of outside agents. Second, women acquire aversions to food that are high in toxins, such as coffee, meat, poultry and alcohol, but rarely to toxin-free foods, such as bread and cereals [Dickens and Trethowan, 1971; Tierson et al., 1985]. Third, pregnancy sickness appears to be universal, being found in both industrial and nonindustrial countries and in women in hunter-gatherer cultures in Africa and South America. In the few cultures where it is rare, diets generally lack animal products and are based mainly on maize [Flaxman and Sherman, 2000; Profet, 1992]. Fourth, research has demonstrated fewer spontaneous abortions for women who experience pregnancy sickness than for those who do not [Flaxman and Sherman, 2000; Profet, 1992; Weigel and Weigel, 1989]. This relationship has been confirmed in a recent metaanalytic study, assessing the co-occurrence of pregnancy sickness and miscarriage in over 20,000 pregnancies [Flaxman and Sherman, 2000]. And, fifth, research on pica, the eating of nonnutritive substances such as clay (a common practice for pregnant women in some parts of the world and in parts of the USA), has shown that when clay is eaten along with certain toxic foods, it prevents the toxins from being absorbed into the blood system and thus reduces pregnancy sickness and the effects of teratogens on the fetus [Profet, 1992]. This convergence of data from different sources illustrates that what has typically been viewed as a dysfunctional state, for which medication is sometimes prescribed, is actually a well-adapted mechanism that serves to foster the development of the unborn. The convergence of evidence does not, of course, prove that pregnancy sickness is an evolutionary adaptation, but the variety of data consistent with this interpretation makes this developmental evolutionary hypothesis more tenable. Describing the Phylogenetic and Sociocultural History of Human Developmental Features Developmental psychologists, as well as historians, believe that knowing the past helps in understanding the present and predicting the future. This is no less true, we argue, for phylogeny. Knowing the evolutionary (and sociocultural) history of a species can help explain present and future behavior, and may, in some cases, provide insight into problem behavior (e.g., child abuse, male-on-male violence, rape, reading/math disabilities). On the one hand, comparative studies of other closely related species (especially those focused on ontogeny of psychological functions, particularly in nonhuman primates) can be especially informative. On the other hand, paleontology and allied disciplines (e.g., evolutionary anthropology, history) can provide invaluable information about how our ancestors lived, permitting the generation and testing of evolutionary psychological hypotheses. With respect to comparative psychology, Parker [2000, 2001] and Parker and McKinney [1999] have recently articulated a framework that we find particularly interesting. They call that framework comparative developmental evolutionary psychology, which they define as ‘the use of psychological stage models to compare cognitive development in monkeys, apes and humans’ [Parker, 2000, p. 2]. In her 270



Table 1. Developmental sensorimotor substages in object permanence in monkeys, great

apes and humans Species

Humans Chimpanzees Gorillas Orangutans Macaques Capuchins Squirrel monkeys

Age, months stage 2

stage 3

stage 4

stage 5

stage 6

2 3 1.5 ? 0.75 1 ?

4–8.5 6 3.5 ? 1–1.5 2–3 ?

7–8 9 7.5 ? 1–1.5 3–12 ?

7–10 12 7–9 ? 3.5 8–12 ?

11–21 18 9–10 ? none or partial none none

Adapted from Parker and McKinney [1999], p. 42.

work, Parker [1996] examines the possible role that ontogeny may have had in human evolution. In their work, Parker and McKinney [1999] presented a synthesis of studies concerning the ontogeny and phylogeny of ‘Piagetian intelligence’ in primates (including the work of Gibson [1990], Jolly [1966], Langer [1998, 2000], Parker [1977, 1990]). In their analysis, they make use of a theory with wellestablished developmental milestones in human infants (Piaget’s sensorimotor theory). To illustrate, table 1 presents the average age of attainment (in months) in humans, great apes and monkeys of substages 2–6 of object permanence, following Piaget’s formulation (based on data presented in Parker and McKinney [1999]). Specifically, Parker and McKinney propose that, in primate evolution, cognitive abilities have been added to the end state of a species’ mental capacities (a form of recapitulation theory). Species’ attainment of Piagetian-measured intelligence is linearly related to their phylogenetic relationship to H. sapiens. Also, whereas the suite of sensorimotor cognitive abilities tends to be integrated and develop synchronously in humans [Langer, 2000], the development of these various abilities is less integrated in nonhuman primates, suggesting one major difference in the cognitive development of H. sapiens and his close genetic cousins. Although Piaget’s account of cognitive development does not enjoy the nearuniversal approval it once did [Bjorklund, 2000; Brainerd, 1996; Siegler, 1998], the replicable stage-like accomplishments of children provide a framework for assessing cognitive development in other species, making the approach an important one. Other comparative and developmental psychologists have investigated other aspects of monkeys’, apes’ and children’s cognitive development, such as theory of mind [Call and Tomasello, 1999; Povinelli and Eddy, 1996], self-recognition [Gallup, 1970; Povinelli et al., 1993], understanding of quantitative relations [Boysen, 1993; Sulkowski and Hauser, 2001], social learning [Bering et al., 2000; Whiten, 1998] and representation [Hare et al., 2001; Whiten, 1996] among other topics [Byrne, 1995; Suddendorf and Whiten, 2001; Tomasello and Call, 1997, for reviews]. Such comparative data, when interpreted from an explicitly evolutionary perspective, can provide important insights, we argue, for understanding human cognitive development and its emergence over the course of evolution. Evolutionary Developmental Psychology


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Concerning the second approach, that of paleontology, archeology and their allied disciplines, we can find some primary examples of works by these historians of our ancient origins relevant to an EDP perspective. For example, Wynn [1989] evaluated the stone tool technology of Homo habilis and Homo erectus in terms of Piaget’s stages of cognitive development. He concluded that the early technology (dating to about 2 million years ago) reflected spatial cognitive abilities corresponding to the early stages of Piaget’s preoperational period, whereas later attainments (about 300,000 years ago) reflected spatial cognition comparable to the level of concrete operations. More recently, Mithen [1996], based on evidence from artifacts (e.g., stone tools) and fossils (particularly those suggesting differences in rates of development of earlier hominid species relative to H. sapiens), has proposed that hominids evolved powerful, domain-specific modules to deal with their natural and social worlds, but that it was not until the emergence of modern humans about 100,000 years ago that H. sapiens was able to integrate these modules to produce a general-purpose intelligence. Likewise, based on similar types of evidence, Donald [1991] proposed four ‘stages’ of cognitive evolution (episodic, mimetic, mythic or narrative, and theoretic), expressed in terms of how knowledge could have been represented in memory. More recently, Nelson [1996] has applied aspects of Donald’s model to cognitive development. Paleontological evidence of the life stages of our ancestors is limited by its very nature, but what data are available is consistent with the interpretation that our hominid forebears developed more rapidly than modern humans. For instance, Bogin [1997, 1999] proposed that, whereas other mammals progress through the three life stages of infancy, juvenility and adulthood, modern humans have added the stages of childhood and adolescence (although it could be argued that other mammals, particularly chimpanzees, also experience something akin to childhood and adolescence). In humans, childhood follows infancy and precedes the juvenile period. This is a time when the child is no longer nursing, but cannot yet forage or prepare food for itself. (For instance, children still possess ‘baby teeth’ and lack the technical skills to obtain high-nutrition food on their own.) Human adolescence, with its characteristic growth spurt, follows the juvenile period, beginning in the second decade and continuing until reproductive maturity, which in traditional cultures is in the late teens or early twenties. Using fossil evidence such as dental development and bone size at different developmental periods, Bogin estimated that our australopithecine ancestors (dating back about 3.5 million years) had life stages similar to that of modern chimpanzees (Pan troglodytes), consisting of a period of infancy lasting 5 or 6 years, followed by a juvenile period with adulthood beginning at about 12 years of age. Evidence of a childhood period is seen only with the beginning of the Homo line (Homo habilis, dating back about 2.5 million years ago); and adolescence, with its distinctive growth spurt, is found only in modern Homo sapiens. Reproductive age apparently also steadily increased through the Homo lineage, ranging from about 12–13 years for H. habilis to 14 or 15 years for H. erectus, to the late teens and early twenties for H. sapiens. There is also evidence that development was more rapid in a more recent Homo relative, the Neanderthals (Homo neanderthalensis) than it is (and was) for H. sapiens. This conclusion is based on a discrepancy between rates of dental and cranial development [Akazawa et al., 1995; Dean et al., 1986; Stringer et al., 1990; Zollikofer et al., 1995]. For instance, based on dental development, the fossilized 272



skeleton of a Neanderthal infant was judged to be about 2 years old when it died. Yet, the cranium size was equivalent to that of a modern 6-year-old child [Akazawa et al., 1995]. The use of archeological and paleontological evidence to help situate cognitive development within an evolutionary perspective does not, of course, make these hypotheses any more correct than hypotheses generated without reference to phylogenetic or prehistoric origins. But when combined with theory and evidence from other sources, these approaches can serve to generate testable hypotheses and provide a more complete picture of human development than perspectives that ignore the ancient history of our species. Designing ‘Evolutionary Experiments’ To be successful, a scientific theory needs not only to be able to organize and explain extant phenomena but should be able to predict new ones. In behavioral sciences such as psychology, this means generating hypotheses and testing them empirically. This is the basis of our proposal for developing what we call, for lack of a better term, ‘evolutionary experiments’: ways of testing hypotheses about evolved behaviors on the basis of their (present) occurrence among a set of different empirical possibilities. We think a good example of this can be found in Belsky, Steinberg and Draper’s application of the parental investment theory to different styles of attachment, rate of physical development and subsequent mating strategies [Belsky, 1997; Belsky et al., 1991] and to more recent research following up on their original hypothesis [Chasiotis et al., 1998; Ellis and Garber, 2000]. As we have described above, the parental investment theory supports the thesis that males and females invest differently in the time and resources they devote to their offspring, based on the assumption of the evolutionary advantages that our ancient predecessors obtained from such differential investment. If this assumption is correct, then a series of ‘evolutionary experiments’ might be designed to test it, based on the idea that this selected differential investment should continue to operate within contemporary societies. So, for example, following the parental investment theory, Belsky et al. [1991] predicted that children reared in homes characterized by inadequate resources, high stress, marital discord/father absence, and harsh and inconsistent childcare would display insecure attachment, reach puberty early, form short-term and unstable pair bonds and invest relatively little in their own offspring. In contrast, they predicted that children from home environments characterized by adequate resources and spousal harmony/father presence would display secure attachment, mature later, postpone sexual activity and show greater investment in the fewer number of children they produce. Subsequent research has generally confirmed these predictions and provided greater insight into the proximal causes of the phenomena. For instance, Ellis and his colleagues have shown that pubertal timing in girls is positively related to maternal psychopathology and the age at which an unrelated adult male (stepfather or mother’s boyfriend) joins the household [Ellis and Garber, 2000] and negatively related to positive aspects of father-daughter interaction [Ellis et al., 1999]. Further research will be needed to understand the complex relationships between childrearEvolutionary Developmental Psychology


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ing practices, critical features of the domestic situation, rate of physical maturation, and teenage and adult sexual behaviors and mating strategies, but such studies can be done and would not even have been contemplated, we believe, in the absence of an evolutionary developmental theory. Another topic that has received considerable attention in nonhuman primates but much less in humans concerns the issue of prepared learning, specifically prepared fears. Animals living in the natural world may be particularly sensitive to cues that signal danger, including potential predators. Monkeys living in the wild, for example, show a fear of snakes. It would be reasonable to ask whether this type of fear was selected over evolutionary time, given its obvious advantage for survival. If this were the hypothesis, we might design ‘evolutionary experiments’ to test if contemporary monkeys show some differential fear behaviors when faced with different experimentally presented stimuli. In a series of such experiments, Cook, Mineka and their colleagues examined rhesus monkeys’ acquisition of fear of snakes versus biologically irrelevant stimuli [Cook and Mineka, 1989, 1990; Cook et al., 1985; Mineka, 1992; Mineka et al., 1984]. First, they demonstrated that laboratory-raised monkeys show no ‘natural’ fear of snakes. They then showed the monkeys videos of other monkeys displaying fear reactions to snakes versus fear reactions to ‘neutral’ stimuli, such as flowers and rabbits. When later tested, these monkeys displayed fear of snakes but not of the neutral stimuli, suggesting that monkeys are prepared to develop fear of snakes, an adaptive response in their natural environment where snakes constitute a major danger. Monkeys are not the only primate to fear snakes, of course. The fear of snakes is common in most human societies and is a frequently observed phobia among adults in industrialized countries [Nesse, 1990]. The logic used to assess the possibility of prepared fears in monkeys could be similarly applied to humans. To our knowledge, however, there have been no published studies with human infants or children similar to the ones conducted by Cook, Mineka and their colleagues with monkeys. However, there is preliminary evidence of this in a study by Rakison [2001], following the logic we have described above, who found that human infants may possess adaptive mechanisms for recognizing certain potentially dangerous animals. In Rakison’s study, 10-month-old infants were habituated to pictures from different categories: predators (snakes and spiders) and nonpredators (frogs and rabbits). Based on subsequent attention to pictures of novel and familiar predators and nonpredators, Rakison concluded that infants treated the perceptually dissimilar predators similarly and differently from the nonpredators. Specifically, on the test trials the 10-month-olds looked longer at the nondangerous animals than at the two dangerous ones, and looked at the spider and snake equally. In other words, they had formed a category for ‘dangerous animals’ despite the fact that the two exemplars they were exposed to (snakes and spiders) looked nothing alike. This suggests that the infants were ‘prepared’ to classify these stimuli as conceptually similar, an argument in support of the prepared-learning hypothesis. These data are not as convincing as the findings of Cook and Mineka with monkeys in illustrating prepared fear, but they reflect the beginning of a potentially exciting research program based explicitly on hypotheses generated from EP.

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Five Proposals, One Purpose We have discussed five ways of applying EDP to contemporary issues of psychological development: (1) classifying developmental features according to their evolutionary or functional status; (2) proposing hypotheses and microtheories to explore the function of developmental traits; (3) collecting data from different sources to test developmental evolutionary hypotheses; (4) describing the phylogenetic and sociocultural history of human developmental features, and (5) designing ‘evolutionary experiments’. These five ways of applying an EDP perspective are not, of course, independent from one another. For example, in order to classify the function of a developmental characteristic (proposal 1), such as sex differences in certain styles of play, one must first propose a theory or hypothesis about the phenomenon (proposal 2). This would likely be followed by collecting data from different sources (e.g., human children in different environments and cultures; nonhuman mammals; proposal 3), describing the phylogenetic and sociocultural history of those features (proposal 4) and/or even designing some ‘evolutionary experiments’ (proposal 5). Moreover, regardless of one’s primary goals (e.g., describing developmental function, testing evolutionary hypotheses, describing phylogenetic/ sociocultural history), they can be best achieved by incorporating all five of the proposals listed above, with the ultimate goal of developing a better understanding of human development. Importantly, we would like to emphasize that the task of collecting data from an evolutionary developmental perspective should be viewed as integral to building a coherent, ‘big picture’ theory of development. We suspect that, at least partially, some earlier evolutionary approaches to psychology (e.g., sociobiology) failed to gain mainstream support because of the absence of a truly theoretical effort that allowed them to integrate their new findings into a comprehensive framework, compatible with more proximate theories of development. The absence of such a broader theoretical account likely led, to some extent, to the dispersion of findings among the different ‘local’ issues (e.g., social vs. cognitive), to different levels of explanation (e.g., microtheories vs. ‘facts’) and to different perspectives (e.g., ethology vs. sociobiology), making it difficult to generate a broader understanding of the significance of the data as well as the possibility of establishing a common agenda for future research and theory. Therefore if, as we propose, an EDP research agenda can serve as a useful tool to improve the understanding of human ontogeny, we strongly support the idea that theoretical efforts must invariably accompany research efforts. In fact, to some extent, we believe that the primary goal of all EDP scientists should be to contribute with their works to the building of a general theory on human functional development. Concluding Comments

Throughout this article we have tried to examine what EDP is, what, from our perspective, its distinctive features are in comparison to other evolutionary approaches to psychology (some ‘old’ approaches, such as classical ethology, and some ‘new’ approaches, such as EP) and, finally, how an EDP approach can be incorporated into current research to improve our knowledge about developmental Evolutionary Developmental Psychology


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issues. Just as evolutionary theory is the foundation for modern biology, we believe that it needs to be the foundation for modern psychology [Daly and Wilson, 1988; Tooby and Cosmides, 1992; Wilson, 1998] and, in particular, for developmental psychology. However, whereas we see EDP as serving as an overarching perspective for developmental psychology [Bjorklund, 1997], we do not conceive of it as an exclusive theory, replacing alternative accounts of development. Rather, we see EDP as a useful tool that can help to improve our understanding of certain aspects of development (especially, of course, the functional ones), but can (and must) coexist with and supplement other more proximal accounts of ontogeny. We acknowledge that evolutionary accounts of human ontogeny may require substantial revisions in the future before they are accepted by a majority of developmentalists and before they more closely reflect the reality of nature. But given the ‘fact’ of evolution (humans are, indeed, the product of a continuous line of evolutionary change), we believe theories of evolution should be incorporated into psychological explication, particularly those concerned with development. Finally, we want to address one frequent criticism of evolutionary approaches, that an evolutionary explanation of a behavior pattern implies the pattern is ‘natural’, ‘normal’ or in some other way justifiable. This is the so-called naturalistic fallacy, and it is precisely that, a fallacy. It is one thing to understand the past and the evolutionary influences on contemporary behavior, but it is quite another thing to justify that behavior or to propose its inevitability or social desirability. So, for example, understanding the evolutionarily based factors that contribute to patterns of child abuse, male-on-male aggression or spousal abuse should not imply that these behaviors are acceptable in contemporary society. Natural selection shaped human behavior for success in a very different environment from the one in which most members of our species now live, making many products of natural selection actually maladaptive for modern life. Furthermore, perhaps the single defining feature of H. sapiens is our flexible intelligence, meaning that we are able to adapt to a wide range of environments and are not destined to repeat the actions of our phylogenetic or sociocultural ancestors. But knowing that past, the social and information-processing biases we have inherited, and their development, can help us not only better understand human behavior but also, perhaps, provide insights for dealing with our baser tendencies. The self-serving assumptions made by social Darwinists and some evolutionary theorists in the 19th and 20th centuries [Shipman, 1994] have understandably made many socially conscious behavioral scientists reluctant to adopt an evolutionary perspective of development. And such an attitude is not due solely to the fact that some proposals were not ‘politically correct’ but to the fact that many of the assumptions underlying them were scientifically unfounded or based on a misreading of the basic biological literature [Lewontin et al., 1984; Lickliter and Berry, 1990; Morss, 1990]. Given the potential implications of an evolution-based theory of psychological development, we must be cautious not to repeat the mistakes of our scientific predecessors. But we emphasize that ‘to understand is not to justify’, and we strongly believe that conceptualizing human development from the perspective of an evolutionary approach has great potential for furthering our knowledge about development, informing us about our evolutionary past and potentially helping to ameliorate some ‘problem’ behaviors.

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