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									                      Child Development, January/February 2000, Volume 71, Number 1, Pages 57Ð65

                            Evolutionary Developmental Psychology
                                     David C. Geary and David F. Bjorklund

     Evolutionary developmental psychology is the study of the genetic and ecological mechanisms that govern the
     development of social and cognitive competencies common to all human beings and the epigenetic (geneÐ
     environment interactions) processes that adapt these competencies to local conditions. The basic assumptions
     and domains of this emerging Þeld, as related to human life history and social and cognitive development, are
     outlined, as are implications for issues of importance in contemporary society.

INTRODUCTION                                                 tal psychology, that is, to outline basic assumptions,
                                                             deÞne domains of study, and discuss future directions.
Evolutionary developmental biology is the study of
                                                                Among the fundamental issues that must be ad-
how genetic systems are expressed during develop-
                                                             dressed by this emerging Þeld are the function of de-
ment, how the ecology of the developing organism
                                                             velopment and evolutionary change in the develop-
inßuences the expression of one genetic system or an-
                                                             mental period, issues discussed in the Þrst section
other, and how modiÞcations in timing of develop-
                                                             below. The second and third sections focus on the
mental processes inßuence evolution (Gilbert, Opitz,
& Raff, 1996; Waddington, 1942). Of particular impor-        two core areas of developmental psychology, social
tance is epigenetics, Òthe sum of the genetic and non-       and cognitive development, respectively, and provide
genetic factors acting on cells to selectively control the   an evolutionary framework for conceptualizing empir-
gene expression that produces increasing phenotypic          ical and theoretical work in these areas. The last section
complexity during development. The genotype is the           addresses the relationship between evolved pheno-
starting point and the phenotype is the endpoint of          types and their expression in contemporary society.
epigenetic controlÓ (Hall, 1992, p. 215). Genes provide
the instructions for guiding the development of the
core phenotypes, such as body structure and social           EVOLUTION AND DEVELOPMENT
behaviors, of the species. Sensitivity to internal (e.g.,
                                                                Function of development. A long developmental
hormones) and external (e.g., population density) con-
                                                             period has a clear risk, death before the age of repro-
ditions ensures that the expression of the genotype is
                                                             duction, and thus would only evolve if there were
responsive to social and ecological factors such that the
individualÕs physical, behavioral, and psychological         beneÞts that outweighed this risk. Comparative
phenotypes are adapted, during the course of develop-        studies suggest that one function and an important
ment, to these conditions (e.g., population density).        adaptive beneÞt of delayed maturation is the accom-
   The development of most human phenotypes and              panying ability to reÞne the physical, social, and
associated individual variability among these pheno-         cognitive competencies that support survival and re-
types will be inßuenced by a mixture of genetic and          production in adulthood (Mayr, 1974). For instance,
ecological mechanisms, that is, by epigenetic pro-           an extended juvenile period is found in all social
cesses (Hall, 1992; Scarr, 1992). These processes ensure     mammals and the length of this period increases
commonality in the basic phenotypes of all humans            with increases in the complexity of the speciesÕ so-
and reßect the evolutionary history of our species but       cial system (Joffe, 1997). These patterns support the
at the same time allow for the ontogenetic adaptation        view that one function of delayed maturation is to
of these phenotypes to the local ecology. The goals of       allow juveniles to practice and reÞne the sociocogni-
evolutionary developmental psychology, the sister            tive competencies associated with survival and re-
discipline of evolutionary developmental biology, are        production (e.g., competing for mates) in adulthood.
to identify the social, psychological, cognitive, and        An extended developmental period is also related to
neural phenotypes that are common to human beings,           enhanced tool use in some species and greater
and to other species, and to identify the genetic and        knowledge of the local ecology, both of which facili-
ecological mechanisms that shape the development of          tate later foraging (Byrne, 1995; Geary, 1998).
these phenotypes and ensure their adaptation to local
conditions. The goal here is to provide the skeletal         © 2000 by the Society for Research in Child Development, Inc.
framework for the Þeld of evolutionary developmen-           All rights reserved. 0009-3920/2000/7101-0007
58   Child Development

   For humans, play, social interactions, and explora-        Table 1   Stages of Human Life History
tion of the environment and objects appear to be the
mechanisms through which these emerging compe-
                                                              Defining feature: For mammals, this is the time of breast-feeding.
tencies are practiced and reÞned during development
                                                              Human characteristics: In hunter-gatherer societies, infants are
(e.g., Pellegrini & Smith, 1998). In theory, these child-       typically breastfed until the age of 3 years. The age of
initiated activities provide experiences with the so-           weaning in humans is shorter than that found in a close
cial, biological (e.g., prey species), and physical world       relative, the chimpanzee, where infants are weaned between
that interact with underlying genetic systems to pro-           4 and 5 years of age.
duce the physical, social, cognitive, and neural phe-                                        Childhood
notypes associated with the survival and reproduc-            Defining feature: The period between weaning and the ability to eat
tion of our ancestors (Geary, 1998; Scarr & McCarthy,           adult foods.
1983). Child-initiated social play and exploration are        Human characteristics: This period is longer in humans than in
intimately linked to cognitive and neural develop-              other primates and appears to be associated with a relatively
ment in that these activities result in the environmental       short infancy. In hunter-gatherer societies, childhood
                                                                encompasses the ages of 3 to 7 years, 7 years being the age at
experiences that are an integral part of the epigenetic
                                                                which the first adult molars appear and dependence on adults
processes that result in adult phenotypes (Greenough,           decreases. During this time, the responsibility for feeding
1991; Hall, 1992). One resulting prediction is that the re-     children often shifts from the mother to the wider community
lationship between early play and social activities and         (e.g., older siblings), although social and psychological
later competencies will be activity speciÞc. For instance,      dependence on parents continues.
exploration of the wider ecology is associated with an                                      Juvenility
improved ability to mentally represent and navigate in        Defining feature: Period between weaning or childhood and sexual
the physical world (Matthews, 1992), but is not neces-          maturation and social independence. This period is common to
sarily associated with improved abilities in all spatial        social mammals and is characterized by social and other forms
domains (e.g., remembering the location of objects).            of play.
   Evolutionary change. In considering evolutionary           Human characteristics: In hunter-gatherer societies this period
                                                                ranges between 7 years and the mid teens. As with other social
change in the developmental period, it is useful to
                                                                mammals, this is often a time of social and other forms of play,
conceptualize human life history as being comprised             as well as a time during which parental dependency decreases
of developmental stages (Charnov, 1993). These stages           and peer influences increase.
are useful for considering general social and develop-
ment goals and changes in these goals at different
                                                              Defining feature: Encompasses the process of physical, social, and
points in the life span and do not, for instance, appear
                                                                personal maturation. In many primates, the transition between
to map onto any developmental stages for the cogni-             juvenility and adulthood is not marked by an obvious period of
tive modules described later. Bogin (1997) provided             adolescence (e.g., no growth spurt).
one such framework and argued that human develop-             Human characteristics: Clear growth spurt, and prolonged period of
ment can be understood in terms of Þve stages: infancy,         physical maturation. This is generally a time to explore adult
childhood, juvenility, adolescence, and adulthood. The          social and sexual roles and refine reproduction-related social
                                                                strategies (e.g., those associated with competing for mates).
basic features of these stages, as found in humans in
                                                                During this time, the social activities common to juvenility
hunter-gatherer societies and in other mammals, are             become increasingly adult-like (e.g., play fighting gradually
described in Table 1 (Bogin, 1997; Lancaster & Lan-             escalates to real fighting). Early physical maturation and
caster, 1983; Leigh, 1996). The stages are characterized        prolonged dependence on parents (e.g., to complete schooling)
by differing degrees of physical development, social            has increased the length of this period in industrial, compared
                                                                to hunter-gatherer, societies.
dependence, and social goals.
   Based on evidence derived from the fossil record                                         Adulthood
(e.g., adult size), it has been estimated that the age of     Defining features: Period of mature reproductive activities, which
maturation for Australopithecus afarensis, A. africanus,        involve finding a mate or mates (e.g., competing for mates or
and Homo habilis was similar to that found in the mod-          choosing mates) and investment in offspring. For 95% to 97%
                                                                of mammalian species, there are large sex differences in these
ern chimpanzee (Pan troglodytes), that is, nine to eleven
                                                                activities, with males focusing most of their reproductive
years (Bogin, 1997; McHenry, 1994). In other words,             energies on mating and females on parenting.
two to four million years ago, our ancestors were sex-        Human characteristics: As with other mammals, the finding of a
ually mature at around 10 years and probably began              mate and investing in any resulting offspring are the focus of
to reproduce a few years later. The estimated age of            this period in hunter-gatherer, and other, societies. Humans are
maturation of our most recent ancestor, H. erectus, was         atypical among mammals, in that men invest in the well-being
                                                                of children, though less so than women do, and women, like
between 12 and 13 years, whereas that of modern hu-
                                                                men, compete for mates.
mans is in the late teens to early twenties (Tanner, 1990).
                                                                                           Geary and Bjorklund    59

Thus, the length of the developmental period has            ßects changes in the nature of social relationships.
nearly doubled since the emergence of A. afarensis. All     Social relationships in infancy and childhood are
of the stages described in Table 1 are longer in humans     largely survival related, whereas those during juve-
than in other primates and almost certainly longer than     nility appear to involve a preparation for adult repro-
in our hominid ancestors (Bogin, 1997; Leigh, 1996).        duction. The nature of social relationships changes
   The substantial increase in the length of the develop-   again during adolescence and adulthood, when they
mental period suggests that our ancestors were more         are focused more directly on reproductive issues.
successful at keeping their offspring alive than were          Infancy and childhood. There is little question that
other primates; child mortality rates in hunter-gatherer    the primary relationship in infancy and childhood is
societies are about 50% compared with 67% to nearly         between the child and his or her parents (Bowlby,
90% in other primates (Lancaster & Lancaster, 1983).        1969). The associated attachment-related behaviors,
Moreover, the lengthening of the developmental period,      such as stranger anxiety and separation anxiety,
along with a threefold increase in brain volume since       emerge at about the same age in all human societies
A. afarensis, suggests a substantial increase in the com-   and at about the same age in the chimpanzee, suggest-
plexity of hominid social systems and in the ability to     ing a long evolutionary history (Bard, 1995). Some
exploit biological and physical resources (e.g., through    form of attachment is, in fact, common to primates
tool use). An extended developmental period, along          and appears to function to reduce infant mortality
with the increased social play and exploratory behavior,    risks by keeping the infant in close proximity to its
would enable the reÞnement of increasingly sophisti-        parent or parents and by increasing the level of paren-
cated physical, social, and cognitive competencies.         tal investment (Bard, 1995; Bowlby, 1969; MacDonald,
   Adaptive value of immaturity. Not all features of the    1992); the latter refers to resources, such as time and
developmental period are a preparation for adult sur-       food, provided to offspring at a cost to the parent
vival and reproduction. A basic assumption of evolu-        (e.g., delayed birth of the next offspring, see Geary,
tionary developmental psychology is that natural se-        2000; Trivers, 1974). There are, however, individual
lection has resulted in cognitive and social traits that    differences in the nature or quality of the attachment
support survival of individuals at all stages of devel-     relationship across dyads of parents and children
opment, and that infants, children, juveniles, and ado-     (Ainsworth & Bell, 1970), suggesting that attachment
lescents may have evolved speciÞc adaptive behaviors        emerges through an epigenetic process. In other
to deal with their developmentally deÞned niches            words, the attachment process is biologically driven,
(Bjorklund, 1997). When such a perspective is applied       but the nuances of this relationshipÑthe measurable
to human cognitive and social development, one sees         phenotypeÑare shaped by the nature of the parentÐ
cognitive or social immaturity in a different light.        child relationship.
Seemingly ÒimmatureÓ behavior may have been se-                One implication, and an important conceptual
lected as a way young organisms can negotiate the           framework for future studies, is that different forms
preadult years (Pellegrini & Smith, 1998). As an exam-      of attachment reßect an adaptation to different social
ple, Turkewitz and Kenny (1982) argued that the im-         contexts (MacDonald, 1992). In theory, these adapta-
mature sensory abilities of infants (e.g., poor eye         tions would reßect behavioral and psychological ad-
sight) may serve as a form of protection from over-         justments to maintain or increase levels of parental in-
stimulation (see Bjorklund, 1997, for a review). Aspects    vestment (Trivers, 1974), although empirical research
of cognitive immaturity, such as an egocentric per-         on attachment generally has not approached these in-
spective and poor metacognition (see Bjorklund,             dividual differences from this perspective. A related
1997), may have an adaptive role for children as well.      issue is whether early attachment patterns inßuence
For instance, their short auditory memory span may          later reproductive strategies (Belsky, Steinberg, &
serve to reduce the amount of language processed and        Draper, 1991). There is indeed a correlation between
thus aid in comprehension.                                  receiving low or inconsistent levels of parental invest-
                                                            ment in childhood, an insecure attachment to parents,
                                                            and later instability in marital relationships and thus
                                                            lower levels of investment in any resulting children.
Sociocognitive competencies, such as the ability to pro-    However, it is not clear if this correlation is caused by
duce and respond to communicative behaviors (e.g.,          early attachment patterns or reßects heritable traits,
vocalizations), are a requisite feature of survival and     that is, genes that inßuence both marital instability
reproduction in all primates (Hauser, 1996). Develop-       and low levels of parental investment (MacDonald,
ment not only enables the reÞnement of these socio-         1997). Future studies of potential social and genetic
cognitive competencies through social play, it also re-     inßuences on attachment and later parenting might
60   Child Development

follow the approach recently used by Rowe and his                 Adolescence and adulthood. Adolescence is deÞned
colleagues to study genetic and social inßuences on            by the physical changes that prepare individuals for
individual differences in social aggression (Rowe,             adult reproductive activities, such as giving birth or
Almeida, & Jacobson, 1999).                                    physical competition for social status, an increasing
    Juvenility. During juvenility, the focus of human so-      interest in members of the opposite sex, and an esca-
cial relationships shifts from parents to peers, as is         lation of the forms of social competition described
found in other social mammals (Bogin, 1997; Harris,            above (Tanner, 1990). As an example, in mammals,
1995). Across primate species, a long juvenile period is       including humans, the play Þghting of males be-
associated with a larger neocortex and a complex social        comes increasingly intense and adult-like during
system (Barton, 1996; Joffe, 1997), suggesting, as noted       adolescence. For humans, early courtship and pre-
earlier, that one goal of juvenility is to practice and re-    sexual (sometimes sexual) heterosexual relation-
Þne sociocognitive competencies. An intriguing feature         ships emerge, and appear to provide the experi-
of human social behavior during juvenility is the ten-         ences needed to establish and maintain the more
dency of children to segregate themselves into same-           stable relationships of adulthood (Bogin, 1997; Mac-
sex groups and to engage in different forms of social          coby, 1998).
play within these groups, although these differences              Early adulthood is the reproductive period and in
begin to emerge in childhood (Maccoby, 1988, 1998).            hunter-gatherer societies usually begins in the late
From an evolutionary perspective, these sex differ-            teens for girls and a few years later for boys (Bogin,
ences are predicted to be a reßection of and a prepara-        1997). Although there are similarities in the reproduc-
tion for sex differences in adult reproductive activities,     tive activities of men and women, there are differ-
at least in hunter-gatherer societies and presumably           ences as well. For instance, men engage in more phys-
during the course of human evolution (see Bjorklund &          ical, coalition-based intrasexual aggression for the
Shackelford, 1999; Geary, 1998, 1999, 2000).                   establishment of social dominance than do women,
    As with juvenile males in most other mammals,              and women spend more time in parenting activities
boysÕ play is largely focused on the achievement of            than do men (Bjorklund & Shackelford, 1999; Geary,
social status and social dominance and includes rough-         1998). In hunter-gatherer and other preindustrial so-
and-tumble play (e.g., play wrestling) and group-              cieties, women continue having children until meno-
level competition (e.g., team sports, Pellegrini & Smith,      pause and spend their post-menopausal years raising
1998; Smith, 1982). These activities are a preparation         their youngest children and investing in their grand-
for the physical and often deadly forms of male-on-            children. Older men, in contrast, often attempt to con-
male aggression in adulthood that is common in pre-            tinue their reproductive activities by marrying younger
industrial societies and related to striving for social        wives (Bogin, 1997; Lancaster & Lancaster, 1983). These
dominance and mate acquisition (Geary, 1998; Keeley,           patterns suggest different life histories for women
1996). The social activities of girls are more communal        and men during human evolution and, as a result, de-
than those of boys and are focused on the develop-             velopmental sex differences in social goals, behav-
ment and maintenance of a small number of intimate             iors, and motivations are expected, and found (Geary,
and reciprocal relationships with other girls. In adult-       1998, 1999).
hood, these networks appear to provide a system of                Finally, natural selection does not eliminate the
social support and stability. The achievement of a sta-        many deleterious changes associated with adult ag-
ble social network in turn is associated with reduced          ing but rather, under some conditions, can operate to
morbidity risks for children and thus appears to be an         delay the onset of these changes (Charnov, 1993).
evolved parenting strategy (Flinn & England, 1995;             During hominid evolution there has been a clear in-
Geary, 1998). This does not mean that girls do not com-        crease in the length of the lifespan, suggesting that a
pete with one anotherÑthey do: they back bite, shun,           slowed rate of maturation and the associated extension
and gossip, the goal of which appears to be to disrupt         of adulthood has resulted in reproductive advan-
the social networks of their competitors (termed Òre-          tages. The advantages associated with a longer devel-
lational aggression,Ó Crick & Bigbee, 1998). Although          opmental period, as described earlier, could only be
it is not certain, girlsÕ relational aggression during juve-   supported by a corresponding increase in the lifespan
nility might provide practice for later competition over       of parents, given the dependency of human children.
mates and other resources, just as boysÕ rough-and-            The increase in the length of adulthood is thus likely
tumble play provides practice for later maleÐmale              to be related in part to increases in the level of paren-
competition. In any case, the evolutionary perspective         tal investment, which in turn would result in repro-
will provide a broader context for conceptualizing             ductive beneÞts, speciÞcally reductions in child mor-
and studying aggression among girls and women.                 tality rates and potential improvementsÑthrough an
                                                                                                 Geary and Bjorklund     61

extended childhood and thus greater opportunity to              ual- and group-level systems (see also Caporael,
reÞne social and cognitive competenciesÑin the later            1997). Individual-level systems are designed for the
social and thereby reproductive competitiveness of              on-line monitoring of dyadic interactions and for es-
children (Geary, 2000; Lancaster & Lancaster, 1983).            tablishing and maintaining interpersonal relation-
   Nonetheless, the power of natural selection wanes            ships. The associated submodules include those that
with age, and Òthe beneÞts resulting from evolution-            support the reading of nonverbal behavior and facial
ary selection evince a negative age correlationÓ (Baltes,       expressions, language, and theory of mind (Baron-
1997, p. 367). This is because traits that are expressed        Cohen, 1995; Pinker, 1994). The group-level modules
only after an individual has reproduced and success-            parse the social universe into kin, friends (in-group),
fully reared offspring are not subject to the same de-          and competitors (out-group).
gree of selection pressure as traits that directly inßu-           Sensitivity to nonverbal behaviors, facial expres-
ence survival to reproductive age and successful                sions, and other communicative behaviors (e.g., vo-
reproduction. As a result, there are no strong selection        calizations) of conspeciÞcs, as well as the preferential
pressures against deleterious genes that have their in-         treatment of kin and the formation of favored and dis-
ßuence in old age (such as those that contribute to             favored social groups, are found in many other spe-
AlzheimerÕs dementia). In fact, many of these genes             cies (Goodall, 1986; Hauser, 1996). A rudimentary the-
or other factors (e.g., hormones) that provide adap-            ory of mind might be evident in chimpanzees, although
tive beneÞts early in life may have more deleterious            this is currently debated (e.g., Povinelli & Eddy, 1996;
effects later in life.                                          Premack & Woodruff, 1978). Either way, it is clear that
                                                                humans have the unique ability to form in-groups and
                                                                out-groups on the basis of social ideologies (e.g.,
The Þrst of two central issues for evolutionary devel-             In addition to managing social relationships, our
opmental psychology is to identify the evolved do-              ancestors had to secure food and other resources from
mains of mind, that is, the constellations of cognitive         the natural environment, which in turn almost cer-
competencies that appear to have been shaped dur-               tainly resulted in the evolution of modules for pro-
ing evolution (Cosmides & Tooby, 1994; Pinker, 1997).           cessing biological and physical information. Biologi-
The second issue concerns the mechanisms that gov-              cal modules are for categorizing and representing the
ern the development of the associated competencies              behavior or growth patterns of ßora and fauna in
and their adaptation to local ecologies (Gelman, 1990;          the local ecology, especially species used as food,
Siegler, 1996).                                                 medicines, or in social rituals (e.g., Berlin, Breed-
   Domains of mind. Geary (1998) recently proposed              love, & Raven, 1973). Physical modules are for guid-
that evolved domains of mind be conceptualized as               ing movement in three-dimensional physical space,
constellations of hierarchically organized modules              mentally representing this space, and using physical
for processing information in the social, biological, and       materials (e.g., stones, metals) for making tools
physical worlds, as shown in Figure 1, although there           (Pinker, 1997; Shepard, 1994).
appear to be other modules that are not represented                An important developmental prediction is that the
in the Þgure (e.g., for numerical processing). Socio-           skeletal competenciesÑthe neural systems that guide
cognitive modules are further divided into individ-             attention to and processing of the associated infor-

Figure 1 Proposed domains of mind. From Male, female: The evolution of human sex differences (p. 180), by D. C. Geary, 1998,
Washington, DC: American Psychological Association. Reprinted with permission.
62   Child Development

mationÑof these modules are inherent but are ßeshed         Table 2 Predicted Developmental Features of Evolved Cog-
out as children engage in social discourse, play, and       nitive Modules
exploration (Gelman, 1990; see Elman et al., 1996, for                            Hierarchical Organization
an alternative view). The prediction is that natural
                                                            1. The modules shown in Figure 1 are very likely to be comprised
child-initiated activities are centered on learning            of a hierarchy of submodules. For instance, the language system
about people and developing social competencies,               includes specialized systems for the comprehension and
learning about other living things, and learning how           production of speech. These, in turn, are supported by sensory
the physical world is organized, that is, gaining com-         and motor systems for processing and articulating
                                                               language sounds.
petencies in the areas of folk psychology, folk biology,
                                                            2. The degree to which submodules are specialized is likely to be
and intuitive physics. The associated activities would
                                                               inversely related to their level in the hierarchy. The most basic
include, for instance, sociodramatic play, exploring           submodules are likely to be highly specialized, designed to
the environment, and object-oriented play. An outline          process a restricted range of stimuli, such as specific language
of the predicted developmental features of evolved             sounds. Modules at the highest level receive information from
cognitive modules is presented in Table 2 (Geary,              many lower-level modules and may show moderate to high
                                                               levels of flexibility in the range of stimuli that can be processed
1998; Gelman, 1990). These predictions may provide a
                                                               and in their output. An example is the apparently infinite
useful framework for guiding future cognitive devel-           number of utterances that can be generated by the language
opmental research and for linking this research to             production system.
parallel studies in the Þelds of ethnobiology and cog-
                                                                        Sensitive Periods and Child-Initiated Activity
nitive anthropology (Atran, 1998).
                                                            1. If cognitive modules emerge by means of an epigenetic process,
    Mechanisms of development. The long developmen-            then the development of the associated neural and cognitive
tal period of humans argues against the position that          systems will be dependent on exposure to domain-specific
any inherent features of these cognitive modules and           information (e.g., language sounds). Experiences interact with
associated behaviors are immutable (Mayr, 1974).               the inherent skeletal features of these modules to produce the
                                                               phenotypic competencies.
Rather, there is a core set of cognitive competencies,
but the associated phenotypes are modiÞable by ex-          2. A bias in childrenÕs activities and the types of information to
                                                               which they attend is expected. These activities are expected to
periences, that is, these competencies emerge by               correspond to the domains shown in Figure 1 (e.g., orienting
means of epigenetic processes. For instance, it ap-            to people, exploring the environment) and provide the
pears that children in all cultures are biologically pre-      experiences that are an intimate feature of the epigenetic
pared to process and respond to the sounds of all hu-          development of the modules.
man languages, but the phenotype that eventually            3. Sensitivity to environmental input is expected to be time-lim-
emerges is the speciÞc language to which they are ex-          ited to some degree; that is, sensitive periods are expected. The
                                                               length of the sensitive period may be related to the
posed (Kuhl et al., 1997). Another prediction is that          submodulesÕ level in the hierarchy. Because the functioning of
these modules will not be fully elaborated unless chil-        higher level submodules may depend on input from lower level
dren are exposed to and engage in the relevant activ-          modules, the sensitive period for lower-level modules is
ities. For instance, Western children have a rudimen-          expected to be shorter and occur earlier in life than that for
tary understanding of folk biology but, presumably             higher level modules. In short, the length of the sensitive period
                                                               may be directly related to the complexity of the information
due to a lack of extensive exposure to ßora and fauna,         processed by the module.
they do not develop the same degree of folk biological
knowledge as is found with individuals in hunter-                                     Implicit Knowledge
gatherer and other preindustrial societies (Atran,          1. The skeletal structure of evolved modules reflects the organiza-
                                                               tion of the underlying neural systems and the types of informa-
1998; Kiel, 1992).                                             tion to which these systems respond.
    The mechanisms that contribute to this epigenetic
                                                            2. Knowledge is built into the organization of these cognitive and
process are not well understood but, like natural selec-       neural systems, that is, they respond to appropriate ecological
tion, appear to involve the generation of variability          information and produce intelligent responses to this information.
and experiences acting on this variability (Siegler,           The functioning of many of these systems is likely to be
1996). The brain and mind appear to generate multiple          automatic and largely outside of the realm of conscious control.
representations and strategies for processing and act-      3. The degree to which the functioning of modules is relatively
                                                               automatic and the associated knowledge implicit may be
ing on social, biological, and physical information. Ex-
                                                               inversely related to the modulesÕ level in the hierarchy.
perience results in one or a few of these representations      Lower-level modules are likely to be characterized by
or behavioral strategies being selected over others. The       automatic processing (assuming adequate attention to this
relative success of one approach or the other for achiev-      information) and a high degree of implicit knowledge. Individuals
ing the desired goal (e.g., inßuencing other people) ap-       are more likely to gain explicit awareness of the output of
                                                               higher-level modules, and the functioning of these modules
pears to be the selective process. However, the under-
                                                               may be more open to top-down control by the individual.
lying cognitive and neural mechanisms that generate
                                                                                             Geary and Bjorklund    63

variability and ensure the selection of successful over      sometimes life-threatening risk-taking (e.g., reckless
less successful strategies are not well understood (see      driving) and deadly forms of male-on-male violence is
Shrager & Siegler, 1998, for related discussion) and         readily understood in terms of competition for social
thus should be one focus of future cognitive develop-        status, which in turn often inßuences mating opportu-
mental and developmental neuroscientiÞc research.            nities (Daly & Wilson, 1988; Geary, 1998). By under-
                                                             standing the goal of these behaviors and the contexts
                                                             that are likely to elicit them (e.g., Òsaving faceÓ when
                                                             confronted by a male peer), psychologists should be in
                                                             a position to reduce the frequency of these behaviors.
A basic tenet of evolutionary psychology is that mod-        One potential strategy is to provide multiple opportu-
ern humans evolved domain-speciÞc cognitive abilities        nities for young men to achieve status in one domain
and behavioral strategies to deal with recurring condi-      (e.g., athletics) or another (e.g., academics). For in-
tions in the environments of our ancestors (Cosmides &       stance, in this view, athletic competition is a culturally
Tooby, 1994), but these abilities and strategies may not     regulated expression of intrasexual competition, and
always be well suited to contemporary conditions.            this competition can be modiÞed (e.g., through rules)
Thus, in addition to studying epigenetic processes that      and channeled through athletics such that it does not
guide the development of evolved social and cogni-           escalate into deadly violence.
tive systems, the goal of evolutionary developmental
psychology is to understand how these evolved bi-
ases and constraints inßuence the expression and de-
velopment of social and cognitive competencies that          A complete understanding of human social and cog-
are important in modern society.                             nitive development, and any associated sex differ-
    As an example, much of formal education is Òun-          ences, requires an understanding of human evolution
naturalÓ in that much of what children are taught in-        and the associated epigenetic processes that guide the
volves tasks never encountered by our ancestors              development of evolved social and cognitive pheno-
(Brown & Bjorklund, 1998; Geary, 1995). Although             types. To argue otherwise is to ignore the vast empir-
humans apparently have been using language for               ical literature supporting DarwinÕs (1859) theory of
thousands of years, it is only in this century that the      evolution (e.g., Weiner, 1995) and to deny the core
majority of people on the planet are literate. Geary         theoretical foundation of the biological sciences. The
(1995) referred to language and other evolved forms          goal here was to provide a skeletal framework for
of cognition, such as those represented in Figure 1, as      linking the extant empirical and theoretical literatures
biologically primary abilities, and skills that build upon   in developmental psychology to the wider Þeld of
these primary abilities but are principally cultural in-     evolutionary biology and, in particular, to evolution-
ventions, such as reading, as biologically secondary abil-   ary developmental biology (e.g., Hall, 1992). This
ities. Biologically primary abilities are acquired uni-      emerging interdisciplinary Þeld of evolutionary de-
versally and children typically have high motivation         velopmental psychology seeks to explain not only the
to perform tasks involving them. In contrast, biologi-       evolutionary and biological inßuences on human de-
cally secondary abilities are culturally determined,         velopment, but also to understand the social and eco-
and often tedious repetition and external motivation         logical conditions that will necessarily affect the de-
are necessary for their mastery. From this perspective,      velopment and expression of social and cognitive
it is understandable that many children have difÞculty       competencies. In other words, an evolutionary per-
with reading and higher mathematics.                         spective should provide a useful framework for con-
    Similarly, an understanding of evolved social biases     ceptualizing and guiding future research across many
has profound implications for understanding and ad-          of the developmental specialties (e.g., social, cogni-
dressing other pressing social issues, such as individual    tive, and neuroscientiÞc).
differences in parenting styles, child neglect and abuse,
the sharp increase in male-on-male violence during ad-
olescence and early adulthood, divorce, and prejudice
(e.g., Daly & Wilson, 1988). Without an appreciation of      We thank Todd Shackelford and three anonymous re-
the evolutionary and biological contributions to the         viewers for helpful comments on an earlier draft. The
epigenetic processes that shape the development of           preparation of this article was supported, in part, by a
these phenomena, psychology as a discipline will never       Summer Research Fellowship awarded to the Þrst au-
address these issues to its fullest potential. As an exam-   thor by the Research Council of the University of Mis-
ple, the tendency of adolescent males to engage in           souri at Columbia.
64   Child Development

ADDRESSES AND AFFILIATIONS                                        Cosmides, L., & Tooby, J. (1994). Origins of domain speciÞc-
                                                                     ity: The evolution of functional organization. In L. A.
Corresponding author: David C. Geary, Department                     Hirschfeld & S. A. Gelman (Eds.), Mapping the mind: Do-
of Psychology, 210 McAlester Hall, University of Mis-                main speciÞcity in cognition and culture (pp. 85Ð116). New
souri at Columbia, Columbia, MO 65211-2500; e-mail:                  York: Cambridge University Press. David F. Bjorklund is at Flor-               Crick, N. R., & Bigbee, M. A. (1998). Relational and overt
ida Atlantic University, Boca Raton, FL.                             forms of peer victimization: A multiinformant ap-
                                                                     proach. Journal of Consulting and Clinical Psychology, 66,
                                                                     337 Ð 347.
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