Evolutionary Developmental Psychology

Evolutionary Developmental Psychology Karin Machluf and David F Bjorklund, Florida Atlantic University, Boca Raton, FL, USA Ó 2015 Elsevier Ltd. All rights reserved.

Abstract Evolutionary developmental psychology posits that natural selection operates across the lifespan and focuses on how psychological mechanisms develop. The basic tenets of evolutionary developmental psychology are outlined, along with detailed discussion of ontogenetic, deferred, and conditional adaptations. These are then applied to mental health, where the mental health model is pitted against the evolutionary developmental perspective of mental health. Lastly, adolescent risktaking behaviors are examined from an evolutionary-informed perspective using life history theory to explore the influence of environmental harshness and unpredictability in early childhood.

Development and Evolution The intimate connection between development and evolution has long been acknowledged. Darwin, in his controversial and revolutionary book The Origin of Species (1859), meticulously described the processes of embryology, as it was understood then, and used it as part of his indisputable evidence for evolution (see Darwinism). In a letter to a his colleague, Asa Gray, Darwin wrote, “Embryology is to me by far the strongest single class of facts in favor of change of forms, and not one, I think, of my reviewers has alluded to this” (1860). He concluded that if slight changes are introduced across different points of time, for different parts of the body, it would produce different forms that are adapted to those particular circumstances. Darwin’s assessment of the importance of development was correct but simultaneously premature. Darwin knew nothing of genetics, and the rediscovery of Mendel’s work in the early 1900s and August Weismann’s doctrine of the separation of the germ and somatic lines (what happens to the body cells of an organism during its lifetime cannot influence the content of its sex cells), resulted in the temporary parting of ways of development and evolution. The field became ‘genecentric,’ integrating Mendelian genetics with Darwin’s ideas of natural selection, leading to neo-Darwinism, or the Modern Synthesis (Huxley, 1942). This reflected the official acceptance by the scientific community that large-scale changes in form could be explained by small genetic changes acted upon by the process of natural selection, over long stretches of time, to produce variations within species. From this perspective, development did not matter, or, as more elegantly expressed by Richard Dawkins (1976/2006: p. 62), “The details of embryological developmental processes, interesting as they may be, are irrelevant to evolutionary considerations.” Development, although of vital importance to the individual, was essentially inconsequential to natural selection and thus evolution. In the past few decades, however, the focus on development has reemerged in evolutionary thinking, with, for example, Stephen Jay Gould’s Ontogeny and Phylogeny (1977), reinvigorating the idea that modifications during development may influence evolution, Eva Jablonka and Marion Lamb’s Epigenetic Inheritance and Evolution (1995), proposing the role

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of epigenetics in evolution (see also Jablonka and Lamb’s 2005 book, Evolution in Four Dimensions), and Mary Jane WestEberhard’s Developmental Plasticity and Evolution (2003), extending epigenetics to developmental plasticity (see Epigenetic Inheritance). Over the last two decades, evolutionary thinking has entered the mainstream of psychology, and most evolutionary psychologists have modified the now-famous quote by Theodosius Dobzhansky (1973: p. 749) stating that “Nothing in biology makes sense except in the light of evolution” to “Nothing in psychology makes sense except in the light of evolution.” The central concept of Darwin’s theory of evolution is natural selection, which assumes that more members of a species are born in a generation than will survive, resulting in competition for a set of limited resources (see Microevolution). Individuals vary, and those that have features associated with greater survival will pass those features to their offspring. In other words, heritable variations in characteristics of an individual interact with the local environment, and, over long periods of time, change in frequency, resulting, eventually, in specieswide traits in the population. Evolutionary psychologists realized that natural selection could be used to explain species differences in psychological as well as physical features, and thus could be used to explain and predict many aspects of human behavior (e.g., Buss, 1995; Daly and Wilson, 1988; Tooby and Cosmides, 1992; see Evolution of Cognition: An Adaptationist Perspective). The principal focus of mainstream evolutionary psychology was understandably on adults, who do the reproducing, the ultimate measure of Darwinian success. Much like biologists of an earlier era, many evolutionary psychologists saw development as an epiphenomenon, interesting and important for the individual, but little more than ‘noise’ when it came to important psychological characteristics that typify Homo sapiens. Yet, individuals must survive the oftenhazardous periods of infancy and childhood before reaching adulthood, where the reproducing (and subsequent rearing of children) takes place. In fact, although the likelihood of a newborn dying before reaching adolescence today is about 1% in Western cultures, it is closer to 50% in traditional societies and was likely as higher or higher for our ancestors (Volk and Atkinson, 2008). This makes childhood the crucible of evolution, such that any adaptations that increase the chance

International Encyclopedia of the Social & Behavioral Sciences, 2nd edition, Volume 8

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Evolutionary Developmental Psychology

of surviving to adulthood during these earlier stages of life would certainly be favored by natural selection.

Evolutionary Developmental Psychology Natural selection operates at all stages of the lifespan; however, it operated at some stages more than others, especially early in development. From this realization was born the field of evolutionary developmental psychology (e.g., Bjorklund et al., 2007; Bjorklund and Pellegrini, 2000, 2002; Bjorklund and Hernández Blasi, 2005; Burgess and MacDonald, 2005; Ellis and Bjorklund, 2005; Geary and Bjorklund, 2000). Evolutionary developmental psychology is defined as the application of the basic principles of evolution 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 (gene–environment 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 also are characteristics of children’s behaviors and minds” (Bjorklund and Pellegrini, 2002: p. 4). It posits that natural selection operates throughout ontogeny (i.e., the lifetime of the individual), not simply during an organism’s reproductive years. In this article, we first outline briefly some of the principles and assumptions of evolutionary developmental psychology (see Bjorklund and Pellegrini, 2000, 2002). We then look at how an evolutionary developmental psychological approach can be applied to the study of developmental psychopathology. We specifically contrast the canonical mental health model, which generally emphasizes costs and largely ignores any benefits of children’s responses to risky environments, with an explicitly evolutionary developmental perspective. Finally, we discuss how this evolutionary developmental perspective can help better understand risk-taking and antisocial behaviors, especially in adolescence.

Some Basic Principles of Evolutionary Developmental Psychology A central concern of evolutionary developmental psychology is how psychological mechanisms develop. In fact, this, some believed, is what mainstream evolutionary psychological theory was lacking. How do inherited, presumably genetically coded behaviors and cognitions get expressed in the phenotypes of adults? The obvious answer seems to be ‘through development,’ and in this section we examine some of the basic principles of evolutionary developmental psychology.

All Evolved Characteristics Develop via Continuous and Bidirectional Gene–Environment Interactions That Emerge Dynamically over Time One problem that many people have with an evolutionary approach to human behavior is that there is the taint of genetic determinism, the belief that if a behavior or type of cognition

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evolved and is influenced by genes (which are, after all, the basic units of inheritance), then there is little one can do to change ‘human nature’; that is, genes directly cause, or determine, behavior (e.g., Spencer et al., 2009; Lickliter and Honeycutt, 2003). Such an approach is generally anathema to developmental psychologists, accounting, in part, for the reluctance of some developmentalists to adopt an evolutionary perspective. However, evolutionary developmental psychologists have adopted a developmental systems approach (Gottlieb, 2002, 2007), which stipulates that genes and environment interact at all levels of organization, from the biochemistry of the cells through culture. From this perspective it is inappropriate to ask ‘how much’ of any feature (physical or psychological) is the result of ‘nature’ vs ‘nurture,’ but rather about ascertaining how do the biological and environmental factors interact to produce those features. At the core of the developmental systems approach is the concept of epigenesis, which Gilbert Gottlieb (1991: p. 7) defined as “the emergence of new structures and functions during the course of development.” New structures or behaviors are the result of the bidirectional relationship between all levels of biological and experiential factors, from the genetic through to the cultural. ‘Experience,’ from this perspective, includes events external to the individual, such as perceptual stimulation and social interaction, but also internal events, such as the presence of neurotransmitters, hormones, and even the firing of one neuron as it influences its neighbors and itself. Functioning at one level influences functioning at adjacent levels, with constant feedback between levels. This relationship can be expressed as follows: genetic activity ðDNA4RNA4proteinsÞ 4structural maturation4function; activity: From this perspective, all development is the product of epigenesis, with complex interactions occurring among multiple levels. Evolved psychological mechanisms, the central tenet of mainstream evolutionary psychology, can be viewed as inherited ‘messages’ that, following epigenetic rules, interact over time with the environment to produce behavior (see Bjorklund et al., 2007). Because the experiences of every individual are unique, there would be substantial individual differences (i.e., plasticity) in development. Despite this, most members of a species share many features. This is because individuals inherit not only a species-typical genome, but also a species-typical environment, starting with the prenatal environment. To the extent that individuals grow up in species-typical environments similar to those of their ancestors, development should follow a speciestypical pattern.

Development Is Constrained by Both Genetic and Environmental Factors Infants are born into a complex world of sights, smells, sounds, and other people. How do they learn to figure out what is important and what is not, and, in general, to make sense of the plethora of sensations that bombard them? It was once believed that infants were born (essentially) as blank slates –

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they had the general human ability to learn and, via the operations of classical and operant conditioning, learned what is relevant and what is not. This seems not to be the case. Rather, newborns are prepared by natural selection to ‘expect’ certain types of environments and to process some information more easily than others. From this perspective, one can talk of infants and children inheriting biases that increase the likelihood of their developing adaptive responses to their environment, with such biases and constraints enabling learning. For example, from shortly after birth, infants show biases in what they like to look at and listen to, and many of these are related to other people. For instance, at birth babies prefer the voices of women to those of men (DeCasper and Fifer, 1980) and prefer to listen to the language that their mothers speak rather than to other languages (Mehler et al., 1988), biases that are surely the result of prenatal auditory experiences. Such biases should not be thought of as ‘innate’ in the way the term is typically used, but rather as the nearly inevitable outcomes that are constrained by how genes and experience interact in a species-typical way during early development.

An Extended Childhood Is Needed in Which to Learn the Complexities of Human Social Communities Humans spend more of their lives as ‘prereproductives’ than any other mammal, something that was surely risky, especially for our ancestors who could not expect a long life. In evolutionary biology, when there are great costs associated with a characteristic (here, an extended juvenile period) there should also be great benefits. For ancient and contemporary humans, many believe that those benefits were in terms of developing a large brain capable of acquiring the skills necessary to navigate the social world (Bjorklund et al., 2005; Dunbar, 1995, 2010). The complexities of human societies are highly varied across the globe, and it takes time for children to acquire the abilities to cooperate and compete with their fellow H. sapiens. Humans’ extended period of immaturity, coupled with a big brain that permits a social learning ability unsurpassed in the animal world, allowed our species to become as socially sophisticated as we are.

Natural Selection Has Produced Adaptations of Infancy and Childhood: Ontogenetic, Deferred, and Conditional Adaptations If natural selection operates at all life stages, then we must look specifically at adaptations associated with infancy and childhood, rather than focusing solely on adaptations in the reproductive adult. From a developmental perspective, however, not all adaptations are created equal. Some evolved characteristics of infants and children were specifically selected to serve an adaptive function at a specific time in development, termed ontogenetic adaptations (Bjorklund, 1997). In contrast, deferred adaptations are those aspects of childhood that were selected for their role in preparing the child for adulthood (Hernández Blasi and Bjorklund, 2003). And conditional adaptations influence the course of children’s subsequent development, depending on conditions in their immediate environment (Boyce and Ellis, 2005).

Ontogenetic adaptations emerge at different times in development and adapt the child for challenges specific to that time in life, and may disappear when they are no longer functional. That is, some aspects of infancy and childhood are not preparations for later adulthood but were selected to serve an adaptive function at specific times in development. The most obvious ontogenetic adaptations are associated with prenatal functioning. The umbilical cord, for example, delivers nutrients to and removes wastes from the fetus but is discarded at birth, as infants’ entire respiratory and consummatory systems become reorganized. An example of a postnatal ontogenetic adaptation is children’s physical immaturity. Research has shown that children’s immature physical features (i.e., ‘babyness’) can help them garner nurturing behaviors from their parents. Some features of physical immaturity include heads that are proportionally large relative to their bodies, foreheads that are large in relation to the rest of the face, a small flat nose, big eyes, and short limbs (Lorenz, 1943). Adults find these features ‘cute’ and rate children with these features as more sociable, easier to care for, and more competent (e.g., Alley, 1981; Glocker et al., 2009; Sprengelmeyer et al., 2009) than children without them. A recent study by Sherman et al. (2009) assessed how ‘cuteness’ influences behavioral carefulness, the increased control of and attention to fine motor movements, after viewing immature (puppies and kittens) and mature (dogs and cats) images. Sherman et al. (2009) concluded that viewing immature faces increased fine motor abilities in adults, which the authors suggest is important for caring for a small, delicate child (see also Nittono et al., 2012). In other research, cognitive immaturity has also been proposed as an ontogenetic adaptation. For example, children who express some forms of immature cognition (‘The sun is not out because it is mad’) are rated higher on measures of positive affect by adults (e.g., cute, likable) and lower on measures of negative affect (e.g., irritable, likely to lie) than children expressing more mature cognition (e.g., ‘The sun is not out because wind blew clouds in front of it’) (Bjorklund et al., 2010b; Periss et al., 2012). In other studies, young children’s tendencies to overestimate their physical, social, and cognitive abilities have been shown to have some positive consequences. Research has demonstrated that preschool children overestimate their memory abilities, their intelligence, and their social standing (see Bjorklund et al., 2010a for a review). This was historically thought of as a handicap, and that children’s immature cognitions would eventually develop into functional, accurate, and mature cognitions (Bjorklund, 1997; Bjorklund and Green, 1992). From an evolutionary developmental perspective, however, this poor metacognition (thoughts about the one’s cognitive abilities) may hold some possible benefits (Bjorklund and Green, 1992). For instance, Bjorklund et al. (1993) assessed preschool children’s pretest predictions (how well they thought they would perform a task) and posttest evaluations (how well they thought they had performed the task) of performance on a series of imitation tasks (e.g., juggling one, two, or three balls). They then correlated the accuracy of children’s estimations with their performance on the Vocabulary subset of the Wechsler Preschool and Primary Scale of Intelligence. Bjorklund et al. (1993) found that verbal

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IQ in the 3- and 4-year-olds was higher in children who overestimated more, while the 5-year-olds with poor metacognition (higher overestimation) tended to have lower verbal IQ scores. Thus, for younger preschoolers, being out of touch with one’s abilities was related to high levels of IQ. Another study by Shin et al. (2007) found that kindergarten, first-, and second-grade children who overestimated their memory performance more on earlier trials actually had higher levels of memory recall and strategy performance on later trials than more accurate children (see also Lopez et al., 1998). These findings suggest those children’s immature cognition, in this case overestimation, may in fact serve an adaptive purpose, and is not simply a ‘deficiency’ that children need to grow out of. In contrast to ontogenetic adaptations, deferred adaptations are those that prepare children for life as an adult and are perhaps most easily seen through the lens of gender differences in children’s behavior. For example, boys and girls tend to differ in their forms of fantasy play (see Pellegrini, 2012). Girls engage in more play-parenting and nurturing roles, while boys play power and dominance games, usually centered on aggression or domination themes. There are also gender differences in the forms of aggression used by boys and girls. Boys are more physically aggressive while girls are more relationally aggressive, revealing a female bias for social–interactional conflict and a male bias for physical conflict. This is relevant from an evolutionary perspective considering that males have, over evolutionary history, competed with other males for mates and status, while females required greater social fluency in order to acquire resources to help rear their offspring. These gender differences can be seen as antecedents for the sexual roles children will assume as adults, or would have assumed in ancient environments (see Human Mate Choice, Evolution of). Deferred adaptations are most likely to evolve when environmental conditions remain relatively stable over time. This provides children with reliable information about the environments that they are likely to encounter as adolescents and adults. However, although gross aspects of children’s environments may be similar over time, and even similar to the environments experienced by ancestral children, many aspects of children’s environments cannot be anticipated by natural selection. Some children may be born into resource-rich environments, whereas others may be born into resourcepoor environments, along with the stresses associated with such environments. Children need behavioral and cognitive flexibility in order to direct their development in anticipation of adult environments. According to Belsky et al. (1991: p. 650), “a principal evolutionary function of early experience – the first 5 to 7 years – is to induce in the child an understanding of the availability and predictability of resources (broadly defined) in the environment, of the trustworthiness of others, and of the enduringness of close interpersonal relationships, all of which will affect how the developing person apportions reproductive effort.” In other words, children need to be sensitive to environmental conditions early in life and adjust some aspects of their physical and psychological development in anticipation to what their future environment is apt to be in all likelihood.

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Thus natural selection should favor plasticity and sensitivity to local environments, and this often takes the form of conditional adaptations. Boyce and Ellis (2005) defined conditional adaptations as “evolved mechanisms that detect and respond to specific features of childhood environments – features that have proven reliable over evolutionary time in predicting the nature of the social and physical world into which children will mature – and entrain developmental pathways that reliably matched those features during a species’ natural selective history” (p. 290). Perhaps the most studied topic from this perspective is the rate at which children (especially girls) attain puberty and their future adult mating strategies as a function of their early rearing environment. Belsky and his colleagues (1991) argued that children’s early environments are good predictors of what their later environments will be like. An adaptive reproductive strategy (‘adaptive’ from an evolutionary perspective, not necessarily from the point of view of mainstream society or of mental health professionals) for children growing up in high-stress, low-resource, and unpredictable environments is to reproduce early and often, because it is substantially less likely that any one child will have the requisite resources needed to meet his or her reproductive (evolutionarily driven) goals. The opposite pattern would develop for children growing up in more predictable and supportive homes, with their most adaptive strategy being to postpone reproduction and to invest heavily in children. Nearly two decades of research has essentially confirmed this hypothesis, finding relationships between various types of socioemotional stress measured during childhood and the age at which girls attain menarche and the particulars of their subsequent sexual behavior (see Belsky, 2007; Del Giudice, 2009; Ellis, 2004, for reviews).

Applying Evolutionary Developmental Psychology to Mental Health Although developmental psychologists were initially reluctant to incorporate evolutionary ideas into their theories and research, the role of evolutionary thinking in explaining important aspects of human development has increased over the past decade. Evolutionary theories can provide new insights into many aspects of development. One important contribution of such thinking is in the area of developmental psychopathology (see, e.g., articles in Ellis and Bjorklund, 2012; Ellis and Boyce, 2011).

The Mental Health Model The current dominant scientific paradigm for explaining the relationship between high-risk environments and subsequent mental health could be referred to as the mental health model, or the diathesis-stress model, which assumes that high-risk environments adversely affect children’s well-being, promoting disturbances in development, even if not clinical disorders per se, and contend that certain individuals are predisposed to develop psychopathologic conditions when exposed to stressful environments. According to this model, supportive environments tend to produce ‘good’ developmental

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outcomes, such as good health, happiness, secure attachment, high self-esteem, emotional regulation, and educational success, whereas negative, stressful environments promote ‘bad’ developmental outcomes, such as illness, insecure attachment, behavioral problems, drug use, alcoholism, conduct problems, depression, early pregnancy, and limited education. This has led researchers to focus on the deleterious effects of familial and ecological stressors on mental health outcomes (e.g., adolescent onset of psychopathology). Dysfunctional behavior in childhood and adolescence is seen as the natural consequence of exposure to harsh, unpredictable, or uncontrollable socioecological contexts. As such, the mental health model emphasizes costs and largely ignores any benefits of children’s responses to their life situations, making it difficult to explain children and adolescents’ motives for their behavior. This leads to the view of children’s reactions (e.g., conduct disorder in childhood, adolescent risk behavior) as inherently pathological, thereby requiring preventive interventions.

Evolutionary Developmental Psychological Models of Mental Health In contrast, an evolutionary developmental perspective assumes that natural selection designed the developing brain to respond to different environmental contexts – good and bad – in an adaptive manner. When children encounter stressful environments, therefore, it does not disturb their development so much as it directs or regulates it toward strategies that are adaptive under stressful conditions – even if those strategies are currently harmful in terms of the long-term welfare of the individual. As stated by Ellis and his colleagues (2012: p. 600), “The evolutionary model posits that natural selection shaped our neurobiological mechanisms to detect and respond to the fitness-relevant costs and benefits afforded by different environments. Most important, these responses are not arbitrary but function adaptively to calibrate developmental and behavioral strategies to match those environments.” These adaptations may well be harmful in terms of the longterm welfare of the individual and society as a whole, but they are well designed to cope with the immediate circumstances. In some cases they are deeply ingrained in our psychology and physiology, based on many thousands of generations of genetic evolution. In other cases they are culturally ingrained, which does not make them any less compelling. Either way, when we understand the local sense in which children and adolescents’ responses to high-risk environments are adaptive, a different agenda for research and intervention emerges, as compared with the mental health model. Using an example summarized earlier, children growing up in high-stress environments with minimal resources and unstable homes would benefit (from an inclusive-fitness perspective) by reproducing early and often, rather than waiting to reproduce (as society would prefer), because it is unlikely that they will have the necessary resources needed to succeed in the long term. It is important to note that just because children and adolescents are adapting to harsh environments does not imply that these are ideal situations or that there are no costs. Harsh environments certainly have costs, sometimes even resulting in death, but this viewpoint

recognizes the possible benefits of high-risk behaviors. Avoiding this naturalistic fallacy (i.e., thinking that ‘natural’ is ‘good’ or ‘justifiable’), one can see how children adapting to high-stress environments allows them to make the best of a bad situation, although it may be seen as a negative outcome by conventional societal standards. In this vein, several theorists have proposed specific evolutionary models based on the idea that conditional adaptations underpin development of contingent survival and reproductive strategies and thus enable individuals to function competently in a variety of different environments. For example, W. Thomas Boyce and Bruce Ellis’ (Boyce and Ellis, 2005; Ellis and Boyce, 2008; Ellis et al., 2005, 2011) theory of biological sensitivity to context attempts to explain the adaptive relation between early life experience and stress reactivity. It proposes that early environment helps in priming the future stress reactivity, and that early experience with stress interacts with individual differences in aspects of children’s personalities, sometimes producing adaptive long-term outcomes and sometimes even producing maladaptive long-term outcomes, depending on the quality of environment children live in. Both genetic dispositions and early experience influence how readily young organisms respond to events in the environment (that is, how sensitive they are to context), which in turn affects their long-term reactions to stress. Those individuals who are especially biologically sensitive to changes in the environment may gain special benefits or experience special deficits from extreme environments, relative to less-sensitive (and more stable) individuals; that is, sensitive individuals are impacted by their environment both for better and for worse. This contrasts with the mental health model’s exclusive focus on possible negative implications of difficult environments. This ‘for better or for worse’ phenomenon can be seen in research from Boyce and his colleagues (1998) with rhesus monkeys. Boyce et al. (1998) measured biological reactivity to novel experiences in a troop of rhesus monkeys living in a low-stress environment. Some monkeys were highly reactive to novel experiences, showing elevated cortisol levels, whereas others were less reactive. Furthermore, an examination of injuries over the course of a year indicated slightly more injures for animals classified as low in reactivity than those classified as high in reactivity. The monkeys then experienced a highly stressful period of confinement to a 1000-square-feet building. The rate in injuries between the high- and lowstress environments did not vary for the monkeys classified as low in biological reactivity. However, rates of injuries skyrocketed for the high-reactive monkeys. Whereas these monkeys had the lowest rate of injuries in the low-stress environment, they had the highest rate in the high-stress environment. In a study with human children, Boyce and his colleagues (1995) measured the incidence of respiratory infections in 3- to 5-year-old children who were classified as either low or high on the basis of cardiovascular and immunologic reactivity. The incidence of respiratory infection for lowreactive children was about the same regardless if they came from low- or high-stress homes (a little less than three infections per 3-month period) (see also Obradovic et al., 2010). In contrast, high-reactive children had the lowest

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incidence of infections if they came from low-stress homes (slightly over two infections per 3-month period), but the highest rate if they came from high-stress homes (about 3.5 infections per 3-month period). In describing high-reactive and low-reactive children, Boyce and Ellis (2005) used the metaphor of the orchid and the dandelion. Stemming from a Swedish idiomatic expression, maskrosbarn, the dandelion child refers to resilient children who are able to thrive in both negative and positive environments. In contrast, the orkidebarn refers to the orchid child, described as the “context-sensitive individual, whose survival and flourishing is intimately tied, like that of the orchid, to the nurturant or neglectful character of the ambient environment. In conditions of neglect, the orchid promptly declines, while in conditions of support and nurture, it is a flower of unusual delicacy and beauty” (Boyce and Ellis, 2005: p. 284). In a related theory, Jay Belsky and his colleagues (Belsky, 2000, 2005, 2007; Belsky et al., 2007) proposed that some children are more sensitive to individual differences in parenting than others, or differentially susceptible to environmental influences. According to differential susceptibility theory, when environments are unpredictable, children who are easily able to adapt to a wide range of contexts (that is, who are highly responsive to differences in parenting behavior) will be able to adjust to novel, often less-than-optimal, environments (for example, father absence, insensitive parenting). They will also do particularly well in enriched environments, those providing better-than-average support, resources, and opportunities. Other children, however, are more stable and less influenced by extreme environments. Parents can hedge their bets, so to speak, by producing both types of children, some who will be receptive to change and others who will thrive in the ‘expected’ environment. Belsky believes that whether children are sensitive to parenting or not is primarily a function of genetics. Who are the children most sensitive to parents’ endeavors? A number of researchers have shown that highly fearful, anxious, and ‘difficult’ children are more influenced by parental behavior (for example, maternal style of discipline) than less-anxious or ‘average’ children (Gilissen et al., 2008; Kochanska, 1993), as are infants with difficult temperaments (Stright et al., 2008; see Belsky, 2005; Belsky and Pluess, 2009). In summary, the mental health model, the currently dominant scientific paradigm, is unable to fully explain the relationship between high-risk environments and subsequent mental health as it focuses exclusively on costs of risky actions and does not account for any of the potential evolutionary benefits that may result from engaging in such behaviors. The evolutionary developmental perspective, however, contends that natural selection has shaped children to be able to respond to both positive and negative environments in an adaptive manner. Children who are most susceptible, the orchid children, will thrive in supportive, stable environments, but will have greater adverse effects in less than ideal environments as well. Less susceptible children, the dandelions are more stable and are less likely to be influenced by external factors, irrespective of whether they are good or bad.

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The Evolutionary Developmental Model and Risky Adolescent Behavior Application of these differential sensitivity models can be seen in examinations of risky behavior in adolescents. Adolescents are more likely than any other population to engage in risky behaviors, such as sexual promiscuity, crime, and aggression (Steinberg, 2008). The mental health model would categorize these behaviors as maladaptive outcomes stemming from a negative environment and genetic disposition. This perspective, according to Ellis et al. (2012), only captures the downside of risk without considering why people take risks. Risky behaviors are not maladaptive if the expected benefits outweigh the expected costs. Adolescent behavior, including risky behavior, should, according to the evolutionary developmental model, be examined for potential fitness benefits. It is important to note again the distinction between risky and maladaptive behaviors; risky behaviors may result in harm to the individual, but may still serve an evolutionary (i.e., fitness) purpose. Given this, however, risky adolescent behaviors still frequently produce negative consequences for the individual and the society as a whole and should therefore be examined in an attempt to minimize them. According to Ellis et al. (2012), viable solutions for interventions require understanding why adolescents engage in risky behaviors, rather than just categorizing them as negative and maladaptive. Ellis et al. (2012) argue that adolescence serves as a critical period for individuals to develop and master their social milieu and reproductive strategies. Puberty is characterized by specific maturational changes such as the emergence of primary and secondary sexual characteristics, marked increase of appetite, height, and weight, increase in sensation-seeking, higharousal experiences, and increases in peer aggression and parental conflict. Also apparent is the emergence of sexual motivation and desire to develop and maintain oppositegender relationships. These changes correspond with many of the behaviors adolescents exhibit: the development of sexual characteristics makes an individual more attractive to the opposite gender, which corresponds to the increase in sexual drive and desire for high-arousal experiences. Higher levels of aggression and parental conflict help adolescents develop autonomy and attain social dominance among their peers, which helps explain the higher-stakes competition that occurs in adolescence over mates (or boyfriends and girlfriends), social status, and friendships. Relative to adulthood, the adolescent years are associated with higher rates of depression, eating disorders, alcohol and drug use, violence, and sexual promiscuity. Ellis et al. (2012) argue that social feedback is particularly crucial to adolescents, with natural selection favoring increased emotional, behavioral, and neurobiological responses to activity within the social realm. For example, Chein et al. (2011) suggest that the presence of peers influences activation in the adolescent brain, but not the adult brain. Using functional magnetic resonance imaging, Chein et al. (2011) measured brain activity in adolescents and adults as they made decisions in a simulated driving task across two conditions. The first condition had the participant in the simulated driving task alone, whereas the second condition informed the participants

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they were being watched by their peers. During the peerobserved condition, adolescent, but not adult, brains exhibited activity in the reward centers of the brain (ventral striatum and orbitofrontal cortex), which predicted subsequent risk-taking behaviors. Consistent with Ellis et al.’s (2012) position, these results suggest that peer influence plays a greater role in regulating social behaviors, particularly risktaking behaviors, more during adolescence than in adulthood. Of course, not all adolescents engage in risky behavior to the same extent. The mental health model has been successful in identifying some of the specific environmental conditions that generally produce stressful or supportive environments, which in turn are associated with adolescents’ tendencies to engage in risky behaviors or poor mental health outcomes. Differences along the continuum of parental sensitivity, parental harshness, marital quality, parental mental health, and socioeconomic status are just a few of the factors that the mental health model examines in order to understand behavioral outcomes. But as Ellis et al. (2012: p. 608) state, the mental health model has not been explicitly informed by evolutionary theory and, consequently, has not focused on or delineated basic dimensions of environmental stress and support that guide conditional adaptation. According to this position, guided by life history theory – which postulates that important developmental events in an animal’s life are shaped by natural selection to promote inclusive fitness (see Fertility Theory: Embodied-Capital Theory of Life History Evolution) – individuals make trade-offs that prioritize resource and energy expenditures, such as reproduction (mating, parental investment) and bodily protection (predator defense, immune function), so that greater investment in one domain occurs at the expense of investment in other domains. Life history theory states that individuals will select a strategy that optimizes their trade-offs over the life course. These strategies lie on a slower–faster life continuum. Earlier pubertal timing, more sexual partners, earlier age of reproduction, more offspring with lower parental investment, and more risky behaviors typically characterize a faster life history strategy. A slower life history strategy, on the other hand, is characterized by slower rates of development, delayed age of first sexual encounter and reproduction, fewer offspring with higher parental investment, and exhibiting fewer risky behaviors (see Figure 1).

An Evolutionary Life History Framework: Harshness and Unpredictability Recent research following these models has elaborated on children’s sensitivity to context by going beyond the study of harsh environments and also examining the unique predictive value of variations in environmental predictability as well (Belsky et al., 2012; Ellis et al., 2009; Simpson et al., 2012). Ellis et al. (2009) proposed an evolutionary life history framework, stating how different environmental conditions early in development affect the fast versus slow life history trajectories, specifically the influence of harshness and early unpredictability. Ellis et al. (2009) defined harshness as agespecific rates of morbidity and mortality, predictably measured by socioeconomic status (SES); they defined unpredictability as the fluctuations of harshness across development. Unpredictability is measured by indicators such as frequent residential changes, parental divorce, and

Figure 1 Faster versus slower life history strategies. Reproduced from Ellis, B.J., Dahl, R.E., Del Giudice, M., Dishion, T.J., Figuerdo, A.F., Gray, P., Hawley, P., Jackson, J.J., Jacobs, W.J., Volk, A.A., Wilson, D.S., 2012. The evolutionary basis of risky adolescent behavior: Implications for science, policy, and practice. Developmental Psychology 48, 598–623.

remarriage. Ellis et al. (2009) proposed that each of these environmental factors should uniquely contribute to differential outcomes in life history strategies. For example, in one recent study, family social disadvantage (harshness) predicted deviant, often criminal, behavior and the number of children one had in early adulthood (Dishion et al., 2012). Other research has shown that the predictability of a child’s early environment can have an independent effect on a child’s developmental trajectory. For example, Belsky et al. (2012) tested a model proposing that higher environmental harshness and higher levels of unpredictability in the first 5 years of the child’s life will independently predict an accelerated life history trajectory. They found that unpredictability of a child’s environment in the first 5 years is a more direct predictor of having a larger number of sexual partners, a signal of an accelerated life history strategy, at 15 years of age, while early harshness had a more indirect effect. Simpson and his colleagues (2012) extended this line of research by looking at multiple life history variables, as opposed to just the number of sexual partners, and extended the outcome measure (number of sexual partners) to the age of 23. They also examined whether predictability and harshness are more influential on development when occurring in early (prenatal to the stage of 5-year-old children) vs later childhood (6- to 10-year-old children). Supporting the results in Belsky et al. (2012), they reported that exposure to harshness and

Evolutionary Developmental Psychology (especially) unpredictability in the first 5 years of childhood significantly and independently predicted an accelerated life history trajectory by having higher levels of aggression and incidences of delinquent behavior, more sexual partners, and higher likelihood of being associated with a criminal act. As expected, exposure to harshness and unpredictability in later childhood (6- to 10-year-old children) were not significant predictors of these outcomes. If the environment is unstable and fluctuates often, it may be more beneficial for the individual to develop a faster life history strategy, reproducing faster and more often, in anticipation that future environments will be unpredictable. Alternatively, if one’s early environment is stable, this is a good indication that future environments will also be stable, and a slower life history strategy would be more beneficial in the long run. From an evolutionary perspective, adolescents exhibiting risky behaviors, sexual promiscuity, and early reproduction are exhibiting adaptive behaviors that are best suited for their environmental context. Exactly these patterns have been observed in several recent studies (Ellis et al., 2009; Simpson et al., 2012), independent of measures of harshness, finding strong support for the evolutionary developmental models.

The Evolutionary Developmental Model and Antisocial Behavior An evolutionary developmental perspective can also shed light on antisocial behaviors, such as bullying or stealing. Research by Hawley (1999, 2002, 2003) suggests that behaviors that would normally be characterized as antisocial are often quite successful in attainting resources for individuals exhibiting them, at least in some environments. Hawley (1999) posited three classes of children with respect to resource control: prosocial controllers, coercive controllers, and bistrategic controllers. Prosocial controllers gain and maintain access to resources by cooperating in a group, are socially skilled, and are typically friendly and well liked by peers. Coercive controllers are strictly aggressive in their attainment of resources. They tend to be impulsive, aggressive, and socially rejected. In contrast, bistrategic controllers use both prosocial/cooperative and antisocial/coercive strategies to attain and maintain resources, but tend to be socially savvy, attractive social partners, who also exhibit aggression and bullying similar to that of coercive controllers. From this perspective, aggressive behaviors, although disruptive in a classroom and potentially leading to injuries, especially among adolescents, serve to obtain resources, access to mates or friends, and achieve social dominance (Sijtsema et al., 2009; Veenstra et al., 2010), with some children using aggressive behaviors strategically in combination with prosocial behavior. Furthermore, the adaptive benefits of bullying are evident by data that children identified as bullies tend to date at younger ages and engage in more dating activities than their nonbully counterparts (Connolly et al., 2000; Faris and Felmlee, 2011; Gallup et al., 2011). In summary, aggression may have evolutionary benefits in attaining resources and social dominance, and ultimately, mating opportunities. The fact that aggressive behavior, often in the form of bullying, frequently has some benefits for children and adolescents in no way condones such behavior,

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but rather provides insights into why some intervention programs are destined to failure and others have a chance of succeeding.

Conclusion Darwin’s theory of natural selection is the driving force for modification and survival over time. Evolutionary explanations have been widely used to examine adult behavior and psychology; however, the field of evolutionary developmental psychology contends that natural selection acts throughout ontogeny and not simply only in adulthood. Individuals must survive the often-hazardous periods of infancy and childhood before reaching adulthood, suggesting that any adaptations that increase the chance of surviving to adulthood during the early stages of life would certainly be favored by natural selection. An evolutionary approach to infancy, childhood, and adolescence provides researchers and practitioners with new insights with which to view both typical and atypical development. In particular, when ‘problematic’ behavior, such as risk-taking or aggression, is observed in children or adolescents, an evolutionary approach causes one to ask “What’s in it for the child?”. That is, although bullying or sexual promiscuity in children and adolescence, for example, are clearly behaviors that society has every right to want to minimize, an evolutionary developmental perspective recognizes that these behaviors may provide some adaptive value for some children in some specifiable contexts. Applying evolutionary ideas to help understand both typical and atypical development does not mean that the other accounts (e.g., learning, social, cognitive) are unnecessary. Rather, an evolutionary perspective supplements other approaches to understanding development and, in the end, produces a more complete, and from an application perspective perhaps more useful, account of development. In fact, one might modify once again Dobzhansky’s famous statement and say that “nothing in childhood and development make sense exception in the light of evolution” (cf Konner, 2010).

See also: Cognition, Evolution of; Darwinism; Epigenetic Inheritance; Fertility Theory: Embodied-Capital Theory of Life History Evolution; Human Mate Choice, Evolution of; Microevolution.

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