Chapter 80. Fundamental Concepts of Child Development

No single construct has been more central in the development of the biologic sciences than homeostasis. Although the term homeostasis was coined in the 20th century, its conceptual origin can be traced to the notion of a stable, relatively unchanging internal environment, which was first described by Claude Bernard in the 19th century. Bernard recognized the fragility of life, surrounded as it is by a constantly threatening, aversive, and often pathogenic environment, and he argued that viability in the face of external challenge depends on an organism’s capacity for protecting its internal milieu. In this context, homeostasis is a dynamic, self-regulating process that ensures constancy and permanence in the internal physiologic state through complex, multilevel feedback systems that respond to a deviation in one direction with a countering adjustment in the opposite direction. Thus, the fundamental goal of a homeostatic system is to maintain an inerrant “set point” that assures stable and continuous biologic functioning. The regulation of body temperature, cortisol suppression of adrenocorticotropic hormone (ACTH) secretion, and glycogenolysis during periods of hypoglycemia are all examples of feedback loops that protect the continuity and equilibrium of an organism’s interior.

Whereas homeostasis governs regulatory strategies within the tissue, cell, or subcellular structures, the closely related concept of adaptation refers to the behavioral and biologic activities that promote the survival of individuals or groups. In evolution, adaptation involves the selective preservation and reproduction of organisms and characteristics of organisms that offer survival benefits in the face of external threats.1 Beyond evolution, however, adaptation has been used to describe complex social and individual developmental processes that respond to specific environmental challenges. For example, daytime continence emerges in a 3-year-old child within a context of growing parental expectations for toilet training; a preschooler clings to a tattered but revered blanket (a so-called transitional object) to calm his or her uncertainties and fears about attending a new childcare center; and a 12-year-old girl exhaustively discusses her first menstrual period with friends as a means to cope with the complications and challenges of sexual maturation. At all stages of development, the capacity to weather, absorb, and find meaning in the vicissitudes of life is one of the defining characteristics of humankind.

Many novel, challenging life experiences are commonplace and are accessible to a range of homeostatic and adaptive strategies. However, children also encounter circumstances that strain their adaptive capacities and may present acute or chronic stressors that exceed their ability to cope. Indeed, psychosocial stress has been defined as those environmental demands or threats that overtax an individual’s ability to adapt. When such conditions are encountered, a variety of biologic and behavioral responses are evoked; if sufficiently intense or prolonged, such responses can lead to the development of a diagnosable disorder.

Research in humans suggests that 2 principal, interrelated systems are involved in the neurobiologic response to stress: (1) the corticotropin-releasing hormone system, and (2) the locus ceruleus–norepinephrine system.2

The corticotropin-releasing hormone system, centered in the paraventricular nucleus of the hypothalamus, stimulates the release of adrenocorticotropic hormone and proopiomelanocortin by cells of the anterior pituitary gland, which in turn trigger secretion of adrenal glucocorticoids, thereby altering blood pressure, glucose metabolism, immune processes, and behavior. Noradrenergic neurons from the locus ceruleus–norepinephrine system of the medulla and dorsal pons activate the sympathetic arm of the autonomic nervous system, raising blood pressure and heart rate and promoting vigilance.

These two highly interactive response systems mediate successful neurobiologic adaptation to stressors but play a contributing etiologic role, as well, in the pathogenesis of stress-related physical and mental health disorders. In some cases, such disorders constitute a failure of homeostatic and adaptive processes; in other cases, they reflect the capacity of dysfunctional, exaggerated adaptive processes to cause disease. Associations between emotionally stressful experiences and adverse, maladaptive health outcomes have become increasingly well substantiated in both adults and children. Although debate continues about whether stressors cause specific pathologic conditions or simply alter generalized host susceptibility, little doubt remains that both chronic adversities and acute stressful events elevate risks of physical and mental disorders.

Both clinical experience and epidemiologic observations suggest that not all children are equally vulnerable to psychosocial stressors. In fact, homeostatic and adaptive capacities appear to be quite unevenly distributed within human populations.3 Some children succumb to a succession of modestly stressful events, while others seem able to sustain normal functioning and health through even the most adverse and emotionally trying circumstances. Observations of children with varying levels of neurobiologic sensitivity to environmental challenges raise questions about the universality of stress-illness linkages and underscore the importance of individual differences in children’s behavioral and biologic responses to the social world.

A common occurrence in pediatric practice is a parent’s expression of elation or dismay on discovering how different in personality and behavior a second child is from the first. In families with several children, one may be highly susceptible to broken bones, falls from heights, unusual rashes, or a seemingly endless series of ear infections or colds. One child may be thoroughly outgoing, while another is timid and withdrawn. A daughter may be an outstanding athlete, while a son has not even a passing interest in sports. Physicians certainly are aware of the range from quiet calm to noisy fearfulness that accompanies the intrusion of a physical examination or the discomfort of an immunization.

Each of these experiences is a reminder that one of the more celebrated and universal characteristics of childhood is its marked diversity. A central task of pediatric medicine is to discover, honor, and respond to this diversity in a manner that supports parents in their efforts to nurture the individual talents of their children.

While observations regarding such differences extend back to ancient Greek civilization, the systematic study of temperamental differences began with the New York Longitudinal Study of Stella Chess and Alexander Thomas. Although researchers differ in the extent to which they view temperament as a stable, inherent characteristic, all agree that the concept itself describes a set of individual predispositions that underlie and modulate the expression of activity, emotionality, and sociability. In this study, clusters of behavioral styles were constructed to identify “easy,” “difficult,” and “slow-to-warm-up” children.4 Further study suggests that temperament has both behavioral and neurobiologic aspects. Behavioral differences generally are arrayed along dimensions such as activity level, adaptability, intensity, and mood; neurobiologic differences include the physiologic responses to stressors and challenges that reflect internal reactivity to environmental events.

Developmental researchers now have a greater understanding of the significant associations between the behavioral and psychobiologic facets of temperament. For example, 3 components of autonomic nervous system reactivity—threshold (ie, the stimulus intensity required to elicit a response), dampening (ie, return to baseline following a stimulus), and reactivation (ie, rearousal with repeated stimulation)—may be biologic analogs of the original behavioral descriptions that defined distinct temperament clusters. Thus, the child with an “easy” temperament might have a high threshold and strong capacity for dampening, whereas a “difficult” child might have a low threshold and capacity for dampening but be strongly predisposed to reactivation.

The origins of differences in behavioral and psychobiologic “style” are not completely known, but they appear to be interactively determined by genetically based predispositions, environmental exposures, and the epigenetic effects of experience on gene expression. For example, individual differences in shyness and timidity may be plausibly derived from constitutional differences in gene sequences, from personal experiences, and/or from experiential modifications of the epigenome (that is, the packaging or structural state of DNA) that are capable of changing gene transcription.5

An appreciation of individual temperament differences is important in the practice of pediatrics not only because of their impact on development and behavior but also because of their potential link to both mental and physical health. For example, preschool children with extreme shyness may be at heightened risk for anxiety disorders during middle childhood or for panic disorder and agoraphobia (ie, fear of being in large, open spaces) as adults. Children with exaggerated cardiovascular or immunologic responses to stressors appear to have elevated incidences of injuries and respiratory infections during periods of naturally occurring stressors such as residential moves or parental divorce.

While the mechanisms underlying these associations are debated, available data suggest that certain subsets of children may have an impaired ability to self-regulate their behavior, physiologic functions, and subjective experiences of somatic pain. One possible explanation for impaired self-regulation is that children who display certain behavioral and neurobiologic phenotypes (eg, shyness and its associated predisposition to autonomic arousal) have an underlying, heightened sensitivity to the social environment and a relative inability to monitor and constrain their behavioral and physiologic responses.

A capacity for recognizing, monitoring, and containing the emotional “coloring” of environmental events is a critical early developmental achievement. Infants’ interactions with caregivers, principally with their mothers, guide and shape the unfolding of affective experience and expression during the first months of life. Later, as maturation proceeds, the regulation of emotional experience becomes less dependent on caregivers and more accessible to a child’s emerging self-control. Such individual differences in the capacity for self-regulation may emerge as an important determinant of mental and physical well-being. In a moment of prescience 250 years ago, Thomas Sydenham wrote that the cause of “nervous disorders” may lie partially in “the temperament of the body . . given us by nature.”

Unlike almost all other species, humans experience a prolonged period of helplessness and total dependence early in life. In fact, it is striking that the species demonstrating the greatest capacity to control its environment is so incapable of meeting even its most basic survival needs on its own during infancy. Consequently, the relationship between an infant and his or her primary caregiver(s) is a fundamental requirement for healthy human development.

The initial bond and growing attachment that characterize an infant-caregiver relationship are grounded firmly in biology. As described by John Bowlby, newborns and their parents are genetically programmed to form strong attachments to each other. Young infants respond preferentially to the image of a human face and to the higher pitched sound of a mother’s voice. In turn, caregivers are naturally captivated by the magnetism of a baby’s smile and the urgency of his or her cry. These core attachment behaviors have been documented in a variety of family configurations and across a broad range of cultures.6

The defining characteristics of healthy, growth-promoting, early human relationships are embodied in the social concepts of reciprocity and contingency. Thus, when young children and their caregivers are “tuned in” to each other, their interactions are adaptive. During the early years of an infant’s life, much of the responsibility for promoting a harmonious relationship rests on the caregiver’s ability to read the baby’s cues and to respond appropriately. When a caregiver’s responses are contingent, predictable, and attuned to the infant’s feelings, the young child experiences an early sense of security, personal efficacy, and positive self-worth. This leads to what Erik Erikson labeled “basic trust,” or the phenomenon through which outer predictability leads to a sense of inner certainty.7 For most parents, getting to know their babies and learning to read their signals is a highly rewarding experience that evolves naturally without the need for professional assistance. However, challenges to this relationship-building process may originate in either partner—from the infant who is relatively unresponsive or “difficult to read” as a result of prematurity, neurologic impairment, chronic illness, or extreme temperamental style; or from the caregiver whose capacity to nurture is compromised by inexperience, psychological disturbance, or severe external stressors such as poverty or social isolation.

Establishment of a secure attachment with a small number of key caregivers provides a firm foundation for healthy cognitive, social, and emotional development. Essential to this process is the need for a secure and trusted base from which the developing child can venture forth to explore the larger environment and to develop a differentiated identity as an autonomous yet socially connected individual. Thus, the average 1-year-old child seeks maternal closeness, the 15- to 18-month-old child begins to stray cautiously from the parental orbit (with frequent returns for “refueling”), and the healthy 2-year-old child is off and running (and rarely looks back!). As the process of separation and individuation unfolds, the adaptive child navigates a delicate balance between the maintenance of strong interpersonal bonds and the mastery of both physical and psychological independence.

During the first 6 months of life, most infants respond positively to anyone; during the second 6 months, they seek preferential closeness with their primary caregivers and begin to show signs of stranger anxiety when confronted by unfamiliar persons. Throughout the second year, children and caregivers negotiate a gradual disengagement from their intense, highly personalized attachment relationship. Whereas the younger infant assumes that “mother is always there,” the emerging toddler becomes acutely aware of his or her own separateness and demonstrates varying degrees of “separation anxiety” behaviors that mark this important phase of development. For many children, a transitional object, such as a special blanket or stuffed animal, serves a vital symbolic function to facilitate the mastery of this fundamental separation challenge. By the end of the third year, most children are able to tolerate the temporary absence of their primary caregivers and accept the company of unfamiliar adults with minimal difficulty.8

Extensive research has demonstrated the far-reaching benefits of strong, early attachments and the adaptive resolution of necessary and inevitable separations. During infancy, children with secure attachments engage in richer exploratory behavior, demonstrate more sophisticated problem-solving skills, and exhibit more positive affect. During the preschool years, secure attachments are associated with better peer relationships, higher self-esteem, and a greater capacity for empathy. The ability to form increasingly mature and stable relationships into adulthood is presumed to be influenced by one’s early attachment experiences. Ongoing tensions between the development of personal autonomy and the nurturance of meaningful social relationships represent a fundamental life challenge.

The concept of individual competence and the intrinsic drive to master one’s environment are basic features of human development throughout the life cycle. This begins in early infancy with the inductive process through which babies learn about the nature of the physical and social world by their own experiences and actions.9

To a certain extent, differences in the level and quality of performance among individuals are manifestations of differences in biologic endowment. However, human abilities do not develop independent of the context in which people live. Thus, individual competencies in children are shaped by the degree to which the early caregiving environment and ongoing life experiences provide both opportunities to learn and the support needed to take advantage of such opportunities. The talented athlete who combines natural grace with a commitment to long hours of practice and the musical prodigy whose parents arrange for piano lessons in the preschool years are examples of this dynamic interplay between biology and the environment. Long-standing battles over the extent to which human competence is influenced by nature (ie, genetics/constitution, as underscored in the maturational model popularized by Arnold Gesell) or by nurture (ie, experience/environment, as emphasized in the learning theory developed by James Watson and B.F. Skinner) reflect exercises in futility. The question is not which is important but how each contributes to ultimate outcomes.

The traditional approach to studying and assessing children’s abilities has been to focus on a core set of performance domains. Although these domains are interdependent and the boundaries among them can be somewhat arbitrary, it is helpful to review each independently.

In the physical realm, increasingly refined motor control develops in a cephalocaudal and proximal-to-distal pattern. Influenced by both neuromaturation and practice, the development of discrete motor skills continues from early infancy through adult life. In the gross motor area, children begin by establishing head control and the ability to roll over, progressing to maintaining a sitting position and walking independently. They then progress through the mastery of increasingly complex skills such as riding a bicycle, participating in competitive sports, and, in rare circumstances, pursuing a successful career as a professional dancer or gymnast. In the fine-motor area, children begin by reaching for objects and establishing a fine pincer grasp. They then move on to more complex tasks such as cutting with scissors and writing legibly and, in some instances, may achieve ultimate success during adulthood in the arts of calligraphy or microsurgery.

The development of cognitive competence reflects a range of intellectual capacities that distinguish humans from all other living creatures. From earliest infancy, children are programmed biologically to learn about the world through their own actions and the ongoing construction of their own internal mental representations. Much learning depends on the extent to which a child’s environment provides appropriate opportunities and supports, but much of the energy fueling the development of cognitive competencies arises from the child’s own initiative.

In the early years, the thinking of young children is characterized as “egocentric” (ie, they are unable to view the world from any but their own perspective). Thus, a great deal of cognitive maturity is embedded in the process of gradual “decentration,” whereby children develop a growing appreciation of how the world is perceived and understood by others.

Based on the seminal work of Jean Piaget, cognition can be viewed as progressing through 4 discrete stages. The first stage, termed sensorimotor, extends from birth to approximately 18 months of age. During this period, the child’s knowledge of the world is grounded in his or her motor activities and sensory experiences. Coinciding with the emergence of a sense of one’s existence as a separate human being, children learn that objects exist even when they are no longer visible (ie, “object permanence”), and they develop an appreciation for the relation between actions and consequences (ie, “causality”). The second stage of cognitive development, termed preoperational, generally extends from 18 months to 7 years of age. The hallmark of this period is development of the capacity for representational thinking, symbolic functioning, and the emergence of fantasy in language as well as in play. The “magical thinking” of the preschool years results in delightful explanations of natural phenomena (eg, “rain comes from God crying”) as well as the risk of viewing illness as a punishment for misbehavior. The third cognitive stage, termed concrete operational, typically extends from ages 7 to 11 years. During this period, children are capable of logical mental manipulations, and their thinking reflects an ability to appreciate several dimensions of an issue at the same time. For example, as demonstrated in Piaget’s classic experiments, the school-aged child can understand that a short, fat cup can hold as much water as a tall, thin glass. Finally, beginning in early adolescence or later, the cognitive stage of formal operations is reached. During this period, individuals engage in abstract reasoning, which gives them the power to manipulate ideas rather than remain restricted to the concrete world. For the reflective adolescent and young adult, this provides a framework for passionate discussions about morality, values, and philosophic principles.

Closely related to the realm of cognition is the development of a symbol-based system of communication known as language. Comparable to the emergence of their knowledge about the physical world, children develop communicative competence because they have the innate ability to “discover” the rules that underlie language function and not simply to imitate what they have heard. Although we generally take for granted these abilities to interpret what we hear and to speak, it is important to appreciate how remarkable it is that young children are able to process the verbalizations of people around them and to produce unique word combinations that they have never before heard.

The study and assessment of communication distinguishes between speech, which refers to the physical act of talking, and language, which refers to the underlying symbol system. During the first 12 months of life, infants progress from cooing (ie, vowel sounds) to babbling (ie, consonant sounds) in conjunction with a growing appreciation of the social context of communication, which is characterized by selective attention to conversation, turn taking, and interpretation of a repertoire of nonverbal signals. By the end of their first year, most children have mastered virtually all the sounds of their native language and have produced their first true words—and those who are raised in a bilingual environment will master the sounds of both languages without difficulty. During the second year, expressive language progresses toward increasing competence in syntax (ie, rules of grammar). By 3 years of age, most children are able to communicate their thoughts through coherent narratives, and by age 5, the structure of their language begins to approximate that of adults.

Beyond the domains of physical and cognitive/linguistic skills, expanding competence in the realms of emotional development and social relationships represents another critical maturational agenda. From the basic reciprocal interactions that characterize the early infant-caregiver bond through the intense process of separation-individuation and the development of core family ties, peer relationships, and mature adult intimacies, humans are essentially feeling and social creatures. Thus, as children develop a growing sense of themselves as separate individuals, they develop the capacity to look both inward and outward.

The identification and understanding of affect, capacity for empathy, emerging sense of morality, ability to form meaningful relationships, and growth of both self-concept and social perspective are some of the central dimensions of socioemotional development that show continued growth from early childhood through the later adult years. Attempts to understand the underlying processes in this domain of personal competence have generated some of the richest theoretical contributions to our understanding of human maturation. In the classic Freudian formulation, social and emotional well-being depend on the successful resolution of intense conflicts between the child’s innate pleasure-seeking drives and the limits placed on the satisfaction of those urges by parental control and cultural mores. Following Sigmund Freud’s seminal theories, Margaret Mahler and her colleagues viewed the early process of separation and individuation as the essential means through which young children achieve “psychological birth” and a beginning sense of their own individual identity. Erikson developed an elaborated life span conceptualization of psychological identity extending from the fundamental establishment of a basic sense of trust in early infancy through the sequential challenges of increasing autonomy, skill, and a sense of personal efficacy, intimate relationships, and generativity.7 As in all other areas of development, success in the emotional and social domains depends on both intrinsic constitutional abilities (eg, temperamental style, sensitivity to social cues) and the influence of supportive relationships (both within and outside of the core family unit).

In summary, human competence is manifested in a wide range of domains involving physical prowess, mental problem solving and abstract reasoning, emotional regulation, and social sensitivity. The range of abilities in the general population is broad, and the relative value assigned to differential achievement varies among families and across cultures. In a highly competitive society, the natural human drive toward mastery is exaggerated by intense social and economic pressures. In contrast, a culture characterized by nonhierarchical, egalitarian values offers broader acceptance and support of a wider range of ability. The transmission of these values, through both the family unit and the wider culture, is likely to have a major influence on the evolution of individual self-concept as well as on how people view each other.

The extent to which human development is a continuous or a segmented process is the focus of much debate. Proponents of a discontinuous model point to the dramatic qualitative changes that are apparent when one takes a long-term view over the course of the full childhood period. Thus, 15-year-old children do not simply know more than 7-year-old children (or 7-year-old children more than 2-year-old children); they also know things in a different way. On the other hand, qualitative changes occur gradually, not by great leaps, which suggests an essentially continuous process.

Many of the greatest contributions to our knowledge of child development have been conceptualized within the context of stage models; Piaget’s work on cognition and Freud’s stages of psychosexual development discontinuities are examples (eTable 80.1). Despite the salience of stage-related theories of development, however, the day-to-day reality of individual human function is certainly not marked by dramatic qualitative shifts. A reconciliation of these seemingly contradictory positions can be found in the concept of a developmental transition, which refers to the transformation from one discrete stage to the next. One way to understand developmental transitions is to think of them as times of structural reorganization (ie, periods of psychological disequilibrium reflecting elements of both the stage being completed and the stage yet to begin). Intense negativism in a toddler who is attempting to reconcile strong feelings of attachment to his mother with the natural drive for autonomy is one example. The need to balance core family ties with adolescent rebellious impulses to achieve a healthy adult identity is another example.

One important consequence of the qualitative change that characterizes human development is the relative instability of individual differences in competence. That is, except in cases of severe disability, the rank ordering of individuals within specific developmental domains often shows dramatic variability over time. Part of the variance in performance relates to the ongoing influences of differential life experiences. However, beyond the role of environmental impact is the inherent nature of stage-related, qualitative developmental discontinuities. For example, the “brightest” 15-month-old child will not necessarily be the “smartest” 10-year-old, because block stacking and the rapid completion of form boards reflect different skills from those needed to excel in verbal and quantitative reasoning.

A related phenomenon is the concept of “critical periods” of development. Derived from the discipline of ethnology, it is postulated that certain developmental accomplishments must be mastered within a particular time frame, after which the “window of opportunity” is lost irrevocably and permanent dysfunction becomes inevitable. Although critical periods have been demonstrated for a number of behaviors in a variety of animal species (eg, imprinting in geese), as well as for the development of binocular vision in cats and humans, absolute critical periods have not been documented for any specific aspect of cognitive competence in humans. Alternatively, the concept of “sensitive periods” has been suggested to indicate that although their later emergence is not impossible, certain developmental achievements are best mastered during particular periods in the development of brain circuitry. The establishment of basic trust in early infancy and the development of language by the end of the preschool period are 2 examples of competencies that are postulated to be difficult to achieve beyond their sensitive periods.

Perhaps the construct that best captures the complex issue of developmental continuity-discontinuity is the concept of epigenesis. Derived from the science of embryology, the epigenetic model depicts a dynamic process in which a highly differentiated organism evolves from a completely structureless germ cell through the complex, reciprocal impacts of environment and protoplasm. Thus, like the dramatic changes that characterize the marked transformation from zygote to embryo to fetus to neonate, human development and behavior unfold through a highly interactive process marked by progressive differentiation, individual continuity, and dramatic qualitative change.10

An important advance in our knowledge of human development over the past few decades has been a growing appreciation of the contextual nature of complex developmental processes. Stated simply, despite its strong biologic underpinnings, the development of children is highly influenced by the multiple environments in which they live and by their individual experiences within those environments, a phenomenon described by Urie Bronfenbrenner as the ecology of human development.11,12 In different circumstances, the environment can be viewed as either supportive or detrimental. Most frequently, different aspects of the same environment can serve as sources of both protection and risk.

The most proximal aspect of a child’s caregiving environment is the family unit, and at its core is the special intimacy characterizing the daily interactions that take place between an infant and his or her primary caregiver(s). These dyadic relationships are themselves embedded within a dynamic family system that can be highly variable in structure and in the way it affects the development of its members. As children grow older, they are influenced by a wider variety of overlapping relationships that make up the larger family unit. These may include ties to parents, siblings, grandparents, and other members of the extended family. Within these relationships, differences in personal investment, overt or covert rivalries, and temperamental matches or mismatches all contribute to the considerable diversity of family experiences shaping the personalities of children as they grow up.

In addition to variations in their membership and structure, other important differences in family characteristics can have significant impacts on the development of children. Families may be cohesive, fragmented, or enmeshed. They may be flexible and highly adaptable or rigid and incapable of adjusting to change. Attitudes toward child rearing may be strict and authoritarian or permissive and nonhierarchical. Standards for individual performance and behavior may be high or low. Approaches to discipline and punishment may be harsh or forgiving. Interactions may foster intense competition or convey a strong egalitarian message.

All children who live in the same household experience both a shared and a unique environment. To the extent that a distinctive ambiance characterizes the family unit, the environment is shared. However, the fact that individual family members influence and experience each other in different ways means that each child occupies a relatively unique environmental niche.

Above and beyond the powerful impacts of their family, children also are influenced by the communities in which they grow up. Similar to the family, a community can be an important source of protection or vulnerability, depending on its material and spiritual resources. For example, a local neighborhood may provide high-quality, easily accessible, and affordable child care, or it may have a fragmented and poor-quality infrastructure for working families. Available recreational facilities may include safe and attractive parks and playgrounds or dangerous and foreboding, abandoned buildings and empty lots. A school system may be well supported and rich in creativity and nurturance, or it may be poor in resources and diminished in morale. A community may embody a sense of pride and joint ownership in its shared way of life, or it may be depleted in spirit and devoid of any sense of meaningful interconnection.

Finally, families and communities are themselves embedded, like nested cubes, in a broader culture that reflects a particular set of values and traditions. Cultural characteristics that are likely to influence the development of children include religious rituals, attitudes toward gender roles, traditional approaches to discipline, and the extent to which ethnic discrimination and racism influence social, economic, and political institutions. For minority groups and newly immigrated families, conflicts with the majority culture present significant challenges to personal development, which may result in both positive and negative outcomes.

The ultimate mystery of human development is the complex process through which threads of genetic endowment and contextual experience are woven together to form the fabric of a maturing, growing person. Indeed, all developing human beings are products of nature, nurture, and what Arnold Sameroff described as their complex transactions.

A vast array of phenotypic characteristics is encoded in the genome of each individual. Such characteristics define an array of physical parameters such as facial features, hair and eye color, and potential for linear growth. Experienced pediatricians are well attuned to the atypical phenotypic features of children with chromosomal anomalies (eg, trisomies, deletions, nondysjunctions) and the metabolic consequences of specific gene mutations (eg, sickle cell anemia, phenylketonuria). Typically less well-known or understood, however, are the genetic influences on psychological and socioemotional development, including those involved in the regulation of intelligence, temperament, and personality.

The heritable portions of psychological characteristics are polygenic in origin (ie, the products of multiple interacting genes). For example, genetic analyses suggest that polygenic influences account for approximately one half of the variance in intelligence quotient scores.

Increasingly, virtually all human developmental variation—both physical and psychological, both normative and pathological—is becoming viewed as the product of interactions among multiple genes and multiple environmental exposures. Even morbidities considered purely genetic (eg, phenylketonuria) or purely environmental (eg, a bicycle accident) can be seen, in larger context, as products of gene-environment interplay (eg, the roles of dietary phenylalanine or of genes predisposing to risk-taking behavior). Such interplay can involve gene-environment correlation (where, for example, genetic and environmental risk are co-conferred by one’s parentage), effects of genes on environments (eg, how an infant’s genetic predisposition to irritability can alter the family context), effects of environments on genes (how stressful early events may change the expression of adversity-related genes), or gene-environment interactions (eg, where allelic variation in the serotonin transporter gene produces risk for depression but only under conditions of exposure to many stressful life events).

Because children in the same family share both genomic and environmental influences on their development, disentangling the effects of each is a challenging task. Furthermore, the techniques of selective breeding and experimental exposure to different environments are restricted to studies of laboratory animals, so human behavioral genetics must rely on the less definitive findings of family, adoption, and twin studies. For example, pairs of monozygotic (identical) and dizygotic (fraternal) twins raised in the same family can be examined to ascertain the degree to which genetic relatedness results in similarities of behavior or psychopathology. Conversely, environmental effects can be assessed by studying differences in outcomes among monozygotic twins who were separated during infancy and raised in different families; to whatever extent variations in outcome are not attributable to environmental causes, the remaining variance generally can be assigned to the genome.

Research on schizophrenia, for example, shows a concordance rate of 30% among monozygotic twins, a figure that is 30 times higher than the 1% base rate of the disorder in the general population. While this finding suggests a strong genetic contribution to schizophrenia, the 30% concordance rate is far from the rate of 100% that would be expected if the disorder were purely inherited. Thus, data on schizophrenia suggest that nongenetic factors also play a strong, complementary role in its pathogenesis. Adoption studies of individuals with bipolar and unipolar depression have shown both genetic and environmental influences as well, as have studies of autism, anorexia nervosa, attention deficit hyperactivity disorder, and delinquency. In some conditions, genetic potentialities are triggered or revealed only in specific environmental contexts. For example, heterozygotes for sickle hemoglobin are resistant to malaria, which conveys a selective advantage that probably augments the prevalence of the gene in certain parts of the world. In each of these examples, the population rates of an illness phenotype depend on the prevalences of both genetic risk factors and environmental triggers.

Another example of the complexity of gene-environment interaction involves the traditional assumption that 2 children raised in the same family experience the same child-rearing environment. This conventional wisdom has been challenged by observations that siblings growing up under the same roof often experience quite different “families” and that these unshared aspects of the family environment can have important effects on behavioral and developmental outcomes. Indeed, it now appears that the most powerful environmental influences on behavior and psychopathology are those that derive from these unshared family experiences. Such differences in experience probably occur through a variety of mechanisms. A parent in an unsatisfying marriage, for example, may single out one child for maltreatment. Alternatively, children may have a different experience because of the unique perspective each brings to the family system and because of the differential responses each child elicits from all other family members.

While context cannot change the structure or sequence of the genome, certain experiences may have direct regulatory effects on the transcription of genetic material. For example, environmental stressors play a much stronger etiologic role in the development of a first major episode of clinical depression than they do in subsequent episodes of the same disorder. One way of accounting for this observation is the possibility that repeated neuronal transmission in certain central nervous system structures sets in motion intracellular processes, known as kindling, that may alter gene transcription. Such alterations in gene expression may leave behind memory traces, at the level of neuronal function, that produce a sustained increase in the risk of subsequent depressive episodes.

C.H. Waddington proposed a visual metaphor for these complex developmental transactions that unfold between genes and environment. He suggested that development is like a ball rolling downhill through a landscape of valleys and ridges. The farther the ball rolls, the deeper the valleys and the steeper the walls of the ridges become, rendering diversion into different paths less likely as the process proceeds. Developmental trajectories that are deeply ingrained, with little potential for environmental influence (eg, limb morphology, development of gender, independent ambulation) are referred to as canalized characteristics. Other pathways, such as those that direct a child into normal or disturbed trajectories of emotional development, have less-steep walls and remain more susceptible to the effects of life experiences. Thus, the likelihood that a child will enter a path of abnormal emotional development is a product of both genetic “momentum” and environmental constraints derived from the influences of family, siblings, teachers, and friends.

The importance of the continuing debate about the origins of variability in human development and behavior lies in the increasing prevalence of psychosocial morbidities in contemporary societies. In far too many circumstances, children and their families are surrounded by a grim harvest of increasingly maladaptive development, the seeds of which can be found among problems such as poverty, racism, generational mental illness, substance abuse, and risk-taking behaviors. A deepening understanding of the science that underlies developmental-behavioral pediatrics may contribute to the preventive interventions and effective management approaches that will offer truly comprehensive care for children and their families.

Additional Readings