Overview of Development, Temperament, and Risk Factors
How we come to be the way we are is through the process of development. Generally defined as the lifelong process of growth, maturation, and change, development is the product of the elaborate interplay of biological, psychological, and social influences. By studying development, researchers hope to uncover the origins of both mental health and mental illness.
This section elaborates and extends concepts introduced above regarding the fundamental workings of the brain at different developmental stages. It then proceeds to explain several seminal theories of development pioneered by Jean Piaget, Erik Erikson, and John Bowlby. Their theories cover cognitive development, personality development, and social development, respectively, although there is some overlap. Their major works, published in the 1950s and 1960s, were pivotal for the psychological and social sciences, galvanizing a huge body of theoretical and empirical research. However, with the advancements of science and the diversity of the population, these models may not apply to all groups without some adaptation for cultural context. The section concludes with a reminder that the brain is the “great synthesizer” of the many biological, psychological, and sociocultural phenomena that make us who we are.
Physical development of the nervous system provides the architecture for mental function (cognition, mood, and intentional behavior). As can be inferred from the discussion of brain complexity in the introductory section, nervous system development is arguably one of the most monumentally complicated developmental achievements. One hundred billion neurons must form elaborate and precise arrays of interconnections. Neurons begin the developmental process as undifferentiated cells, cells so seemingly anonymous that they are almost indistinguishable from other cells in an embryo. On the basis of genetic and epigenetic10 influences, the cells must first specialize, or differentiate, into neurons, migrate to their final position, and then send their growing axons (the branch of a neuron that transmits impulses) to project over long distances in order to form synapses with distant target cells (Kandel et al., 1995).
Most neurobiologists are astounded at the level of precision that neurons achieve in their interconnections. The process of nervous system development has been studied at increasingly complex levels—molecular, cellular, tissue, and behavioral levels. Yet, while researchers have charted many of the behavioral milestones of development because they are so amenable to observation and analysis, far less is known about molecular, cellular, and tissue interactions that underlie them.
Four overarching findings or organizing principles have been gleaned from decades of neuroscience research. The first finding is that the formation of connections between neurons and their target cells depends on axons growing along anatomical pathways that are studded with signaling molecules, much like landing lights illuminate the runway for a descending plane. The second finding is that an axon’s reaching the vicinity of, and locating, its correct target cell depends on diffusable chemical signals being transmitted from the target cell. The third finding is that if an axon does not reach its correct target, it is likely to die. This phenomenon, known as cell death, or apoptosis, is so common that it affects up to half of all developing neurons. The brain overproduces the number of cells it needs, from which it pares down to only the correct connections (Kandel et al., 1995). Finally, neuron activity is essential to strengthening the connections that are formed. In other words, stimulation from the environment—which is translated into neuron activity—is vital for the forging of normal neural development (Shatz, 1993; Kandel, 1995). This is a fundamental principle that is revisited later in this section. This principle helps to explain why, for example, babies who are deprived of a stimulating environment during their first year sometimes suffer irreparable developmental effects.
Behavior at birth consists of a repertoire of simple reflexes, that is, inborn neurological reactions that are involuntary in nature. Two examples are the sucking reflex and the rooting reflex,11 both of which are designed to ensure food intake. Over time, the infant displays an expanded repertoire of fine and gross motor skills (e.g., crawling, walking) that begin to unfold in the first few months and year of life. These include the cherished ability to smile, which helps to solidify a social bond with parents and caregivers. What begins as a child’s biological survival need for food—evidenced by such behaviors as rooting and sucking—can turn into a social, interpersonal experience with the caregiver, as in the smile of an infant at the sight of a nurturing parent. These burgeoning motor capabilities are the forerunners of more complex behavioral and mental functions, but the actual relationships between early and later abilities, and their molecular and cellular basis, are understood only in the most rudimentary terms.
Theories of human development are grounded in the developmental perspective. The developmental perspective takes into account the biological, social, and psychological environment; their interaction; and their combined effect upon the individual throughout the life span. Developmentalist L. Breger (1974) proposes that the developmental perspective incorporates three key precepts:
Each of these precepts is thought to apply to neurobiological development, as well as behavioral/psychosocial development. Moreover, each has implications for whether an individual experiences either healthful or unhealthful development that may lead to a mental disorder.
The three precepts are at the heart of each of the three major mainstream theories of developmental psychology that have guided research and increased our understanding of both normal and abnormal human development across the life span. The following paragraphs offer brief sketches of the developmental theories of Jean Piaget, Erik Erikson, and John Bowlby; again, these sketches are provided to afford the reader an historical perspective of research on psychological development.
Piaget: Cognitive Developmental Theory
Jean Piaget formulated one of the most influential theories of cognitive development (Inhelder & Piaget, 1958). Its focus was on cognitive (intellectual) development, that is, the processes by which children come to know and understand the world. Other aspects of human growth, both physical and emotional, are beyond the scope of his theory. Piaget posited that each step of cognitive development proceeds from the previous step in a fixed pattern, beginning at birth and ending in the teen years.
Erik Erikson: Psychoanalytic Developmental Theory
The psychoanalytic theory of development is best exemplified in the work of Erik Erikson, a psychoanalyst who expanded upon Freud’s original theories of psychosexual development. One of Erikson’s pioneering contributions was that development unfolded throughout the life span, a view that has become widely embraced.
Freud postulated that development proceeded through a series of stages in which children seek pleasure or gratification from a particular body part (i.e., the oral, anal, and phallic stage). In contrast, Erikson’s theories of child development focus on the interrelationship between a developing child’s internal psychosexual development and his or her more external emotional development, emphasizing the interpersonal relationships that arise between the child and parents (Erikson, 1950).
Erikson conceived of the life course, from birth to old age, as a series of eight epigenetic stages that, as other developmental theories, proceed in a stepwise fashion, the next dependent upon how well the previous has been mastered: trust versus mistrust; autonomy versus shame and doubt; initiative versus guilt; industry versus inferiority; identity versus role diffusion; intimacy versus isolation; generativity versus stagnation; ego integrity versus despair.
Erikson portrayed each stage as a crisis or conflict that needed resolution, either at the time or at a subsequent stage. Each successive stage presents its own challenges but, at the same time, offers the opportunity for correction of unresolved challenges of previous stages. At each stage the tension was between the psychosocial and psychosexual—the outward-looking versus inward-looking perspectives. Psychopathology, in the form of a mental disorder, would arise if a stage was ultimately not mastered successfully.
Over the years, Erikson’s theory has had great heuristic value to guide theorists and practitioners in organizing their approach to mental health and mental illness. However, his theory does not readily lend itself to empirical scrutiny. His theory also has been criticized as reflecting the concerns of male European culture (where Erikson was born and trained before moving to the United States) rather than those of women and other cultures. The need for cultural sensitivity and competence is discussed later in this chapter.
John Bowlby: Attachment Theory of Development
Fifty years ago, a new conceptualization of the psychoanalytic approach to development came into the lexicon of human development theory. John Bowlby’s reinterpretation of Freudian development is grounded in both Darwinian evolutionary theory and animal ethology. The previous work of Konrad Lorenz and others, who explored the relationship between other animals and their caregivers, determined that the bonds of infant care and the attachment of young to their caregivers are seminal in the drive for survival. Similarly, Bowlby theorized that for humans, attachment to a caregiver had a biological basis in the need for survival (Bowlby, 1951). Moreover, he suggested that this attachment drive exists alongside the drive for nutrition and the sex drive, yet distinct and separate from them. Attachment is seen as the anchor that enables the developing child to explore the world.
With the comfort and security of a stable and routine attachment to the mother—or other primary caregiver—a child is able to organize other elements of development in a coherent way. In contrast, instability in the caregiving relationship—whether physical distance, erratic patterns of parental behavior, or even physical or emotional abuse—may interfere with the sense of trust and security, potentially giving rise to anxiety and psychological problems later in childhood or even decades later in life.
For over a century, an intense debate among developmentalists and other scientists has pitted nature (genetic inheritance) against nurture (environment) as the engine of human development and behavior. Francis Galton, a 19th-century geneticist and cousin of Charles Darwin, declared that“there is no escape from the conclusion that nature prevails enormously over nurture” (cited in Plomin, 1996). As the debate raged, either nature or nurture gained ascendancy. During the 1940s and 1950s, for example, behaviorism held sway over American psychology with its argument that nurture was preeminent.
The pendulum now is coming to rest with the recognition that behavior is the product of both nature and nurture (Plomin, 1996). Each contributes to the development of mental health and mental illness. Nature and nurture are not necessarily independent forces but can interact with one another: nature can influence nurture, and nurture can influence nature (Plomin, 1996).
Studies comparing identical and fraternal twins have shed light on the contributions of nature and nurture. These studies show that for many behavioral traits, as well as mental disorders, there is a noticeable heritable component (see earlier discussion of heritability). Yet even with the most highly heritable traits or conditions, identical twins who share the same genetic endowment display marked differences. Identical twins, for example, are concordant for schizophrenia in 46 percent of pairs (NIMH, 1998), meaning that more than 50 percent of pairs are not concordant. Something yet unknown about the environment protects against the development of schizophrenia in genetically identical individuals (Plomin, 1996).
How do nature and nurture interact? This question cannot be directly answered by twin studies. Animal models have proven to be fertile ground for study of the mechanisms—at the molecular and cellular level—by which nature and nurture interact. As reviewed earlier, research in different animal models has established that the environment can alter the structure and function of the central nervous system (Baily & Kandel, 1993). This holds true not only during early development, but also into adulthood. Nurture influences nature, right down to detectable changes in the brain.
During development of the nervous system, each neuron forms myriad intricate synaptic connections with other neurons, the outcome of the interaction of genes and the environment described above. In this case, the environment is a very general term—it denotes the local extracellular environment surrounding the growing neuron, as well as what we traditionally think of as the environment (sensory environment, psychosocial environment, diet, etc.). When a neuron forms a synapse with its target cell, the pattern of activity, usually furnished by external environmental stimulation, strengthens or weakens the developing synapse. Only strengthened synaptic connections survive early development to form enduring connections, while weakened synaptic connections are eliminated (Shatz, 1993; Kandel et al., 1995). For example, kittens deprived of visual experience early in life sustain permanent disruption to synapses in parts of their visual cortex (Hubel & Wiesel, 1970).
Later in the course of development, established patterns of connections still can be altered by the environment—through learning. Studies in a variety of animal models have found that certain forms of learning lead to changes in the structure and function of neurons. With long-term memory—the long-term storage of learned information—these changes take the form of an enhanced number of synaptic connections and increased gene expression (Kandel et al., 1995). Increased gene expression appears to be for synthesis of new proteins needed for the structural changes occurring at the synapse (Bailey & Kandel, 1993).
Researchers continue to probe for changes in the brain associated with mental disorders. They have found, for instance, that repeated stress from the environment affects the hippocampus, an area of the brain located deep within the cerebral hemispheres. Research in animals has shown that repeated stress triggers atrophy of dendrites of certain types of neurons in a segment of the hippocampus (Sapolsky, 1996; McEwen, 1998). Similarly, imaging studies in humans suggest that stress-related disorders (e.g., post-traumatic stress disorder) induce possibly irreversible atrophy of the hippocampus (McEwen & Magarinos, 1997). Anxiety disorders also alter neuroendocrine systems (Sullivan et al., 1998). These are some of the tantalizing ways in which nurture influences nature.
The mental health field is far from a complete understanding of the biological, psychological, and sociocultural bases of development, but development clearly involves interplay among these influences. Understanding the process of development requires knowledge, ranging from the most fundamental level—that of gene expression and interactions between molecules and cells—all the way up to the highest levels of cognition, memory, emotion, and language. The challenge requires integration of concepts from many different disciplines. A fuller understanding of development is not only important in its own right, but it is expected to pave the way for our ultimate understanding of mental health and mental illness and how different factors shape their expression at different stages of the life span.