Mental Health: A Report by the Surgeon General

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.

¹⁰ Epigenetic influences are those that arise from outside the genes and lead to emergent, as opposed to predetermined, properties.

¹¹ Newborns turn their head towards things—typically the breast—that touch their cheek.

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