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Psychiatric disorders occur in every socioeconomic, racial, and cultural group in the world and are among the top 10 contributors to the global burden of disease.1 In the United States, 8–18% of children and adolescents and 26% of adults recently experienced a psychiatric or substance use disorder.2,3 Alzheimer's disease is found in 8–15% of people over age 65.4 In the last several decades, a great deal has been learned about the distribution of psychiatric and substance use disorders in the population, their familial, biologic, social, and psychologic risk factors, illness course and prognosis, and the socioeconomic costs of these disorders.

Psychiatric disorders account for a large proportion of all chronic health problems. Moreover, an individual's mental state greatly influences general health status and ability to access needed health care services. Four issues underscore the importance of mental health issues for public health and preventive medicine: (a) quality of life is largely determined by a person's mental state; (b) a large proportion of people in primary care have undetected psychiatric or substance disorders; (c) many physical disorders have an important mental component; and (d) as the risk of premature death recedes, the risk of chronic impairment rises. Given the significance of mental health problems, the search for causes is urgent.5 As modifiable risk factors are identified, primary prevention becomes increasingly appropriate and cost-beneficial.

Progress in our understanding of the public health significance of mental disorders follows on the heels of the advances in classification and diagnosis. Current concepts of mental illness are rooted in the diagnostic characterizations of Kraepelin and Bleuler.6 The current classification system used in the United States is the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders, now in its fourth edition (DSM-IV).7 This document reflects a consensus about the types of mental disorders present throughout the life cycle and the constellation of signs and symptoms characterizing each disorder. In making a diagnosis, a clinician depends mainly on the results of a comprehensive mental examination, which focuses on the patient's (a) cognition; (b) current state of consciousness, confusion, or contact; (c) mood or affect; (d) connectedness of thought patterns, hallucinations, delusions, or distortion of thoughts and ideas; (e) personality (e.g., passivity, aggression, helplessness, rebelliousness); (f) behavior patterns, and (g) the complaint bringing the patient into treatment. In epidemiologic research, these domains are systematically evaluated using structured or semistructured diagnostic instruments.

Epidemiologic research designed to estimate the incidence and prevalence of mental disorders evolved over the past 100 years. Three generations can be identified: (a) the period before World War II, in which information about mental illness typically was provided by key informants and agency records (the median prevalence rate in these reports was 3.6%); (b) World War II to the 1970s, in which representative samples of the population were interviewed with extensive psychological and psychosomatic symptom inventories from which psychiatrists made ...

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