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The late George Engel promulgated a theoretical model, based on general systems theory, of the etiology of mental disease that remains central to epidemiologic investigations into the twenty-first century. Research demonstrates that unitary explanations are not adequate to explain disease etiology or thus inform appropriate prevention and treatment strategies. Engel suggested an interrelatedness among biological, psychological, and social factors. Biological factors include hereditary, anatomic, and molecular factors and those factors related to gender, age, and ethnicity. Psychological factors include temperament, personality, motivation, emotion, attention, and cognition. According to Engel's theory, social factors included family, society, culture, and environment; other authors would include religious and spiritual as well as economic factors in this group. Engel also believed that the physician's contribution, through a psychosocial presence, to this “collaborative pathway to health” was often inseparable from that brought by the patient. From this perspective, psychiatric epidemiologists explore the frequency, distribution, outcome, and causation of psychiatric disorders. Identifying a case becomes the first task of the epidemiologist. An essential component of all these uses is the determination of valid denominators to compare the characteristics of populations with and without disease. For example, to determine the prevalence of a case (i.e., the frequency of the case in a given population at a given point in time) one must know the number of persons both with and without the disorder in the population. To determine the incidence of a case (the number of new cases that emerge in the population over a given interval (usually 1 year), one must know the number of people in the population at the beginning of an interval who do not experience the disorder. A list of key terms in epidemiology can be found in Table 2–1.

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Table 2–1. Key Terms in Epidemiology

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