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.

In recent years, psychiatric epidemiologists have recognized that cases often occur simultaneously in the same person (the cases are comorbid). Recent studies have emphasized not only the prevalence and incidence of individual cases but also comorbid cases.

Concept of the “Case”

The Diagnostic and Statistical Manual of Mental Disorders, fourth edition, Text Revised (DSM-IV-TR), and other psychiatric diagnostic systems disaggregate psychiatric disorders into discrete cases. For example, either an individual meets criteria for a diagnosis of major depressive disorder or he or she does not. To identify a case, one must have criteria for identifying cases, but these criteria may vary from one nomenclature to another. For example, the criteria for a case in DSM-IV-TR differ from those in DSM-III in some circumstances.

The use of the concept of a case in epidemiology makes it easier for practicing clinicians to interpret the types of studies performed by epidemiologists, although by arbitrarily assigning an individual to a category of either “case” or “noncase,” one loses considerable data. Several early epidemiologic studies were cognizant of this dilemma and attempted to assign patients to groups based on how well they met the predetermined criteria for each group; these researchers recognized that the ability of clinicians to assign individuals as either cases or noncases was not perfect and was more applicable to a probability function than to a simple yes or no decision. Similarly, the use of symptom rating scales does not require that an individual be assigned to a case or noncase category but rather permits the assessment of depressive psychopathology as a continuum. (Examples of “case finding” in psychiatric epidemiology are provided below in the description of individual studies.)

Other Epidemiologic Concepts

To discern the relationship(s) between factors that contribute to the emergence of a case, epidemiologists explore what has been described as the web of causation. The concept of a web of causation is that specific relationships, such as the relationship between social stressors and a mental disorder, may be connected through a variety of intervening variables that interrelate in a way best illustrated by a web consisting of nodes (etiologic factors) and strings (interrelationships of these etiologic factors). For example, genetic factors may lead to endophenotypes (such as dysfunction of a neurotransmitter system), which lead to intermediate phenotypes (such as a depressed mood), which in turn are shaped by the social environment of the individual. Social factors, in turn, may alter genetic expression. Epidemiologic studies assist investigators in sorting out the different nodes and interactions within this web of causation.

According to Morris, epidemiology has several vital uses: (1) study of the historical health of communities and the estimation of morbidity for different disorders, (2) assessment of the efficiency of health programs and services, (3) determination of individuals at risk of acquiring a disease or disability in all their various presentations, (4) identification of syndromes as the unified collection of related signs and symptoms, and (5) assisting in “…the search for the causes of health and disease.” Proper epidemiologic studies can promote sound health policy, enable more rational health care planning, and facilitate cost-effective prevention and treatment.

First Generation Studies

The earliest formal psychiatric epidemiologic studies were undertaken during the first part of the twentieth century. They were generally of limited scale, relied on institutional records, and used small groups of informants for their data. These were “convenience” studies in which, instead of initiating the surveys themselves, epidemiologists assembled health data from those persons who had already received treatment for a medical problem or had committed suicide. Faris and Dunham’s relatively large pre–World War II study examined the geographic distribution of patients with mental disorders in mental hospitals in the Chicago area. They found that manic–depressive illness was distributed equally throughout the geographic area, whereas schizophrenia clustered in the lower socioeconomic areas.

Second-Generation Studies: The Stirling County & Midtown Manhattan Studies

In comparison to pre–World War II investigations, the studies that followed World War II took advantage of the considerable health information gathered on the military forces during the war. This was the beginning of the “community survey” era of epidemiology. Postwar studies—such as the Stirling County (Nova Scotia) Study, the Midtown Manhattan Study, and the Baltimore Study of mental illness in an urban population—were second-generation studies that attempted to determine the prevalence rates of mental illness in community residents with the help of nonpsychiatrist clinical interviewers. The postwar studies examined general health as well as psychiatric disorders and tended to gather and interpret rates of symptom presentation in groups rather than assess the presence of discrete cases. The gathering of often-isolated symptoms, or the finding of psychopathology or emotional illness by using the data collection system of the World War II era, was not helpful to health planners or policymakers.

Contributing to the ascension of psychiatric epidemiologic research during the early postwar period was the realization that the increase in mortality and morbidity associated with chronic disease (including that of mental disorders) was more important than was the mortality and morbidity associated with acute, generally infectious, disorders. Difficulty with case identification in the community continued to preclude the determination of prevalence rates for specific clinical disorders.

The initial paradigms for these newer studies were often quite different. The Stirling County Study attempted to determine rates for qualitatively different disorders as well as for overall impairment. The Midtown Manhattan Study assumed that mental disorders were on a continuum and—reflecting the thinking at that time (that mental illness differed in degree and not kind)—that all clinical manifestations of illness could be evaluated in terms of functional impairment. The overall prevalence of psychiatric impairment from both of these studies was approximately 20%. Leighton and colleagues demonstrated in Stirling County that the mental illness of the individual could be influenced, for benefit or detriment, by the attributes of the community, thus ushering in an emphasis, during the late 1950s and early 1960s, on social psychiatry.

Third-Generation Studies

Third-generation epidemiologic studies were based on more advanced epidemiologic and statistical techniques and on a move toward scientific or evidence-based medicine. These studies began with the important development of operational criteria for mental disorders (specifically DSM-III). Newer methodological techniques helped address the increasing need for more exact rates of specific disorders for specific persons in specific settings. Indeed, effective treatment has been shown to be related directly to accurate and thus specific assessment and diagnosis. Similarly, appropriate mental health policy planning for the unique health needs of persons with various psychiatric disorders depends greatly on an accurate and precise definition of boundaries between disorders. Further, research into the etiology and thus effective treatment and, it is hoped, eventual prevention of psychiatric disorders must derive from the specificity of operational criteria. Otherwise, the blurring that has occurred between symptom patterns can lead only to similar blurring in the assessment of treatment and prevention effectiveness.