What has been accomplished over the last several decades, and what are prospects for future progress? Even as late as one-quarter century ago, the epidemiology of schizophrenia was nearly a blank page. There was even argumentation about the value of the concept itself. The only risk factors which seemed strong and consistent were the conditions of lower social class life, and the family history of schizophrenia. Since that time, there has been considerable progress delineating a more or less consistent picture of the descriptive epidemiology and the natural history of schizophrenia. Research in analytic epidemiology has generated a series of heretofore unsuspected risk factors, as described above. In general, the risk factors have been considered in the context of theories of how schizophrenia might actually be developing in the psychological and physiological life of the individual— even if the linkage is sometimes speculative. These developments are healthy.
In the future there will be concerted efforts to study risk factors in combination. This process has begun already. For example, Mortensen et al have studied the combined effects of season of birth, urbanization of birthplace, and family history of schizophrenia. The combination is informative in evaluating the importance of the risk factors. Although the relative risk for urban birth is much smaller than the risk associated with having a parent who has schizophrenia, the importance of urban birth is greater, because a much larger proportion of the population are born in urban areas than the proportion with parents who have schizophrenia—the situation of relative versus population-attributable risk (Mortensen et al. 1999). If the causal path connected to urban birth could be identified, the prospects for prevention would be much stronger.
The combination of risk factors will facilitate prospective studies of high risk individuals, in which the high risk is not simply due to family history, as in earlier high risk studies. Furthermore, combination of risk factors will raise the positive predictive value of the risk formulation, to the point where it may be ethically feasible to approach the individual, identify the risk, and begin efforts to protect them from the catastrophic effects of the first episode of schizophrenia. Studies such as these have begun, albeit very cautiously (McGorry et al. 1996; Tsuang et al. 1999; Woods et al. 2003). In general, epidemiologic research has built a strong knowledge base over the past quarter century, and this knowledge base will continue to contribute to public health efforts at prevention of schizophrenia in the coming decades.