A defining feature of the scientific method is the vigorous, transactional process between data and belief systems. As data accumulate, we revise our research models and set up hypotheses that, ideally, allow us to reject or refine our models. When evidence allows us to reject a research model, science can make progress. However, on occasions certain beliefs become dogma (i.e., a strongly held belief that is proclaimed without data). This article argues that schizophrenia research has been disproportionately influenced by the dogmatic belief that there is little variation in the incidence of schizophrenia.
In 1986 the World Health Organization published the preliminary report of a landmark multicenter study of schizophrenia.1
This study employed uniform methodology in order to generate schizophrenia incidence rates from 8 sites (in 7 nations). The incidence of ICD-9 schizophrenia ranged from 16 to 42 per 100,000. When a subset of these patients was extracted according to narrow criteria, the incidence ranged from 7 to 14 per 100,000. Both definitions found at least a twofold difference between the highest and lowest sites, and this difference for the broad (but not narrow) definition was statistically significant. However, in spite of their own data, the authors of this study conclude, “The results provide strong support for the notion that schizophrenic illnesses occur with comparable frequency in different populations.”1(p909)
While the full report of this study is more circumspect in its interpretation of the issue of between-site variation,2
the preliminary report has been frequently cited by researchers and has contributed to a broader belief that schizophrenia has a “flat” epidemiological profile across space and time. Such beliefs may have contributed to an undervaluing of the relative contribution of environmental (and gene × environmental) factors to the etiology of schizophrenia. For example, Crow has stated: “The evidence points to the singular conclusion that, contrary to almost any other common condition, the incidence of schizophrenia is independent of the environment and a characteristic of human populations.”3(p119)
It has been argued elsewhere that this “equal incidence” belief may have tapped into a deeper, unspoken myth about schizophrenia being an “egalitarian disorder.”4
This myth, in its strongest form, suggests that schizophrenia occurs with equal incidence in all nations (rich and poor), in all races and creeds, and in men and women equally. While the notion that schizophrenia respects human rights is vaguely ennobling, it is also frankly bizarre. A generation of researchers has been inoculated with these false beliefs, which has led to an ideological resistance to data that challenge the underlying myths.5
However, there are now robust data showing that schizophrenia is characterized by prominent variations across time and place. This article will use data from several recent systematic reviews in order to describe these variations.