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Large survey suggests schizophrenia less prevalent than textbooks say

May 31, 2005

Schizophrenia is a devastating mental illness and a major contributor to the global burden of disease, but how many people are actually suffering from the disease worldwide? John McGrath and colleagues from the University of Queensland have conducted the most comprehensive analysis on the topic and now report their findings in the May issue of the international open-access journal PLoS Medicine. The researchers have done what is called a systematic review of the medical literature, which turned out a few surprises. And knowing about prevalence-defined as the number of people suffering from the disease at a given time or within a time interval-of such an important disease is crucial to health policy strategies. They found that schizophrenia is more common in developed than in poorer countries but overall less common than previously thought. Across countries, immigrants have higher rates of the disease than native-born individuals.

The scientists have analyzed a total of 1,721 estimates from 188 studies covering 46 countries and calculated the following median prevalence estimates: 4.6 per 1,000 for point prevalence (defined as prevalence during any interval of less than a month), 3.3 for period prevalence (defined as prevalence during a period from 1 to 12 months), 4.0 for lifetime prevalence (the proportion of individuals in the population who have ever manifested the disease and who are alive on a given day), and 7.2 for lifetime morbid risk (which attempts to include the entire lifetime of a birth cohort, both past and future, and includes those deceased at the time of the survey).

The point prevalence numbers are consistent with key policy documents, but the 0.5% estimate for lifetime prevalence given in many textbooks is a significant overestimate. "This," the authors suggest, "is another example where the research community needs to review their belief systems in the face of data." Another often quoted statistic, namely that "schizophrenia affects about one in a hundred" most sensibly refers to lifetime morbid risk data. Here as well, the systematic analysis suggests that the reality is somewhat lower, and the authors suggest that "if we wish to provide the general public with a measure of the likelihood that individuals will develop schizophrenia during their lifetime, then a more accurate statement would be that about seven to eight individuals per 1,000 will be affected."

However, the analysis makes clear that "many people with schizophrenia have persisting symptoms, despite the best mix of interventions we can offer." It has been estimated that current interventions can at most reduce 25% of disease burden. Thus, the authors conclude that "this is a powerful argument for investing in applied and basic research."

Public Library of Science