Risk Factors for Schizophrenia

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Abstract
Mortensen et al. (Feb. 25 issue)1 acknowledge that a family history of schizophrenia is the best-established risk factor for the disorder but suggest that environmental factors, including the place and season of birth, are major determinants. This argument is based on estimates of the population attributable risk regarding factors found to be associated with schizophrenia in their population-based cohort. Unfortunately, their assertions about the relative importance of various risk factors overlook the fact that the calculation of population attributable risk is dependent on the frequency of the risk factor in a population. The calculation of relative risk, however, is not dependent on the population base rate. Thus, for a given relative risk (RR), the population attributable risk can be calculated with use of the following equation: population attributable risk=P(RR–1)÷P[(RR–1)+1], where P is the prevalence of the risk factor in the population.2

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