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The effect of season of birth on the risk of schizophrenia was of the expected magnitude and had the expected periods of maximal risk (February and March) and minimal risk (August and September). We replicated a previous finding that there was no interaction between season of birth and family history of schizophrenia. However, we did not replicate a previous finding that the association between winter birth and schizophrenia occurred only among persons born in urban areas. Lewis suggested that an association between the season of birth and schizophrenia is a methodologic artifact due to the so-called age–incidence effect — that is, persons born in January are older than those born later in the year within the same age category and thus have spent more time at risk for schizophrenia. This concern was not relevant to our cohort study, however, since all age-specific person–years were calculated exactly for each person.

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