Schizophrenic birth seasonality and the age-incidence artifact.

Lewis (1989) dismisses a large number of articles in which schizophrenic winter birthrate excesses have been reported as the result of either design problems or the age-incidence artifact. We review about 20 studies in which the effects of age incidence have been controlled in one way or another. The majority support the conclusion that the seasonality effect is due neither to artifact nor deficient research design. We also correct two significant errors in Lewis' description of our prior work.