The impact of latitude on the prevalence of seasonal depression

Background: This study sought to determine whether the prevalence of the seasonal subtype of major depression (SAD) in the community varied as a function of latitude. Methods: Random telephone numbers were generated across 8 degrees of latitude (41.5degreesN to 49.5degreesN) for the province of Ontario. Eight strata of I degree each were sampled equally throughout a 12-month period. Using a validated and structured diagnostic interview, we inter-viewed by telephone respondents over 20 years of age who had lived in the region for 3 years or more. We evaluated patterns of symptom change across seasons to establish a diagnosis of SAD according to DSM-IV criteria. Results: Of the 2078 households that were assessed for eligibility, 1605 (77%) completed the interview. The crude prevalence of lifetime SAD was 2.6% (95% CI, 1.9 to 3.5). There was no impact of latitude on prevalence of either major depression or the seasonal subtype across the 8 strata, although the global measure of the severity of seasonal change in mood was significantly negatively correlated with latitude. Conclusions: SAD is a common subtype of major depression in Ontario, but there is no evidence to support an increase in prevalence with increasing latitude.