title: The impact of latitude on the prevalence of seasonal depression.
abstract: 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 (41.5 degrees N to 49.5 degrees N) for the province of Ontario. Eight strata of 1 degree each were sampled equally throughout a 12-month period. Using a validated and structured diagnostic interview, we interviewed 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.
authors.affiliation: Department of Psychiatry, Department of Nutritional Sciences, Fellowship Program, University of Toronto, Sunnybrook and Women's College Health Sciences Centre, Toronto, Ontario. anthony.levitt@swchsc.on.ca
authors.forename: Anthony J
authors.initials: AJ
authors.lastname: Levitt
authors.forename: Michael H
authors.initials: MH
authors.lastname: Boyle
mesh_terms: D000328:Adult; D000368:Aged; D003430:Cross-Sectional Studies; D005260:Female; D006306:Health Surveys; D006801:Humans; D015994:Incidence; D008297:Male; D008875:Middle Aged; D009864:Ontario / Q000453:epidemiology; D012307:Risk Factors; D016574:Seasonal Affective Disorder / Q000175:diagnosis / Q000453:epidemiology* / Q000523:psychology; D019371:Topography, Medical