Differences in prevalence of seasonal affective disorder that are not explained by either genetic or latitude differences.

The prevalence of winter SAD was measured in two groups of a(lult Manitobans of wholly Icelandic (lescent, 210 resident in Winnipeg (50 degrees N) and 252 resident in the nearby Interlake district (50.5 degrees N), using the Seasonal Pattern Assessment Questionnaire (SPAQ). These groups live practically at the same latitude and are according to all indications genetically identical. The age-and-sex-standardized prevalence rates of winter SAD proved to be markedly higher in the Winnipeg population than in the Interlake population: 4.8% and 1.2% (p<0.001), respectively. This four-fold dif ference is evidently unexplained by genetic factors or a difference in latitude; its causes have yet to be discovered.