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.

Seasonal affective disorders: relevance of Icelandic and Icelandic-Canadian evidence to etiologic hypotheses.

OBJECTIVE: This study tests the suggestion of earlier studies concerning the importance of genetic factors in the etiology of winter seasonal affective disorders (SADs) and subsyndromal winter SAD (S-SAD). METHOD: Two study populations of Winnipeg, Manitoba residents were canvassed: 250 adults of wholly Icelandic descent and 1000 adults of non-Icelandic descent. We distributed the Seasonal Pattern Assessment Questionnaire by mail to these 2 populations, yielding 204 and 449 valid responses, respectively. RESULTS: Rates of SAD and S-SAD proved markedly lower in the Icelandic population than those in the non-Icelandic population. CONCLUSIONS: These differences seem unexplained by differences in ambient light or climate, thus indicating that genetic factors contribute to the expression of SADs. Compared with earlier findings from a group of adults of wholly Icelandic descent living in nearby rural Manitoba, the etiologic importance of as-yet-undetermined environmental factors unrelated to latitude or ambient light is also indicated.