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.