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.

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.