Objective: To examine whether a genetic selection within the Icelandic population helps it to adapt to the long arctic winter.
Participants and setting: The target population was a group of adults in the Interlake district of Manitoba, Canada, wholly descended from Icelandic emigrants. The ancestry of every individual in this group can be traced back to 1840.
Design: The Seasonal Pattern Assessment Questionnaire was mailed to a random sample of the study population. The data were compared with results obtained with similar methods in populations in Iceland and on the eastern seaboard of the United States.
Main outcome measures: Prevalence rates of seasonal affective disorder and subsyndromal seasonal affective disorder.
Results: The prevalence rates of seasonal affective disorder and subsyndromal seasonal affective disorder were found to be 1.2% and 3.3%, respectively, in this group of Canadians of wholly Icelandic descent. These are significantly lower than those measured with similar methods among people living along the east coast of the United States (chi 2 = 12.6 and 14.4, respectively, P < .001). Standardized rate ratio for this group compared with the American group was 0.18 for seasonal affective disorder and 0.38 for subsyndromal seasonal affective disorder.
Conclusions: This is the second study to find the prevalence of seasonal affective disorder and subsyndromal seasonal affective disorder to be lower among Icelanders or their descendants than among populations along the east coast of the United States. The results indicate that the relationship between prevalence of these disorders and geographic latitude is more complex than has previously been suggested; genetic adaptation in Icelandic populations may play an important role.