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Andrés Magnússon, MD, Jóhann Axelsson, DPhil

Andrés Magnússon, MD; Jóhann Axelsson, DPhil
From the Department of Psychiatry, National University Hospital, Reykjavik, Iceland (Dr Magnússon) and the Department of Physiology, University of Iceland, Reykjavik (Dr Axelsson). Dr Magnússon is currently with the Neurochemical Laboratory at the University of Oslo (Norway).

Arch Gen Psychiatry. 1993;50(12):947-951.

Abstract
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 prevelance 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 (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.

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