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To the Editor: Links between suicide, seasonal affective disorder, and latitude are known. We present new and potentially useful information linking seasonal light variation to suicide. To our knowledge, there have been no previous reports of the association between latitude and suicide rates in the extreme southern hemisphere. We found no studies in MEDLINE that included countries south of 45° south latitude. Argentina is the only country with an appreciable population below 45° south latitude. We analyzed suicide rates in the three southernmost provinces of Argentina (Chubut, Santa Cruz, and Tierra del Fuego); suicide rates were 5.9 (in women) and 21.4 (in men) per 100,000 compared to national rates of 3.1 (in women) and 15.5 (in men) (1). Native American population distribution in Argentina shows that the northernmost province, Salta, had the highest percent of Native Americans (15%), far higher than the national average of about 0.5% (Instituto Nacional de Asuntos Indigenas, Buenos Aires). This information should encourage researchers studying the effect of seasonal or diurnal light variation on suicide rates.
Since Durkheim’s work in 1897 (2), there have been many studies linking latitude and suicide. The latitude-suicide connection is also shown by data from 12 U.S. Indian Health Service administrative areas. Alaska has close to the median rates for homicide and unintentional injury but has three times the median suicide rate (3).
Explanatory factors may include race, socioeconomic differences, temperature, and diurnal or seasonal light variation. Examples of ethnic differences are higher rates in Hungary than in England and in Finland than in Sweden at the same latitudes. The common factor of Finno-Ugrian ethnicity has been proposed. The suicide rates in Native Americans are higher than in the U.S. general population (3).
A possible neurophysiological explanation for the suicide-latitude link is the recent discovery of retinal melanopsin receptors directly linked to the circadian rhythm center in the suprachiasmatic nucleus (4). The extreme seasonal variation of light and dark at high latitudes may be linked to high suicide rates and seasonal affective disorder and suggests the possibility of preventive light therapy.
Further high latitude studies, both north and south, could explore potential prevention of suicide and seasonal depression.
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