Evidence That Latitude is Directly Related to Variation in Suicide
Objective: To use available suicide-rate data from
20 countries to see patterns and relations more clearly.
Method: We obtained raw suicide rates from the Organization
for Economic Cooperation and Development (OECD) database from
1960 through 1997 and calculated averages and standard deviations.
Results: There is a positive linear relation between
the variation in suicide rate and geographic latitude.
Conclusions: The variation in light-dark cycles is
superimposed upon human mood.
(Can J Psychiatry 2002;47:572574)
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Key Words: suicide rate, variation, latitude, light-dark
cycles, seasonality, depression, environment, standard deviation
: Preuve du lien direct entre la latitude et la variation
des taux de suicide
We are aware of a seasonal component in the
exacerbation of affective disorders, the most obvious being seasonal
affective disorder (SAD). Research has shown a relation between
season of birth and first-time admission in patients with schizophrenia,
and also in predilection for violence (14). These reports
suggest that ambient light affects various psychiatric disorders.
Figure 1 plots
the suicide rate (SR) of the 20 countries with the most complete
data over nearly 4 decades and compares the SR with the average
latitude of each countrys major population centre (5). The
initial impression suggests a scattergram, and indeed,
there is no relation of SR to latitude. However, we can form groupings
of SR that are, for the most part, geographically contiguous. The
countries within the groups outlined in rough circles in Figure
1 are more likely to be linked genetically. The grouping containing
Canada, New Zealand, the US, and Australia, although not geographically
close, comprisesapart from relatively low populations of indigenous
peoplesemigrants from Northern Europe. The Scandinavian countries
are the most widely separated in terms of SR, but Icelands
SR is similar to Norways. Icelandic people are likely to have
strong genetic similarities with Norwegians as the result of earlier
Viking migration from Norway. Japan, an island nation isolated for
centuries until relatively recently, appears to stand apart from
the other groupings.
Results and Discussion
Figure 2 plots
the standard deviation (SD) of the mean annual SR for each country
from 1960 to 1997. What is surprising is the linear relation of
latitude to SD, a measure of variation in SR. The higher the latitude,
the greater the variation in rate, at least in countries where data
are available. We also know that the higher the latitude, the more
variation in seasonal light-dark (L-D) cycles. It appears that variation
in the L-D cycle, which is zero at the Equator and maximum from
the Arctic Circle to the North (or South) Pole, is superimposed
upon our collective affect. Therefore, the variation in SR appears
to exhibit an environmental effect.
It is also apparent from the regression equation in Figure
2 that we can expect little or no variation in SR below 37°
N or S latitude. However, we can expect that there will be a minimum
or baseline SR due to the intrinsic uncertainty of living, even
at the Equator. The SR of the Greece, Spain, and Italy grouping
could well represent that baseline (approximately 4 to 5 suicides
per 100 000 population). We emphasize that this paper is not about
the SR itself, which may be influenced by religious proscription
of suicide and by differing data collection methods. The SD (that
is, variation) in SR is less susceptible than the SR as long as
basic mores and methods remain relatively stable over time. As data
from countries closer to the Equator become available, we may be
able to obtain evidence proving that these regions have the lowest
SR variations. We should also expect that indigenous populations
in countries in boreal or tundral latitudes might develop a degree
of genetic resistance to variable light, therefore mitigating the
effect of the long dark periods at and around the winter solstice.
There is some anecdotal evidence that immigrants to Alaska have
more depression than do Native populations (unpublished observations).
This suggests that genetic adaptation probably plays a role in reducing
the adverse effects of variable seasonal light on neurophysiology.
For example, one published report shows a lower incidence of SAD
in emigrants from Iceland (approximately 66° N) to Canada (approximately
55° N) (6).
We can also speculate that if depression and suicide can be influenced
by latitude (that is, by variable exposure to photons) then other
diseases may be similarly modulated. Examples already in the literature
include multiple sclerosis, neural tube defects, lupus erythematosis,
and lymphoma (710). The discipline of photoimmunology is beginning
to capitalize on these findings (11). The effect of variable light
in human psychiatric and nervous system disorders will undoubtedly
continue to be the subject of future research.
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disorder is low among descendants of Icelandic emigrants in Canada.
Arch Gen Psychiatry 1993;50:94751.
7. McMichael A, Hall A. Does immunosuppressive ultraviolet radiation
explain the latitude gradient for multiple sclerosis? Epidemiology
8. Jablonski N. A possible link between neural tube defects and
ultraviolet light exposure. Med Hypotheses 1999;52:5812.
9. McGrath H. Ultraviolet A1 (340-400nm) irradiation and systemic
lupus erythematosis. J Investig Dermatol Symp Proc 1999;4:7984.
10. Bentham G. Association between incidence of non-Hodgkins
lymphoma and solar ultraviolet radiation in England and Wales. BMJ
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Manuscript received July 2001, revised, and accepted April 2002.
1 Physician, Augusta Mental Health Institute, Augusta, Maine.
2 Director of Information Services, Maine Department of Behavioral
and Developmental Services, Augusta, Maine.
Address for correspondence: Dr GE Davis, Augusta Mental Health
Institute, PO Box 724, Augusta, ME, 04332
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