American Journal of Epidemiology Advance Access originally published online on May 22, 2007
American Journal of Epidemiology 2007 166(2):237; doi:10.1093/aje/kwm150
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LETTERS TO THE EDITOR |
RE: "SEASONAL ASSOCIATIONS BETWEEN WEATHER CONDITIONS AND SUICIDE—EVIDENCE AGAINST A CLASSIC HYPOTHESIS"
1 Neuropsychiatry Society of Minas Gerais, Belo Horizonte, Minas Gerais, Brazil
2 Department of Psychiatry, Federal University of Minas Gerais, Belo Horizonte, Minas Gerais, Brazil
(e-mail: fmvolpe{at}terra.com.br)
The extensive and well-conducted study by Ajdacic-Gross et al. (1) is indeed a valorous contribution to the understanding of the determinants of suicidal behavior. The authors' approach raises an important, but quite unexplored issue: seasonal variations in psychiatric symptoms and human behavior are not completely explained by the climate.
Ajdacic-Gross et al. claim that "the most straightforward approach to enhance the case for climatic conditions as a determinant of seasonality in suicide" (1, p. 562) would be to delete the seasonal component of variance from the rest of the climatic effects. Although we do agree that this method is very useful for evidencing the effect of a singular variable, it may not be the most direct way of isolating climatic from nonclimatic seasonal effects on behavior.
In a recent paper on the seasonality of admissions for mania, Volpe and Del Porto (2) proposed an alternative approach: 1) the seasonality of the phenomenon (admissions) was estimated, testing observed data against a predefined sinusoidal model; and 2) once seasonality was actually demonstrated, a best-subsets multiple regression model was constructed, which maximized the coefficient of determination by including the best set of variables among all studied, despite colinearity or individual significance. Thus, we were able to estimate the amount of variance explained by climatic conditions as a whole. This approach considered that climatic variables are usually intercorrelated and that it may not be wise to select a variable based solely on its statistical significance. We have found, for instance, that although there was a distinct seasonal distribution of admissions for mania (pseudo-R2 = 15 percent), peaking in late winter/spring months, climatic variables altogether explained less than 25 percent of the total variance. This finding was considered evidence that seasonality could not be totally attributed to varying climatic conditions, corroborating previous similar reports (3, 4).
The analytic method preferred by Ajdacic-Gross et al. (1) would be adequate to answer an inverted question: how much of the influence of temperature on suicide rates can be attributed to seasonality? We think that our alternate approach is more direct, since it addresses the following question: how much of seasonality could be explained by the climate?
Both studies bring attention to the subject of which other (nonclimatic) variables determine seasonality in psychiatry. This issue should be addressed more thoroughly in large epidemiologic studies that also take into account variables related to individual features (e.g., genetic, congenital, measured exposure to light and temperature), using more comprehensive hierarchical mixed models.
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ACKNOWLEDGMENTS |
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Conflict of interest: none declared.
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References |
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 TOP References |
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- Ajdacic-Gross V, Lauber C, Sansossio R, et al. Seasonal associations between weather conditions and suicide—evidence against a classic hypothesis. Am J Epidemiol (2007) 165:561–9.
[Abstract/Free Full Text] - Volpe FM, Del Porto JA. Seasonality of admissions for mania in a psychiatric hospital of Belo Horizonte, Brazil. J Affect Disord (2006) 94:243–8.[CrossRef][Web of Science][Medline]
- Sayer HK, Marshall S, Mellsop GW. Mania and seasonality in the southern hemisphere. J Affect Disord (1991) 23:151–6.[CrossRef][Web of Science][Medline]
- Suhail K, Cochrane R. Seasonal variations in hospital admissions for affective disorders by gender and ethnicity. Soc Psychiatry Psychiatr Epidemiol (1998) 33:211–17.[CrossRef][Web of Science][Medline]
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  V. Ajdacic-Gross and M. Bopp TWO OF THE AUTHORS REPLY Am. J. Epidemiol., July 15, 2007; 166(2): 237 - 238. [Full Text] [PDF]
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