Research reportAnalysis of the seasonal pattern in suicide
Introduction
Most studies have found that suicide rates tend to peak during spring and summer (Petridou et al., 2002). Suicides are not evenly distributed over particular periods of time, since social or psychological factors have immediate or long-term influence on the individual’s decision to commit suicide. In addition to social and endogenous time-givers, the physical ones such as changes in local weather conditions can modify human behaviour and influence the occurrence of death from suicide, possibly by interacting with social factors (Preti, 1997, Salib and Gray, 1997). However, changes in aerospace weather conditions (Russell and McPherron, 1973) are beyond this interaction and open up a possibility to test the hypothesis of physical factors having a direct effect on the risk of suicide, and to estimate the magnitude of the effect, if any.
Changes in the geomagnetic activity have been suggested to influence the human circulation, as measured with blood pressure and heart rate (Watanabe et al., 1994), and the onset of depressive episodes in male patients with bipolar disorder (Kay, 1994). A link between circulation and mood may well exist, since there are decreases in cerebral blood flow in the prefrontal cortex, limbic systems and paralimbic areas that may account for impaired attention as well as cognitive and emotional responses in patients with affective disorder (Ito et al., 1996).
Since geophysical factors may affect daily behaviour patterns, it is important in terms of prevention to study whether they can affect the occurrence of death from suicide. An individual’s failure to adjust to stimuli from the natural habitat may lead to death from suicide during certain periods of the year more frequently than by chance. We therefore set out to study the time series of death from suicide in the population of a northern region where extreme changes in the intensity of geophysical factors such as the daylight and ambient temperature routinely take place. Specifically, we aimed at studying the seasonal effect on suicide, and the effects of solar radiation and of geomagnetic activity on the frequency of suicides.
Section snippets
Methods
The material consisted of all suicides (n=27,469) committed from 1 January 1979 to 31 December 1999 in Finland, a northern European country with ∼5 million inhabitants. Data derived from the official death certificates were obtained from Statistics Finland and contained the following information for each case: the date of death, date of birth, sex, residence, birthplace, and marital status. The detailed content of the Finnish death certificate provides excellent accuracy and reliability of
Results
The number of suicides, incidence per 1000 person-years, and risk ratios with 95% confidence intervals from the Poisson regression model are given in Table 1. The combination of linear and quadratic terms of year confirmed that the peak of suicide incidence was in 1990 when there were 1512 suicides and the suicide mortality was 30.3 per 100,000 inhabitants.
The seasonal oscillation was most pronounced when the number of suicides was relatively low, first in the beginning and again in the end of
Discussion
Our main finding was that there was marked fluctuation in suicide occurrence by season in a nationwide analysis of a northern population. The seasonal effect on suicide rates concerns the population. Thus, this finding extends our previous analysis (Partonen et al., 2003) and disagrees with those recent reports claiming the disappearance of the seasonal effect on suicide (Yip et al., 2000, Parker et al., 2001).
We also found that the seasonal effect was most pronounced when the number of
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