Are seasonalities in suicide dependent on suicide methods? A reappraisal.

In general, suicide frequencies peak in spring and early summer and depict a low in autumn and winter months. Suicide seasonality is a ubiquitous phenomenon; however, recent studies have indicated that it is related particularly to violent suicide methods. We investigated differences in suicide seasonalities by suicide methods using Swiss mortality data. The suicide data were extracted from the Swiss cause of death statistics and cover the period 1969-1994. In this period 37,518 suicides (ICD-8 codes 950-959) were registered in the mortality statistics. The data were analysed by the Edwards' method, which examines unimodal sinusoidal patterns in aggregated monthly data, and by spectral analysis with disaggregated monthly data. Overall, Swiss suicide data in 1969-1994 depict the expected pattern with a peak in May and June and a low in December. However, analyses by suicide methods show that there is no common pattern. Suicide seasonalities are absent or weak in some violent as well as non-violent methods (firearms, cutting, crashing, poisoning). Even if clearly present (hanging, drowning, jumping from high places, unspecified/other), the seasonalities show distinct patterns and cycles. Apart from sinusoidal patterns, there are also outliers: in overall data as well as in most method-specific data, the December frequencies fall more than 10% below the neighbouring months. The decrease of the frequencies culminates at Christmas/New Year and then returns to average levels. Hence, the seasonality of the overall Swiss suicide frequencies is but a compound seasonality. It largely depends on specific suicide methods and different cyclical dynamics.