Secular Trends of Suicide Seasonality: A Comparative Study of Various Countries

Abstract

Secular trends of suicide seasonality, both in Japan and in various other countries, were studied. In early periods studied, a peak in suicide deaths appeared in early summer, around June, in most countries. In Japan, however, two peaks in May and July were observed for the periods from 1900-09 to 1920-29. A small trough in June is probably related to the rainy weeks of June, which is characteristic of the Japanese climate. The magnitude of suicide seasonality has gradually decreased with time and the peak month has shifted towards earlier months around April in many countries. A linear relationship between the magnitude of suicide seasonality (S) and the proportion of suicide deaths occurring in the cooler months from November to February (Δ) was derived on a hypothetical curve for suicide seasonality. The observed values of suicide seasonality of Japanese males and females and in many other countries have decreased with time along this calculated curve for S as a function of Δ, where the lesser values of suicide seasonality were observed in the more developed countries. As the suicide seasonality has been changing with time, with a velocity proper for each country, its secular trends should be taken into consideration when two countries are compared.