title: Variation and seasonal patterns of suicide mortality in Finland and Sweden since the 1750s.
abstract: OBJECTIVE
  Suicide mortality varies in both the short and long term. Our study examines suicide mortality in Finland and Sweden from the 1750s until today. The aim of our study is to detect any seasonal peaks in suicide rates and examine their temporal evolution to suggest a mechanism that may explain such peaks.
  
  
  METHODS
  We acquired the study material from the Finnish and Swedish cause of death statistics (257,341 deaths by suicide) and the relevant population gender structure data. We then separately calculated the annual male and female suicide rates per 100,000 inhabitants. We analysed the suicide peaks, calculating factors of proportionality for the available data by dividing the suicide rates in the peak months (May and October) by the annual suicide rates.
  
  
  RESULTS
  Suicide rates in Finland and Sweden peak twice a year. Both men and women in both countries most often commit suicide in May. There is another peak in October, with the exception of Finnish men. These suicide peaks coincide with a temperature increase in May and the biggest annual drop in temperature in October. We also observed a monotonic long-term change in the Swedish statistics, but not in the Finnish data. Our hypothesis is that seasonal variation in suicide rates may be caused by abrupt temperature changes twice a year that trigger the activity in brown adipose tissue and deepen depression.
  
  
  CONCLUSIONS
  While the overall suicide mortality rates varied considerably, the monthly proportions in May did not. This finding suggests a routine factor underlying the spring peak in suicide mortality.
authors.affiliation: Department of Geosciences and Geography, University of Helsinki, Gustaf Hällströmin Katu 2a, 00014, Helsinki, Finland, jari.a.holopainen@helsinki.fi.
authors.forename: Jari
authors.initials: J
authors.lastname: Holopainen
authors.forename: Samuli
authors.initials: S
authors.lastname: Helama
authors.forename: Charlotte
authors.initials: C
authors.lastname: Björkenstam
authors.forename: Timo
authors.initials: T
authors.lastname: Partonen
mesh_terms: D005260:Female; D005387:Finland / Q000453:epidemiology; D006801:Humans; D015994:Incidence; D008297:Male; D015995:Prevalence; D012307:Risk Factors; D012621:Seasons; D012727:Sex Characteristics; D013405:Suicide / Q000706:statistics & numerical data; D013548:Sweden / Q000453:epidemiology; D013696:Temperature; D013997:Time Factors