Suicide aggregation in relation to socio-demographic variables and the suicide method in a general population: assortative susceptibility.

One area of research in suicidology aims at understanding the processes underlying aggregation or clustering of suicide cases within a limited period of time or space (suicide epidemics). Susceptibility to, or propagation of, suicidal behavior due to given risk factors may be operating through media other than space, and its susceptibility for the receiver may be different within different types of strata that are determined by socio-demographic, personality-related or biological-susceptibility differences. We use the term "assortative susceptibility" for this phenomenon. Aggregated cases, comprising calendar months with an unusually large number of suicides after adjusting for seasonal and yearly variations, were defined in the register of all 1093 completed suicides during 1969-93 in the county of Västerbotten in northern Sweden. Binary multiple logistic regressions were performed to compare the aggregated cases with the remaining cases. Compared with the remaining cases, the aggregated cases included significantly more of males and of those living in the rural forested regions. Also, suicide by firearms was significantly more aggregated than the other methods. Our results suggest that middle-aged or older men from the rural areas, who have access to firearms, are likely to belong to the socio-demographic stratum that is susceptible to the processes that give rise to aggregations or clusters of suicides in this county (assortative susceptibility).