Birth order and suicide in adulthood: evidence from Swedish population data.

Each year, almost 1 million people die from suicide, which is among the leading causes of death in young people. We studied how birth order was associated with suicide and other main causes of death. A follow-up study based on the Swedish population register was conducted for sibling groups born from 1932 to 1980 who were observed during the period 1981-2002. Focus was on the within-family variation in suicide risk, meaning that we studied sibling groups that consisted of 2 or more children in which at least 1 died from suicide. These family-fixed effects analyses revealed that each increase in birth order was related to an 18% higher suicide risk (95% confidence interval (CI): 1.14, 1.23, P = 0.000). The association was slightly lower among sibling groups born in 1932-1955 (hazard ratio = 1.13, 95% CI: 1.06, 1.21, P = 0.000) than among those born in 1967-1980 (hazard ratio = 1.24, 95% CI: 0.97, 1.57, P = 0.080). Further analyses suggested that the association between birth order and suicide was only modestly influenced by sex, birth spacing, size of the sibling group, own socioeconomic position, own marital status, and socioeconomic rank within the sibling group. Causes of death other than suicide and other external causes were not associated with birth order.