Birth order and mortality in two ethno-linguistic groups: Register-based evidence from Finland.

Previous research has documented an association between birth order and suicide, although no study has examined whether it depends on the cultural context. Our aim was to study the association between birth order and cause-specific mortality in Finland, and whether it varies by ethno-linguistic affiliation. We used data from the Finnish population register, representing a 5% random sample of all Finnish speakers and a 20% random sample of Swedish speakers, who lived in Finland in any year 1987-2011. For each person, there was a link to all children who were alive in 1987. In total, there were 254,059 siblings in 96,387 sibling groups, and 9797 deaths. We used Cox regressions stratified by each siblings group and estimated all-cause and cause-specific mortality risks during the period 1987-2011. In line with previous research from Sweden, deaths from suicide were significantly associated with birth order. As compared to first-born, second-born had a suicide risk of 1.27, third-born of 1.35, and fourth- or higher-born of 1.72, while other causes of death did not display an evident and consistent birth-order pattern. Results for the Finnish-speaking siblings groups were almost identical to those based on both ethno-linguistic groups. In the Swedish-speaking siblings groups, there was no increase in the suicide risk by birth order, but a statistically not significant tendency towards an association with other external causes of death and deaths from cardiovascular diseases. Our findings provided evidence for an association between birth order and suicide among Finnish speakers in Finland, while no such association was found for Swedish speakers, suggesting that the birth order effect might depend on the cultural context.