Invited commentary: birth order and suicide in a broader context.

In this issue of the Journal, Bjørngaard et al. give us new insights into the etiology of mental health by studying birth order and suicide risk (Am J Epidemiol. 2013;177(7):638-644). Although the authors provided empirical evidence that each increase in birth order (i.e., from first-born to second-born, second-born to third-born, etc.) is associated with a 46% higher suicide risk, they gave us very little information on the likely explanations. In our commentary, we draw attention to the possible mechanisms underlying a causal relationship between birth order and suicide. Given that Norway is one of the richest countries in the world, the findings of Bjørngaard et al. in a Norwegian cohort also call for a discussion of whether their results are representative of other societies that are similar or dissimilar with respect to economic institutions, social conditions, and political environment. We suggest that there are several plausible mechanisms to explain higher suicide rates among later-born children, but other mechanisms might also operate in the opposite direction, that is, have beneficial outcomes among later-born children. Specifically, there are reasons to expect a different relationship between birth order and psychiatric outcomes in poorer societal contexts.