Geographic proximity is associated with transmission of suicidal behaviour among siblings.

Abstract

The aim of this study was to clarify the role of 'contagion', or social transmission, in risk of suicidal behaviour (SB) among siblings. We followed Swedish sibling pairs until one of them (S1; N=111,848) was registered for a suicide attempt or completion. We tested the effect of geographic proximity between siblings on risk of a first SB registration of S1's sibling (S2). To control for familial confounding, we conducted complementary analyses of sibling trios (N=701), comparing risk in different siblings as a function of their respective proximity to S1. The best-fitting model across sibling pairs included an effect of distance between siblings (HR=0.96, 95% CI=0.93-0.99). Hazard ratios declined quickly up to 25km and largely stabilized beyond 150km. Across all pairs, a larger age difference between siblings was associated with reduced SB risk (HR=0.96 95% CI=0.93-0.98). Findings were consistent within the sibling trios. Consistent with the concept of suicide contagion, risk of suicidal behaviour subsequent to a sibling's suicide completion or attempt is higher as a function of sibling closeness. These findings are robust to potentially confounding familial factors.