The causal impact of older siblings' academic achievement on younger siblings' risk for drug use disorder: instrumental variable and propensity score analyses.

BACKGROUND

Drug use disorder (DUD) clusters in families due partly to shared environment, including sibling influences. Low academic achievement (AA) in adolescence increases DUD risk. This study examined whether low AA in an older sibling causally increases DUD risk in younger siblings.

METHODS

We studied all Swedish full sibling pairs (n = 309,666) born 1972-1985 and ≤ 5 years apart. Older sibling AA was assessed at age 16. Using Month-of-Birth (MoB) as an instrument, we conducted instrumental variable (IV) analyses and propensity score (PS) models to evaluate the causal impact of older sibling AA on younger sibling DUD risk, assessed by DUD registration in national medical, criminal, or pharmacy registries.

RESULTS

Older sibling AA significantly predicted younger sibling DUD risk across models. Beta coefficients (±95% CI) were 2.04 (1.97-2.12) in raw analysis, 1.88 (0.74-3.02) in IV, and 1.26 (1.17-1.34) in PS models. Together with the strong first-stage association, the IV estimates remain positive under small departures from the ideal identifying assumptions. Effect sizes declined with increasing sibling age differences (p = 0.036 for IV; p < 0.0001 for PS) and were strongest in male-male pairs (IV: 4.01 [1.42-6.61]; PS: 1.74 [1.55-1.93]). Mediation by older sibling DUD was modest.

CONCLUSIONS

Findings from two causal inference approaches support a largely causal link between low AA in an older sibling and increased DUD risk in younger siblings. Stronger effects in close-aged and male-male pairs further support this conclusion. Interventions to improve AA in older siblings may yield indirect preventive benefits for younger siblings.