LAUSR

Abstract Background Although alcohol use disorder (AUD) runs strongly within families, studies examining the impact of rearing environment, unconfounded by genetic effects, are rare and, to date, contradictory. We here seek to conduct such a study using an adoptive co-sib control design. Methods Defining high-risk as having ⩾1 biological parent with an externalizing syndrome (AUD, drug abuse or crime), we identified 1316 high-risk full-sibships and 4623 high-risk half-sibships containing at least one member who was home-reared and one who was adopted-away. Adoptive families are carefully screened in Sweden to provide high-quality rearing environment for adoptees. AUD was assessed from national medical, criminal and pharmacy registries. Results Controlling for sex, parental age at birth, and, for half-siblings, affection status of the non-shared parent, hazard ratios (±95% CI) for AUD in the matched adopted v. home-reared full- and half-siblings were, respectively, 0.76 (0.65–0.89) and 0.77 (0.70–0.84). The protective effect of adoption on AUD risk was stronger in the full- and half-sibling pairs with very high familial liability (two high-risk parents) and significantly weaker when the adoptive family was broken by death or divorce or contained a high-risk adoptive parent. Conclusions In both full- and half-sibling pairs, we found evidence that the rearing environment substantially impacts on the risk for AUD. High-quality rearing environments can meaningfully reduce the risk for AUD, especially in those at high familial risk.