The causes of parent–offspring transmission of drug abuse: a Swedish population-based study

Abstract

BackgroundWhile drug abuse (DA) is strongly familial, we still have limited knowledge about the causes of its cross-generational transmission.MethodWe examined DA ascertained from national registers in offspring of three family types from the Swedish population [intact (n = 2 111 074), ‘not-lived-with’ (n = 165 315, where biological parents never lived with their offspring) and ‘step’ (n = 124 800 offspring)], which reflected, respectively, the effects of genes + rearing, genes only and rearing only. We replicated these results in three high-risk co-relative designs.ResultsCombined across mothers and fathers, the hazard ratio (HR) for DA in offspring given DA in parents was 3.52 in intact, 2.73 in ‘not-lived-with’ and 1.79 in stepfamilies. In 968 biological full or half-sibling pairs one of whom was reared by and the other never lived with their parent with DA, the HR for DA was greater in the reared than ‘not-lived-with’ child (HR 1.57). In 64 offspring pairs of a parent with DA, the HR for DA was greater in a reared biological v. step-parented non-biological child (HR 3.33). In 321 pairs of offspring of a parent with DA one of whom was a not-lived-with biological child and the second a step-parented non-biological child, the HR for DA was greater in the biological v. stepchild (HR 1.80).ConclusionsBoth genetic and environmental factors contribute substantially to parent–offspring resemblance for DA. The general population contains informative family constellations that can complement more traditional adoption designs in clarifying the sources of parent–offspring resemblance.