Among individuals with alcohol use disorder (AUD) and drug use disorder (DUD), is their genetic liability and its specificity moderated by substance availability?

Offspring (born 1960–1995) and their biological parents from three family types [not-lived-with (NLW) biological father, mother and adoptive] and their AUD and DUD diagnoses were ascertained from Swedish national registers. Parent–offspring resemblance was calculated by tetrachoric correlation.

In Swedes born from 1960 to 1995, prevalence rates of AUD were stable while DUD rates increased substantially. Best-estimate tetrachoric correlations (±95% confidence intervals) between AUD in biological parents and AUD and DUD in their offspring were, respectively, +0.19 (0.18–0.20) and +0.18 (0.17–0.20). Parallel results from DUD in parents to AUD and DUD in children were +0.12 (0.10–0.13) and +0.27 (0.26–0.28). When divided into older and younger cohorts, the specificity of DUD transmission increased substantially over time, while the genetic correlation between AUD and DUD significantly decreased.

Raised when alcohol was the preferred substance of abuse and illicit drugs highly stigmatized, AUD in parents reflected a general liability to substance use disorders, as they transmitted similar genetic risk for AUD and DUD to their children raised when both substances were widely available and relatively acceptable. DUD in parents, by contrast, reflected a more specific liability to DUD and, when transmitted to offspring, produced a considerably stronger risk for DUD than for AUD that increased over time. The magnitude and specificity of the genetic liability to psychoactive substances can be influenced by the availability of that substance.

While drug abuse (DA) is strongly familial, we still have limited knowledge about the causes of its cross-generational transmission.

We 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.

Combined 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).

Both 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.