The heritability of both OCD and SUDs is well established,23,24,25 but studies are only beginning to clarify the contribution of familial influences to their association. Although some studies found no familial coaggregation,10,11 an elevated risk of OCD among first-degree relatives of individuals with alcohol dependence has been reported.26 A study using genome-wide association study (GWAS) summary statistics found a statistically nonsignificant negative genetic correlation between OCD and SUDs,27 whereas the latest GWAS of OCD shows a positive genetic correlation with alcohol dependence.28 Theoretically, a shared endophenotype for OCD and SUDs should manifest as a positive genetic correlation between the two. On the other hand, a direct effect of OCD on SUDs (eg, via self-medication) would be detected as a nonshared environmental correlation, independent of shared genetics. Quantitative genetic studies can test these competing hypotheses, but to our knowledge, no prior sibling or twin studies exist.
