Cannabis use in the last year in a US national sample of twin and sibling pairs.

BACKGROUND Three prior population-based twin studies, none of which was nationally representative, suggested that both genetic and familial-environmental factors contribute to family resemblance for lifetime cannabis use. We seek to replicate these results in a US national probability sample of twin and sibling pairs examining only last year cannabis use. METHODS Cannabis use in the last year was assessed by self-report questionnaire. Biometrical twin analyses were performed. RESULTS Twin and sibling resemblance for last-year cannabis use was substantial, and much higher in monozygotic pairs than in dizygotic and sibling pairs, where levels of resemblance were similar. Modeling suggested that sibling resemblance was due to genetic factors--with a heritability of at least 60% and probably family environmental factors. No evidence was found that cannabis use was influenced by a special twin environment. CONCLUSIONS Consistent with prior studies, use of cannabis is substantially influenced by genetic factors but family-environment is also possibly of importance.