Notwithstanding limitations, this study offers a significant advance in our understanding of the relation between the age of substance use initiation and symptoms of later substance use disorder. Findings provide evidence of overlap in both the familial and unique environmental liability to early substance involvement and later disorder, suggesting that there are shared pathways of risk for nicotine, alcohol, and cannabis. Further, some of this risk may be transmitted via causal mechanisms. Continued investigation of substance-specific and cross-substance processes will allow for a more complete understanding of the mechanisms underlying polysubstance involvement and comorbid disorders.
