The current analyses also demonstrated a strong inverse relationship between age of onset of cannabis use and risks of a range of drug-related outcomes including use of (other) illicit drugs and development of abuse or dependence on cannabis, other illicit drugs, and on the licit drugs tobacco and alcohol (Figure 1). For example, 36.8% of those who reported using cannabis before age 16 reported lifetime use of cocaine, compared with only 13.2% of those who commenced cannabis use after age 20 (and 1.1% of those who reported never using cannabis). Additionally, the association between age of onset of cannabis use and these outcomes was approximately linear with risks of these outcomes increasing steadily as age of onset of cannabis use declined. While such associations have commonly been reported in the literature, they remain controversial and, in particular, interpretation of these associations and of the mechanisms underlying them remains a topic of considerable debate. In addition to the advantages to studying this important question inherent in the twin design, the current study also contained several unique aspects of data collection that
