Our finding of a potentially causal association between some substance use/dependence measures (early alcohol use, alcohol dependence, and daily nicotine use) and educational attainment, but no support for such a relationship between education and other substance measures (nicotine dependence, multiple cannabis measures, and illicit drug dependence), was unanticipated and warrants further attention. It is possible that the initiation of alcohol use and daily nicotine use occurred during a key developmental time for these individuals, whereas the cannabis initiation fell outside this period. There is some support for this, given that our mean age of onset for alcohol use was 18.5 years and for cannabis initiation was 20.1 years. Thus many of these individuals were drinking while still in high school, but most initiated cannabis use after leaving high school. Another potentially important distinction between our measures is that while we examined repeated alcohol use and daily nicotine use, we examined ever use for cannabis. It is possible that repeated use is more critical than initiation in looking at long-term effects on educational attainment. Finally, it is also possible that unmeasured
