As has been discussed in detail by others (e.g., Agrawal et al., 2004; Kendler et al., 1993; Vitaro et al., 2009), depending on the nature of the association between risk and outcome variables, the pattern of association between substance use (the risk factor) and educational attainment (the outcome) will vary systematically for the general population vs. MZ discordant pairs vs. DZ discordant pairs, and the pattern of association for these groups can indicate which processes are important. If genetic factors fully account for the association between substance use and educational attainment, then there will be a significant association between the measures in the general population, a moderate association among DZ pairs discordant for substance use, and no association among MZ pairs discordant for substance use (i.e., population > DZ discordant > MZ discordant = 1.0, with 1.0 being an odds ratio and thus indicating no association). If the association is solely due to shared environmental characteristics, then population > DZ discordant = MZ discordant = 1.0. And if the risk factor causes the outcome (or both are attributable to unmeasured
