Answer to Question 1
A substance (e.g., cigarettes) that is used before illicit substances AND use of which is associated with increased likelihood of later use of illicit substances. Assuming a person is generally more likely to engage in deviant or problem behavior, the apparent gateway substance might just be the easiest thing for a young person to start with, so it is most likely to be the first. Bonus: discussion of whether preventing or delaying cigarette smoking would reduce later use of marijuana or other substances.
Answer to Question 2
Most large surveys find little or no relationship to most personality measures, such as self-esteem. The most consistent correlations have been found for impulsivity. Bonus: distinguishing between studies of rates of use in the general population vs. comparisons with dependent users, with both impulsivity and personality disorders more strongly associated with dependence/abuse.
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