What factors and forces lead someone to "turn on" to marijuana? More important, what causes someone who tried the drug to become a regular user? It would be fallacious to assume that any behavior as complex as the use of drugs, or any one drug, can be completely explained by a one factor or variable, or even a single integrated theory.

Many factors, forces, and mechanisms contribute to the use of drugs in general, and even to use by a specific individual. We have a number of empirical regularities describing illegal psychoactive drug use generally, and marijuana specifically. These correlations are not in doubt; they are, as statisticians are fond of saying, "robust" relationships, solidly documented, independently confirmed by different researchers in a variety of locales (within the United States and Canada, at any rate), and constant over time. The validity of the various explanations to account for these regularities, however, is still being debated, as we saw in Chapter 3.

A team of researchers (Radosevich et al., 1980) distinguished three interrelated sets of variables that are causally related to marijuana use:

• structural variables, which include sociodemographic factors such as age, sex, social class, race, and community or region of residence;
• social-interactional variables, which pertain to interpersonal relationships, or the likelihood of associating with and relating to individuals with varying degrees of involvement with marijuana or its correlates and accompaniments (an example would be one's friends' use of marijuana, or use patterns in one's peer group);
• attitudinal variables, or attitudinal (and, ultimately, behavioral) factors pointing to one's views both of the drug itself and behavior associated with its use—beliefs about whether the drug is harmful and willingness to break the law are two examples of this dimension.

Of course, these sets of variables overlap a great deal; they cannot be sharply or cleanly separated.