IME, it's never been a discipline issue.
It's always been a "Crack is for the rich; but we're not rich; we're working class" issue.
Simply having money leads to better/healthier sources of pleasure, a more stable emotional state, and another, competing "addiction": a paycheck.
Taking away someone's source of income is the quickest way to turn them into a drug addict (especially if they have no safety net). Add in a dash of hopelessness for the future, and we're all set to someone never getting off their feet, as they spiral into self-destruction.
I would also wager that there's a confounding "drug users in lower socioeconomic strata cannot maintain their expensive habits for long, before either: running out of money, self-destructing and blowing up what little of their lives is left, or simply OD'ing." Someone in a higher socioeconomic strata is likely to be hooked on the dopamine drip of a steady cashflow, and the "need" to keep their present level of material comfort (lest their behavioural conditioning starts throwing "fear" at them!), and thus less liable to be broke; thus, less likely to be undisciplined; thus, less likely to score poorly on IQ tests; and thus, able to keep an active drug habit.
