A shitty environment certainly contributes, but drug abuse has risen in every demographic, and genetic predisposition is a huge factor (40-60% cited here[1]).
>Scientists estimate that genes, including the effects environmental factors have on a person's gene expression, called epigenetics, account for between 40 and 60 percent of a person's risk of addiction
That's why I phrased it as "genetic predisposition". As I understand, environment is of a major factor in gene expression, but the underlying genetics also vary between individuals, eg. Native Americans and other ethnic groups have a genetic predisposition towards alchoholism, or others that are bad with lactose, etc.
I agree with your example, but the number cited is not for innate genetic predisposition. It includes including the effects environmental factors have on a person's gene expression.
Some of these may be relatively uniform through the population, but manifest due to different environmental factors.
e.g. Child abuse could cause epigenetic changes to gene expression associated with a drug predisposition. This may be conserved between individuals.
>Scientists estimate that genes, including the effects environmental factors have on a person's gene expression, called epigenetics, account for between 40 and 60 percent of a person's risk of addiction