All that aside, the obvious point of the example is to promote my theory that Jane is more likely to become addicted.

And until we can find a pair of genetically identical twins, separated at birth, one living in upper-middle-class and the other living in a ghetto, who are willing to participate in this experiment, it will remain a theory.

I see your point that addiction has environmental factors as well as genetic ones. I'm just seeing it from a different perspective: Even in a drug-abuse-friendly local culture, some people will be more genetically predisposed to become addicted than others. It's just in that culture, they're more likely to get more doses of the drug, which will trigger addiction even in those not predisposed to it.