Sue and Jane are hypothetical construtcts in your example. And as you said, the genetic factors are hard to put into a controlled test since they are still only marginally understood. So which one of these two women becomes addicted is going to have to remain a theory for now.

As I said, ginned-up. All that aside, the obvious point of the example is to promote my theory that Jane is more likely to become addicted.

I don't feel bad about not being able to predict any of this more definitely or to offer a better example. I think that this is still the kind of stuff that addiction "experts" argue about all the time.