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

Tony, another thought: I'm not sure that it satisfies all of your conditions, but I was reminded of the many famous "twin studies" that were conducted in the study of schizophrenia and, sure enough, there are a series of twin studies such as this one sponsored by NIDA. From what I can tell from reading the abstracts, the studies confirm the genetic component of addiction (we knew that, right?) and, to some extent, include environmental factors. So, perhaps the prospect of a Sue-and-Jane analysis aren't so far-fetched.

With almost any research, I like to give it a while to "level out". As with studies of hormone therapy, it often seems that various studies can see-saw in contradictory fashion until they reach some sort of equilibrium!