crookedshoes, here is a neat twin study that suggests a genetic component to not only addiction, but also a predisposition for abuse (seen more in identical twins). This is specifically about cocaine.

http://bjp.rcpsych.org/content/173/4/345.abstract

There is also research showing a genetic component for not becoming addicted (to alcohol in this case) because of a deficiency in a particular gene in some Asian populations. This group can't metabolize alcohol (well or at all) and become sick quite easily so the chance of becoming addicted is lowered, presumably.

Of course, there is also the environment. If a person has a genetic predisposition for addiction but is never exposed to the drugs/alcohol, he/she obviously won't become addicted.

Mike

PS. I am not in the field of drug dependency. I can't contribute expert advice.