Geno, The problem with that is you are blaming people only with Addictive personalities. Which isn't fair or true in most cases.
Humans have what is called CURIOSITY for the unknown. We are curious creatures by nature. So if we do something, and enjoy, then are presented with something that creates different feelings than we've felt, we are naturally going to be curious. Now some people are "brave" enough, or I guess enjoy the high enough, to follow through in their quest for knowledgment/getting high to try something else. Other people are scared/smart and don't go any further.
It's not based on the fact that someone has a personality disorder. EVERYONE I've known that has smoked weed, has atleast done Acid = VERY bad drug. Now is this because of Curiosity or because Weed is a gateway drug? A lot of them have done Coke or X - no crack or H. But I'm talking just normal regular college kids from back in the day here, not some Addictive personallity or strung out addict. All of them had their heads on straight, thankg god, and have quit their stuff. It's all because one of them said "lets smoke some weed" one time, and we all smoked and ended up having a good time (plus a case of the mad munchies!). So we did it again, and again. But it's not because we had problems, we liked to have fun.
What group do they fit in? They tried Acid, X, and maybe even coke too. So weed in this case was a gateway drug. But for something to be a gatway, the person has to ALLOW it to be a gateway. Obviously if they have the personality to do an illegal drug in the first place, they have the potential to do others. But some people stop, while most try other things. I don't think a medical study will ever be able to tell you what makes humans do things. With so many different reasons for each individual person, it's not black and white, there is a huge gray area.
Peer Pressure is another reason for turning to hard drugs. If they are looking to get into the "in croud" or if they have people leaning on them all the time, it's human nature to trust and to break down and occationally do stuff we wouldn't normally do.