Yeh, which came first? There is the old expression of "drowning your sorrows". When I grew up it was the kids who came from broken homes or loose parental control, these were kids who "hung out" at young ages who were the first to get into the drug culture. Could it be these kids were not happy before they starting partying? And speaking of pot all these kids started drinking before they got into weed. Is booze a gate way to harder stuff that should be prohibited?
All these studies should be treated with skepticism. Chocolate causes heart disease or cancer one day and it's good for you the next. Coffee? Second hand smoke? global warming? What's your agenda? I've got a study to back you up.