In my experience it's a preexisting mental health disorder that leads people to smoking habitually ie self medicating. My brother is bi-polar but he refuses medical help so he smokes a hell of a lot of pot. I have social anxiety disorder and for about a year and a half i smoked 3-4 times everyday with no breaks to numb the anxiety. I started taking paxil about a month ago and its allowed me to cut way back and even go days in a row without smoking.

I don't know if i'd believe that smoking pot leads to schizophrenia or bi-polar disorder. I'd think the people willing to smoke enough that it could lead to the behaviors present in those conditions are people who are predisposed to having them anyway. I'd be willing to bet that many habitual smokers have other family members with abuse problems be that alcohol, food, marijuana, etc.