[lojack]

correlation != causation

OP was commenting on why there may not necessarily be a causation, and you commented on how the doctors tend to think there's a correlation. It just as well could be that schizophrenia tends to lead to increased marijuana consumption as people are self medicating. I'm no expert, so I don't have any strong beliefs around causation, but I do think both lines of thought (marijuana causing schizophrenia, or vice versa) seem pretty logical to me.

It's pretty widely believed that there is a correlation between the two, but the reason behind the link is where there's a lot of debate.