Idk...I think the studies that 'reveal' the correlation between marijuana and early onset of mental disorders..should be more subjective. Think about it, if a teenager/early adult is going through crazy fucking mood swings and major stress, what will they turn to? Weed.
Its like this...there was a survey that looked at kids' grades and the time they spent watching TV. As grades went down, hours of TV watched went up....but that doesn't mean the TV caused the bad grades. Maybe the kids that were not as good in school did not study as much in there other free time, and just watched TV more as school wasn't as important to them.