Does marijuana contribute to psychotic illness?

Current Psychiatry on marijuana and schizophrenia:

Cannabis and psychosis: 4 clinical pearls Cannabis use increases the risk of developing psychosis and is estimated to double the risk for later schizophrenia (5 to 10 new cases per 10,000 person-years) The association is not an artifact of confounding factors such as prodromal symptoms or concurrent use of other substances (including amphetamines) The risk increases with the frequency and length of use (a dose-effect relationship) Self-medication is not the connection between cannabis use and schizophrenia, according to empiric evidence

I'll keep an eye out for responses to this. It's so hard to know what to trust on something like this. This article seems reasonable, but it feels a little reminiscent of Reefer Madness. Also, the increased risk of schizophrenia may be statistically significant but I find it difficult to get too alarmed about the risk going from 1 in 2000 to 1 in 1000. (Maybe I should be more alarmed. I suppose that you start talking about large numbers when you multiply these numbers by large numbers of users.)