The correlation between cannabis and stupidity is 100 percent.
It isn't at all suprising that bathing the adolesecent brain in a poisonous psychoactive chemical like THC would increase the risk of mental illness. The late teens is when the last great growth of brain tissue and development takes place. The brain is marvelously complex and intricate. Put it through a chemical cuisinart and you can expect to have problems.
I wouldn't be surprised if scientists eventually find a link between nicotine and schizophrenia too.