Why is weed illegal?, page-32

As well as 'undeveloped' brains, by which I assume you mean immature,cannabis use has been shown to enhance psychosis in those particularly young users with an inherent tendency, be it genetic or behavioural/environmental.
With many of those in the above category they would possibly gone on to become psychotic even without the dope.

Really heavy and long term use is also stated to be a factor in psychosis.