A puff of logic

Genes, cannabis and schizophrenia. Correlation is not necessarily causation
The Economist (UK)
Saturday, June 28, 2014

That cannabis and schizophrenia are linked is widely accepted. Several studies suggest the drug can set off short-term psychotic episodes in those already suffering from the condition. Other research, though, does more than this. It shows that people with schizophrenia are twice as likely as others to use cannabis. This leads some to argue that the drug is actually a cause of schizophrenia rather than just a trigger—a line of evidence sometimes employed by those who wish to keep it illegal.