There was a story last Friday about a new study in Britain that supposedly showed that smoking one joint would increase your chances of developing schizophrenia and other mental illness by 40 percent, at least as some papers reported it. Scary? It sounded that way, but wait a minute.
Some papers got it right -- this was a correlation, not a demonstration of causation. People who smoke pot are more likely to have schizophrenia. Is this because people with schozophrenia are more likely to smoke pot, for whatever reason? The numbers simply don't tell us.
In addition, as Mark Kleiman, over at The Reality-Based Community (scroll down to Exhale!)points out, if there were anything remotely resembling a cannabis-causes-schizophrenia phenomenon, you would expect those born in 1953, who came of using age at a cannabis peak, to have more schizophrenia than those born in 1963, who came of age during a cannabis-using trough. Sorry, no difference. In addition, an Australian study actually showed a decline in schizophrenia correlated with an increase in cannabis usage.