This latest piece of research (Proal et al. 2013) is a great example of an erroneous assumption that correlation = cause. Previous research has linked marijuana to schizophrenia, suggesting that use of the drug may trigger an episode of schizophrenia (see for example here). However the recent study suggests that causality may be in the other direction – people with a vulnerability for schizophrenia may be more likely to use marijuana.
The study compared families with and without a history of marijuana use, recruiting four groups of participants:
- Non-psychotic people who had used no drugs (N=87).
- Non-psychotic people who had used marijuana (N=87).
- People who had schizophrenia but hadn’t used drugs (N=32).
- People with schizophrenia who had used marijuana (N=76).
They then looked at the relatives of those with schizophrenia (psychosis) in comparison to the relatives of those in the control groups (non-psychotic people).
The results showed an increased risk of developing schizophrenia in the relatives of patients who already had schizophrenia, whether or not those patients used marijuana. This suggests that marijuana does not cause schizophrenia – it may be that people who have a genetic vulnerability for schizophrenia are unduely affected by marijuana or it may be that such a vulnerability leads to a desire for drug use.