This article was originally posted on RealClearScience.
It is known that there is a link between schizophrenia and smoking marijuana. However, much to the chagrin of researchers, it is not ethical to randomly assign people to smoke pot to determine if they develop schizophrenia at some point. As a result, it remains unknown if smoking pot causes schizophrenia, if schizophrenics are more likely to smoke pot, or if some unknown third variable links the two.
A new paper in Molecular Psychiatry attempts to answer this question. It appears that, to at least some extent, genetics can predispose an individual to both schizophrenia and marijuana use.
The authors determined the “polygenic risk score” for schizophrenia in 2,082 healthy people. (The polygenic risk score determines how likely a healthy person is to develop schizophrenia based on his genetic profile.) Their prediction was that people with higher risk scores would also be likelier to smoke pot. And that is exactly what they found, though the causative link was vanishingly small. (More on that below.) Interestingly, they also found that if a pair of twins had a high average polygenic risk score, both of them were likelier to smoke pot. (See graph.)
Overall, the link between a genetic predisposition for schizophrenia and smoking cannabis was statistically significant, but very small: Less than 1% of the variance in cannabis use could be explained by an underlying genetic predisposition to schizophrenia. That means more than 99% of the variance in cannabis use cannot be explained by such a genetic predisposition.
But, there are a few points worth making:
First, it is typical for these types of studies to show only very small links. It reflects our poor understanding of genetics and gene-environment interactions. Indeed, the polygenic risk score only explains about 2-7% of the variance in schizophrenia itself, so it is not surprising that the authors only found a very small causative link. Second, this study looked only at genes linked to schizophrenia. It is quite possible that there is a genetic predisposition to smoking pot, independent of a predisposition to schizophrenia. Third, as the authors indicate, there is plenty of evidence to suggest that smoking pot increases the risk of developing schizophrenia, though the risk here also appears small.
So, potheads (especially you teenagers), it looks like you should worry more about possibly losing 8 IQ points than developing a mental illness.
Source: RA Power et al. “Genetic predisposition to schizophrenia associated with increased use of cannabis.” Molecular Psychiatry. Published: 24 June 2014. doi:10.1038/mp.2014.51