First of all, let me point out that I used the word "precipitates" and not "causes" for a reason, and that is because the causative effects of marijuana use on these disorders is still unclear. However, the evidence that it causes their manifestation is strong. This could mean that these disorders would happen anyway, but not normally manifest until later, in which case it could exacerbate their severity. Or it could mean that people in the early stages of those disorders tend to turn to marijuana use for whatever reason (the phenomenon often termed "self-medication").
MRI studies that show changes in brain structure hardly qualify as "correlation." Furthermore, that saying "correlation does not equal causation" is meant to inform those who see X and Y happen in close temporal proximity and automatically assume that Y is a result of X, when there is not necessarily a relation at all. OTOH, when you grasp that correlation does not always equal causation, it does not mean that you can automatically reject a causative relationship--because some correlations ARE a result of causation. When two observable variables show a strong correlation, and there is a mechanism by which X can cause Y, then correlation tells you to what extent X is involved in causing Y. Other relationships can also be analyzed through correlation--for instance, X and Y might both be related through some other variable or series of events.
There is a strong correlation between marijuana use and schizophrenia. There is also some evidence that marijuana actually causes psychotic disorder. That is, it is well established that marijuana causes transient psychotic episodes, but how much it might contribute to permanent psychosis is still under investigation.
I am well aware that those who have advocated for legalized marijuana love to spread the story that it is perfectly safe, and often compare it to alcohol (as if, somehow, the effects of alcohol have any bearing on the effects of marijuana). However, this vast experiment in legalization has inspired many researchers to conduct studies, so that where information was lacking before, it is now accumulating at a rapid rate. The often touted "safety" of marijuana is not supported by actual evidence, and more evidence is piling up against it all the time. www.pubmed.org, search term "marijuana", a lot of suggestions come up as to specific sub-topics.
This book is 16 years old, so some of the information is out of date, but still gives an idea of some of the research that has taken place: http://www.ncbi.nlm.nih.gov/books/NBK230716/
First of all, let me point out that I used the word "precipitates" and not "causes" for a reason, and that is because the causative effects of marijuana use on these disorders is still unclear.
So noted.
Or it could mean that people in the early stages of those disorders tend to turn to marijuana use for whatever reason (the phenomenon often termed "self-medication").
Indeed.
MRI studies that show changes in brain structure hardly qualify as "correlation."
Those studies did not show changes, since they were not longitudinal - they did not track users over time. Instead they simply compared the MRIs of users and non-users at single times.
When two observable variables show a strong correlation, and there is a mechanism by which X can cause Y, then correlation tells you to what extent X is involved in causing Y.
If there are multiple mechanisms by which X can cause Y, or Y cause X, or Z cause X and Y, then correlation does not favor any one mechanism over the others - nor over 'none of the above.'
There is a strong correlation between marijuana use and schizophrenia.
At some levels - on the other hand, while marijuana use rose in the USA over the past few decades there was no increase in schizophrenia.
There is also some evidence that marijuana actually causes psychotic disorder. That is, it is well established that marijuana causes transient psychotic episodes, but how much [or if at all - CF] it might contribute to permanent psychosis is still under investigation.
Their very names state the substantial difference between transient psychotic episodes and permanent psychosis - so it's a speculative leap from marijuana causing the former to marijuana causing the latter.
I am well aware that those who have advocated for legalized marijuana love to spread the story that it is perfectly safe,
I've never heard that said, and I've certainly never said it.
and often compare it to alcohol (as if, somehow, the effects of alcohol have any bearing on the effects of marijuana).
The differences have never been shown to be relevant to the legalization debate. You might as well say that the legal status of alcohol is irrelevant to the legal status of marijuana because alcohol is a liquid and marijuana is not.
This book is 16 years old, so some of the information is out of date, but still gives an idea of some of the research that has taken place: http://www.ncbi.nlm.nih.gov/books/NBK230716/
An excellent reference - here's what it says about psychological harms: "A major question remains as to whether marijuana can produce lasting mood disorders or psychotic disorders, such as schizophrenia." "The association between marijuana and schizophrenia is not well understood."