Why Correlation is not Causation: Cannabis Use and Schizophrenia

As so often happens, a post from Science on Google+, a community I help moderate, has got me thinking about how easy it is for headlines to quickly lead to #ScienceMediaHype . A post with a link to a news story has the headline, “Teen Marijuana Use Linked with Schizophrenia.” As a sociologist with an interest in mental health, this sets off alarm bells. The discussion on our community quickly turned into a debate about the correlation presented in the headline. As a few of our community members pointed out, correlation does not equal causation.  My post provides a summary of the actual study and I discuss the sociological problems associated with media coverage of mental illness.

Study on Working Memory

The linked article does an okay job of describing the study but the headline and its focus over-extends the study’s findings. This is the problem with media stories: headlines can shape the way the public understands scientific findings. Journalists present quotes from scientists that fit the angle of their story, putting less emphasis on other aspects of the research. 

The study is published in Schizophrenia Bulletin. The sample includes 44 healthy controls, 10 people with a history of cannabis use disorder (CUD), 28 schizophrenia participants with no history of substance use, and 15 schizophrenia patients with a CUD history. Ninety percent of the participants who had schizophrenia already had a CUD history prior to their mental illness. Most of the cannabis users were heavy users, smoking cannabis daily or at least weekly, and most also smoked cigarettes heavily, a variable that the researchers wanted to test. 

The study actually tests working memory deficiency not the cause-effect relationship between schizophrenia and cannabis use. Participants were given memory activities and then brain imaging was used to see their brain patterns. I’ve included the three images from the study. Neuroscience is not my area of research, but I include the diagrams in case other researchers should be interested.

The researchers note that without substance abuse, schizophrenia inhibits cortical development. They note less is known about how cannabis affects brain symmetry, though it is known to disrupt the hippocampus,  which is related to our limbic system. The hippocampus is linked to information retention for both short and long-term memory as well as other functions like spatial navigation. 

The study finds that use of cannabis at an early age impacted memory function. The sample had an average age of 24, so patterns associated with longer-term development need further study. 

The study observes that cannabis users and the participants living with schizophrenia both have problems with memory tasks. At the same time, the researchers not that the brain asymmetry observed when carrying out memory tasks may be linked to a” neurobiological vulnerability” among schizophrenia sufferers. That is, that the observed pattern may be the outcome of a predisposition to substance abuse. For example, the researchers note that similar brain patterns are found amongst cocaine users. So, to put it another way, schizophrenia users may be drawn to substance abuse. More on this below.

Finally, the authors conclude what many of our community members had been discussing: that there is no direct cause and effect relationship.

“Although our data may be compatible with a causal hypothesis, the cross-sectional data do not allow us to test causal relationships or reject alternative explanations. Thus, the shape differences could be explained as either due to the effects of chronic cannabis abuse or the presence of biomarkers that characterize a vulnerability to the effects of cannabis.”

Their research notes that with laws changing, cannabis may be more readily available to youth with a predisposition towards schizophrenia. This makes their research all the more pivotal. This is both in terms of better understanding how brain development is affected by schizophrenia and the social, health and subjective reasons why youth may engage in cannabis use at different stages of their disease. 

Research on Correlation

There are many studies that have linked self-reported cannabis use in early adulthood to an increased risk of developing schizophrenia later in life. One of the most widely cited studies involves Swedish conscripts of 1969, with a follow up study confirming the results. Nevertheless, the direct association between cannabis use and schizophrenia is disputed. For example, there is more to be learned about the relationship between the time that someone starts using cannabis and their first schizophrenic episode. 

There are three hypotheses to explain why young people with schizophrenia use cannabis. 

1) Cannabis triggers the disease in people with a predisposition, as is suggested by the study at hand.

2) People with schizophrenia use cannabis as a way to self-medicate or manage their experience of the disease. The research does not support this strongly, though cannabis use may give sufferers a perception of control over their disease. This is not something to be dismissed and requires further research. One study from 2012 suggests that one component of marijuana, cannabidiol, may be used to treat schizophrenia. This component does not contain THC which is responsible for the intoxication effects of traditional marijuana. Instead, cannabidiol may reduce the symptoms of psychosis, but further research is needed to fully test this treatment.

3) Cannabis use can trigger schizophrenia through confounding variables. This is the prevailing medical view, though the association is not so neatly weaved together. 

Problems with Media Coverage of Mental Illness

The article that covered this story went for a shock value headline. Headlines prime audiences about what they should expect from a scientific article. Sensationalised headlines invite personal opinion based on individual experience. This may range from “I smoke pot and I’m fine” to disparaging comments about “crazy” people. This is the problem with the way in which news headlines shape public discussions of science. Should people read an article before discussing the soundbite? Of course. Does this happen in practice? Not as much as it should. The idea that sensational headlines “sell” is flawed. Shock headlines sometimes get people to click on a link. On a social media site like Google+, some people will go off the headline and the text in a post. In fact, most of the social media research shows that people rarely want to click away from the social site they’re currently on (more on this in another post). This is why we stress in our community that posts should summarise the science behind an article in detail. The link should be there for people who want to read further (and if you write a good post, people will be more willing to click on a link!).

While research suggests that people are sceptical of media reports, not everyone is trained to think about research the way scientists do. Paywalls also stop people from reading the study for themselves (as well as the technical language used in academic journals). This is why scientists need to step up and debunk bad science journalism.

Moving Beyond Individual Speculation

The research has established a correlation, but causation is disputed. The evidence strongly suggests that cannabis compounds schizophrenia and the gravity of this finding cannot be reduced. Nevertheless, media stories that run with a causation headline only serve to spread misinformation.

Bad science writing invites cannabis users to say: “I use it and I’m fine!” It also serves to reinforce a cultural stereotype that people with mental illness may have avoided their condition if they’d only stayed away from recreational drugs. In the end, the causation narrative does more damage and serves only to stigmatise both cannabis users and schizophrenic sufferers as deviants. People living with schizophrenia are doubly stigmatised as their cannabis use makes it seem as if they are wilfully contributing to their illness. Research is seeking to better understand why some people with schizophrenia rely on cannabis. 

Mental health is a serious matter. It shouldn’t be reduced to an alarmist headline. Mental illness is part of the human condition. Addressing the treatment of schizophrenia requires compassion, not judgement. Most of all it needs solid scientific research, not dismissive condemnation based on personal or social conjecture.    

10 thoughts on “Why Correlation is not Causation: Cannabis Use and Schizophrenia

  1. Great post Zuleyka Zevallos. You covered a three topics (correlation vs. causation, sensationalized headlines, and science outreach without jargon) that many of us have been working hard on. The study itself is fascinating and your discussion of it puts a different emphasis on it that helped me gain more from it. Thanks for that. I naturally focused on the imaging aspect.

    The images that they show are volume differences from one group to another. The very complex part of it is that you have to use a brain atlas to make sure you are comparing the hippocampus in subject A with the hippocampus in subject B. The atlas is necessary because, as the study demonstrates, not everyone has the same size substructures of the brain. John Csernansky is one of my boss’s collaborators, so I can talk to him about the study.

    I want to emphasize and applaud this statement.

    This is why we stress in our community that posts should summarise the science behind an article in detail. The link should be there for people who want to read further (and if you write a good post, people will be more willing to click on a link!).


  2. Chad Haney Thanks for your thoughtful and interesting comments. I’ve enjoyed your posts as you know and I agree with you – I think that Google+ is really special as there are lots of scientists on here doing good work. I’ve learned so much about other fields specifically because so scientists make their research and insights accessible. But of course, for every active scientist on Google+ there are dozens that are less so. I want to understand why this is the case and hopefully encourage more scientists to critique bad science journalism and to share their work with the public.

    It’s very useful to read about the brain atlas. As I mentioned, the brain imaging aspect of the research is something I could not comment on, but something I’m profoundly curious about. I’d love to learn more once you speak with John Csernansky!


  3. L McGarity The researchers show that various studies have examined the effects of cannabis on the cerebral cortex, but very little has been done on subcortical regions of working memory. There’s an established body of evidence that cannabis does affect working memory separately for regular (that is, heavy) cannabis users, especially those who smoke from a younger age. The same goes for people living with schizophrenia. So the researchers wanted to see the connection between these phenomena on people who both smoke marijuana (and have done so since youth) and who have schizophrenia. Hope that clears up the study’s focus! 


  4. Zuleyka Zevallos – An off-topic comment on one of your comments:

    “But of course, for every active scientist on Google+ there are dozens that are less so. I want to understand why this is the case and hopefully encourage more scientists to critique bad science journalism and to share their work with the public.”

    Is there a place here for more active outreach to scientists? I envision invitations in the form of QR-coded business cards handed out at conferences and symposia, web pages, emails, as well as simple handshakes and lunch discussions. And similar outreach to colleges and universities, encouraging students to participate through discussion and submission of papers. Ideally, and in the long term, Science on Google+ could be a means for scientists and scientists-to-be to network and to gain both public and professional credibility, adding what venture capitalists and investors call a “market differentiator.”

    I see a constant upward spiral: scientists helping scientists, scientists helping the public, and the public (including the popular press) gaining a better grasp of the workings of science.


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