August 18, 2013

Correlation, genetics, and causation

There’s an interesting piece on cannabis risks at Project Syndicate. One of the things they look at is the correlation between frequent cannabis use and psychosis.  Many people are, quite rightly, unimpressed with the sort of correlation, since it isn’t hard to come up with explanations for psychosis causing cannabis use or for other factors causing both.

However, there is also some genetic data.  The added risk of psychosis seems to be confined to people with two copies of a particular genetic variant in a gene called AKT1. This is harder to explain as confounding (assuming the genetics has been done right), and is one of the things genetics is useful for. This isn’t just a one-off finding; it was found in one study and replicated in another.

On the other hand, the gene AKT1 doesn’t seem to be very active in brain cells, making it more likely that the finding is just a coincidence.  This is one of the things bioinformatics is good for.

In times like these it’s good to remember Ben Goldacre’s slogan “I think you’ll find it’s a bit more complicated than that.”


Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »


  • avatar
    megan pledger

    One of the quotes “However, a recent study that followed people from birth to age 38 found that those who started using cannabis early, every day, and for several years had a permanent 8-point drop in IQ scores. (The study has not yet been replicated.)” is based on data from the The Dunedin Multidisciplinary Health and Development Study.

    That article got ripped apart by Andrew Gelman et al here

    4 years ago

  • avatar
    megan pledger

    Some people with psychotic disorders say they self-medicate with cannabis ***. That means that all the correlation studies that link cannabis and psychotic disorder really have to be *really*, *really* clear about what happened first – the mental health issues or the cannibis use.

    That’s really hard because some of the prodromal symptoms of mental health disorders are really easy to overlook and some people will want to hide them anyway because of the stigma of having mental health issues.

    That’s why it’s worth taking some of the correlational studies with a grain of salt, no matter that they have many tens of thousands of participants.

    ***Scholfield D, Tennant C, Nash L, Degenhardt L, Cornish A, Hobbs C and Brennan G. Reasons for cannabis use in psychosis. Aust NZ J Psychiatry. 2006; 40: 570-574.

    4 years ago