Causality vs. correlation in the relationship between alcohol and suicide

Last week in my ECONS101 class, we covered the difference between causation and correlation. It is important to recognise that just because two variables appear to be related, that does not mean that a change in one of the variables will cause a change in the other variable. Even if we can tell a convincing story that a change in Variable A causes a change in Variable B, the relationship we observe in the data might not arise instead because:

• a change in Variable B actually causes the change in Variable A (reverse causation);
• a change in some other variable (Variable C) causes the changes in both Variable A and Variable B (confounding, or a common cause); or
• the two variables are completed unrelated and the relationship showed up by chance (spurious correlation).

Understanding the difference between correlation and causation is important, because if you create some policy based on the false conclusion that changing Variable A will change Variable B, but the relationship is not actually causal, then you won't get the change you expected or wanted.

Now, although it is a simple maxim to say "correlation is not causation", in practice it is actually quite difficult to avoid attributing causality to correlational data. A lot of research uses language that implies causality, like "the impacts of X on Y" or "the determinants of Z", even when the research design is really only showing a correlation (I know I've done this myself). So, we need to take extra care in interpreting results to avoid people drawing the wrong conclusions. And the media, in particular, are fond of over-stating conclusions. Take this article in Stuff from earlier this month:

One in four New Zealanders who die by suicide do so with excess blood alcohol – with the figures even worse for Māori and Pasifika, new analysis of coronial data has found.

The findings raise major red flags and should prompt urgent changes to Aotearoa's suicide prevention strategy and the Sale and Supply of Alcohol Act, according to the authors, from the University of Otago.

New Zealand’s “alcohol-saturated culture" meant the numbers were disheartening, but not surprising, study author and lecturer Dr Rose Crossin said.

The original research paper, by Rose Crossin (University of Otago at Christchurch) and co-authors, published in the New Zealand Medical Journal, is here (gated). It finds that:

...around one quarter (26.6%) of all suicides over the study period involved acute alcohol use.

We also found significant ethnicity differences, with Māori and Pacific peoples more likely to die by suicide involving acute alcohol use than European and Asian ethnicities.

Now, even though a good story can be told about how alcohol use reduces inhibitions and may lead to increased risk of suicide, there is nothing in this study that demonstrates that link. And, no matter how much we might abhor the statistics, they still don't (by themselves) show that alcohol use causes suicide.

Why is this study not demonstrating causality? For one thing, there is a potentially confounding third variable that is not accounted for - mental health state. People in poor mental health are more likely to use alcohol excessively. They are also more likely to commit suicide. So, high alcohol use may be incidental to suicide, because both are caused by a third variable (mental health). Now, that doesn't mean that alcohol is completely off the hook as a cause, only that there may be other explanations that haven't been excluded in this study. In fact, in the coronial data that they use, the coroners themselves indicate that alcohol was "a contributory cause of death" in about one-third of those that involved acute alcohol use, or about 9 percent of all suicides.

Notice also from the Stuff article quote above that the authors are advocating for policy change, based on the correlations observed in their study. There is good reason to believe that we need a change in the way that alcohol sale and supply is controlled, and my past (and possibly, future) research has contributed to this. However, this study on its own does not contribute much to the case for reform. Even if we assumed that changes in alcohol regulation eliminated suicides with alcohol as a contributory cause, that would reduce suicides by only 9 percent. That sounds like a worthwhile goal. Indeed, any reduction in the suicide rate would be a great victory. However, before we adopt the policy change on the basis that it would reduce suicide risk, we'd want to know how much (if any) of the correlation arises from a causal relationship. And that's not something we know right now.