Identifying Causal Relationships vs. Ruling Out All Other Possible Causes

I was in Washington last month to discuss my work on food prices, in which I look at whether food prices cause social unrest, at an event whose goal was to discuss the link between climate change and conflict.

As many readers of this blog know, disentangling causal relationships from mere correlations is the goal of modern science, social or otherwise, and though it is easy to test whether two variables x and y are correlated, it is much more difficult to determine whether x causes y.

So while it is easy to test whether increases in the level of food prices are correlated with episodes of social unrest, it is much more difficult to determine whether food prices cause social unrest.

In my work, I try to do so by conditioning food prices on natural disasters. To make a long story short, if you believe that natural disasters only affect social unrest through food prices, this ensures that if there is a relationship between food prices and social unrest, that relationship is cleaned out of whatever variation which is not purely due to the relationship flowing from food prices to social unrest. In other words, this ensures that the estimated relationship between the two variables is causal. This technique is known as instrumental variables estimation.

Identifying Causal Relationships vs. Ruling Out All Other Causes

As with almost any other discussion of a social-scientific issue nowadays, the issue of causality came up during one of the discussions we had at that event in Washington. It was at that point that someone implied that it did not make sense to talk of causality by bringing up the following analogy:

Imagine a house on stilts. Mold has been accumulating on the stilts for a number of years. One day, a hurricane comes along, and the house collapses. Can we really say that the hurricane caused the house to collapse when the mold has been eating away at the stilts for a long time?

I am paraphrasing, but the idea was that it was impossible to talk of causality given that most things had both proximate causes and distal causes — the hurricane and the mold, respectively, in the example above.

When the house-on-stilts analogy was brought up, many shrank and responded that establishing whether x causes y is not the same as saying “x is the only cause of y.”

Likewise, establishing that food prices cause social unrest is not the same thing as saying that food prices are the only cause of social unrest. Of course there are other factors.

When food prices rise sharply, people might be rioting in Lagos, but they are unlikely to riot in Milwaukee, which tells us that there must be something else going on.

But the fact that something else might going on does not mean that we cannot ask whether x causes y. When we talk about identifying causal relationships, we are not talking about ruling out all other possible causes. Sometimes a cigar is just a cigar, and when we ask whether x causes y, what we are really asking is “Does x cause y?,” and not “Is x the only cause of y?” The former is answerable; the latter is akin to asking about the unmoved mover of Aristotle’s Metaphysics.