when a scientific study ends by stating that there’s uncertainty about whether a correlation proves a cause, it doesn’t mean that correlations are meaningless in every circumstance. Everyone knows that creating false correlations between two unrelated elements is easy. But it can be that a correlation is so powerful and reliable that it may actually point to that rare thing in the social sciences, a demonstrable causal relation. As a wise man once said, “Correlation is not causation, but it sure is a hint.” When you can separate out a truly robust correlation between two elements in our social life, it’s a big deal.
What makes a correlation causal? Well, it should be robust, showing up all over the place, across many states and nations; it should exclude some other correlation that might be causing the same thing; and, ideally, there ought to be some kind of proposed mechanism that would explain why one element affects the other. There’s a strong correlation between vaccines and less childhood disease, for instance, and a simple biological mechanism of induced immunity to explain it. The correlation between gun possession and gun violence—or, alternately, between gun control and stopping gun violence—is one of the most robust that you can find.