Every time you find yourself saying that there must be some causal relationship between two strongly correlated variables, you should go back and look at this graph:

As Atlantic Business contributor Derek Lowe, from whom I stole that graph, notes,

I've seen a lot shakier plots used to justify some sweeping conclusions, and if those were justified, well, then I'm forced to conclude that Mexican lemons have improved highway safety a great deal. The vitamin C, maybe? The fragrance? Bioflavanoids?

This is particularly tricky when you bring time into it, because things trend--as we get richer, we buy safer cars, get better emergency rooms, etc.  We also import more lemons to make our chi-chi cocktails and lemon meringue pies.  Overlay the two, and you've got a hell of a causal relationship.

But I expect that four years from now, we'll still be having the same conversations with proponents of "cancer clusters" and Democrats convinced that they can scientifically prove that Democrats are better for GDP by doing ham-fisted regressions of Democratic presidencies with a few tightly correlated economic variables.  What's the mechanism?  What makes electric power lines cause cancer, but not the earth's vastly more powerful magnetic field?  What policies did Harry Truman and Bill Clinton have in common (but not with Richard Nixon) that caused this marvelous confluence?  Well, maybe we don't know the mechanism exactly, but never you mind:  just look at that bee-yoo-ti-ful correlation!