Countries that eat more chocolate per capita win more Nobel Prizes per capita. Ice cream sales and drowning rates are strongly positively correlated. The number of Nicolas Cage films released in a year correlates with the number of swimming pool drownings. Shoe size correlates positively with reading ability in children. These examples are amusing precisely because the causal interpretations they might seem to invite are obviously absurd: nobody seriously believes that eating chocolate causes scientific genius, that ice cream causes drowning, or that Nicolas Cage’s filmography influences pool safety. Yet the statistical relationships are real. The correlation exists in the...