Dolores R. sent along an illustration of the nature of spurious relationships. A spurious relationship is one in which two variables appear to be related, but are in fact both caused by a third, “confounding” variable. Drawing on Andrew Sullivan’s map of passport ownership and data on Type 2 diabetes posted at the US News and World Report, Mark Frauenfelder at BoingBoing suggested, with tongue-in-cheek, that owning a passport prevents Type 2 diabetes.  In fact, both are probably related to a third variable, class: having the money to travel also usually means having access to healthy foods and sufficient health insurance.

Lisa Wade, PhD is a professor at Occidental College. She is the author of American Hookup, a book about college sexual culture, and a textbook about gender. You can follow her on Twitter, Facebook, and Instagram.