Did you know that the sale of ice cream is strongly correlated to the number of muggings in a given locale? Could it be that consuming ice cream leads us to attack our fellow citizens? Or perhaps miscreants in our midst mug strangers to get the money to buy ice cream? We have two variables, X and Y. Which one causes which? In this case, there’s a third variable, Z, that causes both X and Y. It’s the temperature. As the temperature rises, we buy more ice cream. At the same time, more people are wandering about out of doors, even after dark, making them convenient targets for muggers.
What causes what? It’s the most basic question in science. It’s also an important question for business planning. Lowering our prices will cause sales to rise, right? Maybe. Similarly, government policies are typically based on notions of cause and effect. Lowering taxes will cause the economy to boom, right? Well… it’s complicated. Let’s look at some examples where cause and effect are murky at best.
Home owners commit far fewer crimes proportionally than people who don’t own homes. Apparently, owning a home makes you a better citizen. Doesn’t it follow that the government should promote home ownership? Doing so should result in a safer, saner society, no? Well… maybe not. Again, we have two variables, X and Y. Which one causes which? Could it be that people who don’t commit crimes are in a better position to buy homes? That not committing crimes is the cause and home ownership is the result? The data are completely tangled up so it’s hard to prove conclusively one way or the other. But it seems at least possible that good citizenship leads to home ownership rather than vice versa. Or maybe, like ice cream and muggings, there’s a hidden variable, Z, that causes both.
The crime rate in the United States began to fall dramatically in the early 1990s. I’ve heard four different reasons for this. Which one do you think is the real cause?
Which of the four variables actually caused the declining crime rate in America? A lot is riding on the answer. Unfortunately, the data are so tangled up that it’s difficult to tell what causes what. But here are some rules for thinking about correlation and causation:
Actually, the only way to prove cause and effect beyond a shadow of a doubt, is the experimental method. Which leads us to our question for tomorrow: does smoking cause cancer in humans?