Post Hoc Ergo Propter Hoc is Latin for “after this, therefore because of this,” or in other words, associating correlation with causation.
People are notorious for making this error in logic, because when something significant happens we want to be able to know & explain why it happened.
I was guilty of this error the other day when my laptop broke (I lost all of my files, I cried and learned my lesson – back them up!!!). I was going about my computer business as usual, and remembered that my iPod battery was nearly drained, so decided to plug it in. As soon as the cord hit the USB port, the screen went black, it crashed, and I couldn’t turn it back on.
I blamed this crash on the iPod, I even yelled at it and threw the cord against the wall! But when my husband took it in to get fixed, they said it wasn’t possible that the iPod was what caused the problem. Sorry iPod, it was just a coincidence that my computer happened to die at the exact moment that I plugged it in. I think this is a great example that helps to understand why many people buy into the anti-vaccine movement in spite of the complete lack of evidence correlating vaccines with autism.
Just like a certain amount of peoples’ computers will crash shortly after they plug their iPods into their USB ports, a certain number of children will be diagnosed with autism shortly after they are immunized. Just because it happens before, doesn’t mean it was the cause.
There’s a story that Paul Offit, author of Autism’s False Prophets, likes to tell that illustrates this beautifully:
My wife is a privately practicing pediatrician in the suburbs. And she was in the office one day and there was a four-month-old sitting on her mother’s lap. And my wife was drawing a vaccine into a syringe that she was about to give this child. Well, while she was drawing the vaccine into a syringe the child had a seizure, and actually went on to have a permanent seizure disorder—epilepsy. And there had been a family history of epilepsy, so she was certainly at risk for that. If my wife had given that vaccine five minutes earlier, I think there’s no amount of statistical data in the world that would have convinced that mother that anything other than the vaccine caused the seizure, because I think those sort of emotional events are very hard to argue against.