Friday, October 12, 2007

Fallacy Friday: Post Hoc

Another Friday, another fallacy. This time we examine the confusing of correlation and causation, more formally named post hoc ergo propter hoc (latin for "after this, therefore because of this").

Gambler's Fallacy
The logical fallacy is committed when we fall into laziness and leap to conclusions about what caused something to happen. Have you ever blown on a pair of dice before you tossed them, and they come up with exactly what you wanted? If you were to conclude that your breath of "luck" made you roll sevens, then you would be guilty of Post Hoc reasoning.

And we've all done this at one time or another. You got up on the wrong side of the bed, and spilled your drink at lunchtime. Or you take an Advil and three hours later your headache goes away. Or you catch a cold and stay home from work, and two weeks later you're better. A lot of us jump to conclusions all the time, mostly because it's easier than checking all possibilities of causation.

Imagine an ancient tribe frantically ripping out the hearts of their human sacrifices to appease the gods and stop the phenomenon we now call a solar eclipse. The end of the eclipse would prove to them the efficacy of their actions, and year after year, they'd be cutting out hearts. You should be able to see here where the connection can be made with superstitious thinking, namely in the form prayer.

Close Your Eyes and Make A Wish
Suppose you ask your deity for a new car, and a month later you get a raise at work, allowing you to purchase one. Rather than ponder other factors, such as your job performance or your company's motives, you make the leap and confuse correlation with causation: the prayer caused the car. Or you lose your keys, so you ask Baby Jesus to help you. Then you go looking for your keys, and find them under the couch. Prayer must have worked, right?

Simply following sequentially from an event isn't enough to adequately say that it was the direct cause of that event. Shit happens. Consequences happen. Lucky charms and little stupid personal rituals all follow from this fallacy. It's the reason for any religious ritual or rite.

You do a funny little dance and it starts to rain. You're amazed so the next day you do it again, but it doesn't rain. What happened? Were you not doing it right? You alter your dance a bit, maybe bowing to the sky every other step. It worked! Now, do that any time you want it to rain. What's that? Didn't work again? Try something else. Maybe just wait till April, then try again.

Or you get a rather large wart on your finger. Based on a story your great grandmother tells you, you cut a potato in half, rub it on the wart and then bury it under the light of a full moon. Over the next month your wart shrinks and eventually vanishes. How right your elders were about the cure!

You get the point on coincidental correlation by now, I'm sure. We've come a long way from the time of the ancient tribes; we know better. Sequences don't establish a probability of causality any more than correlations do. If you want to know the real cause for something, do a controlled, scientific study. And GET OFF YOUR KNEES!

-STA