Logical fallacy #5: Post hoc ergo propter hoc ("After this, therefore because of this")

 

                                                 

We might think of this as the "superstition fallacy." It gets its name from a Latin phrase that translates as “after this, therefore because of this.” For example, I might think, “Every time I wear my lucky shoes, I have a great day!” Obviously, my shoes probably aren’t the reason my day goes well. But our minds are always seeking patterns...

 

Definition: Assuming that because B comes after A, A caused B. Of course, sometimes one event really does cause another one that comes later—for example, if I register for a class, and my name later appears on the roll, it’s true that the first event caused the one that came later. But sometimes two events that seem related in time aren’t really related as cause and event. That is, correlation isn’t the same thing as causation.

Examples: 

Angelys writes: I get that post hoc ergo propter hoc every time my dad and I finish washing our cars, because whenever we wash them, that same day it rains! That happens to us very often.

The argument here would be something like, "It rained because I washed my car." Or even, "I shouldn't wash my car because I don't want it to rain." Or the opposite: "If I want it to rain, I should wash my car."

Pennélope writes: I currently work at a retail store and the air conditioner doesn't work properly. It's never cold enough to wear a jacket, still, my first few weeks working, I brought my jacket everyday, just in case. I remember the one day I decided to leave it at home to wash it, I swear instead of arriving at the store that morning I was at the North Pole. It was extremely cold. I kept repeating myself: "Of course, the day I didn't bring my jacket the air conditioner decided to work at it's fullest capacity." 

The argument here, of course, would be something like: "Whether or not I bring my jacket determines how well the AC will work." When we put it like that, we can see the faulty logic clearly.

Both great examples!

Tip: To avoid the post hoc fallacy, the arguer would need to give us some explanation of the process by which the tax increase is supposed to have produced higher crime rates. And that’s what you should do to avoid committing this fallacy: If you say that A causes B, you should have something more to say about how A caused B than just that A came first and B came later.

 

(Credit: The Writing Center, University of North Carolina at Chapel Hill)