The car washing causes rain fallacy: Post hoc ergo propter hoc

January 23, 2013
carwash fallacy post hoc
Rain’s a coming now – the car wash fallacy (Image via Flickr)

Have you ever thought about washing your car and then seen that it is about to rain?

Recently, one of my sisters popped over and mentioned how typical it was that it was about to rain because she had just washed her car.

While this is a common form of logical fallacy – A happened, then B happened, therefore A CAUSED B – she is not alone. Post hoc ergo propter hoc is Latin for “after this, therefore because of this”.

Part of my interest in cataloguing examples of logical fallacies on this site is not because such indexes are scarce, but because it will help me increase my awareness and root out as many examples of lazy thinking from my own thoughts as possible.

What is typically going on in this type of logical fallacy is ‘confirmation bias‘ – we note occasions when the sequence occurs and don’t give it a second thought when it doesn’t.

One of the most common examples is the myth that hospital emergency wards are busier during a full moon. Wikipedia debunks this unfounded belief, Lunar Effect, solidly, and a passage on the page contains an easily digestible description of confirmation bias:

To the extent that nurses and police officers do indeed claim to observe patterns, this is most likely to be explained in terms of confirmation bias: People notice if something dramatic happens during a full moon, but do not notice when nothing dramatic happens;[2] furthermore, dramatic occurrences that do not occur during full moons are typically not counted as evidence against the belief. Believers are further bolstered in their belief through communal reinforcement: The more people talk about the effect, the more people notice spurious relationships.

If you become aware of this dynamic, it becomes easier to catch yourself practicing confirmation bias.

Gotta run, my lawn needs watering, so I’ll wash my car 🙂