Post hoc ergo propter hoc is a logical fallacy where one event is assumed to cause another. It’s important to establish a correlation or demonstrate cause through examples. Erin Brokovich’s case against PG&E for contaminated water is an example of establishing a correlation with scientific evidence.
The idea that one event causes another can be a logical faux pas when arguing. If you’ve ever taken a critical thinking course, you may recognize it as the Latin phrase post hoc ergo propter hoc, or you may have heard something called a post hoc argument or comment. The Latin term translates to “after this, then caused by this,” and this determination of causality, also called false cause or coincidence correlation is considered a logical fallacy.
Post hoc ergo propter hoc is an easy assumption to make, and it’s not always based on illogical thought patterns. If your pup hides under the bed the first time he hears fireworks, you might think fireworks scare him. This may or may not be true. You should test this several times before assuming that fireworks are the direct cause of the dog’s fear. Also, just because your puppy is afraid of fireworks the first time he hears them doesn’t mean he will be afraid of them, especially if you teach him not to pay attention to the sound.
Also, sometimes people take the leap to post hoc ergo propter hoc thinking believing they can eliminate a problem by eliminating the supposed cause of the problem. You might be thinking, “If I don’t light the fireworks this year, the puppy won’t be afraid.” That’s not entirely true: the puppy may be afraid of many other things that have nothing to do with fireworks. While it’s true that fireworks are one cause of dog fears, they’re not the only cause, and a car backfire, a door slamming, or someone yelling could find your hiding puppy again. under the bed.
The essential structure of the post hoc ergo propter hoc is as follows:
Event A occurred, followed by event B.
So Event A must have caused Event B.
Finally, if I don’t want event B to repeat, I will avoid event A.
We could make a very strong case using this fallacy that drinking water can cause cancer. We could go through all the data on cancer patients and find that everyone has had a glass of water at least once in their life. Using post hoc ergo propter hoc, we then assume that drinking water causes cancer. You can see the problems inherent in this, because many people who drink water don’t get cancer. Just because something occurs doesn’t mean it has any relation to something that occurs at a later time.
On the other hand, if you wanted to correct this error, you could establish a correlation between one event and another, or be able to demonstrate the cause through a huge number of examples. When Erin Brokovich took on California’s Pacific Gas & Electric Company for allowing residents to live near a water supply (which they used) that was contaminated with hexavalent chromium, there was a clear correlation, given the number of cancer cases emerging there, that drinking water contaminated with hexavalent chromium increased the risk of getting cancer. Not all people who drank or bathed in water had cancer, but the preponderance of cases helped establish a correlation between drinking contaminated water and a higher risk of cancer.
Furthermore, Brokovich and Edward Masry were able to use the scientific data to strengthen their argument. In the end, Brokovich’s argument won out because it was not a simple post hoc ergo propter hoc hypothesis. It wasn’t just “A caused B”. Instead it relied on a lot of evidence that there was a direct relationship between A and B.
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