Also Known as: Post Hoc Ergo Propter Hoc, False Cause, Questionable Cause, Confusing Coincidental Relationships With Causes
Post Hoc is a causal fallacy in which it is concluded that one thing must have caused another simply because the first occurred prior to the second. It has the following form:
Premise 1: A occurred before B.
Conclusion: Therefore, A is the cause of B.
The Post Hoc fallacy derives its name from the Latin phrase “post hoc, ergo propter hoc.” This has been traditionally interpreted as “after this, therefore because of this.” This fallacy is committed when it is concluded that one event causes another simply because the alleged cause occurred before the alleged effect. More formally, the fallacy involves concluding that A causes or caused B because A occurs before B and there is not sufficient evidence to warrant such a claim. As with any fallacy of reasoning, the conclusion could be true.
In some cases, it is obvious that A occurring before B does not indicate a causal relationship. For example, suppose Jill, who is in London, sneezed at the exact same time an earthquake started in California. Almost no one would believe that her sneeze caused the earthquake. In many cases, though, this fallacy can be quite appealing. For example, there are often cases in which there might be a connection. For example, if a person’s computer crashes after they install new software, they would reasonably suspect that the software. But if they concluded the software caused the crash simply because it was installed before it occurred, then they would be committing the Post Hoc fallacy.
The fallacy occurs because the evidence provided fails to justify acceptance of the causal claim. As noted earlier, the fallacy can be committed when A really does cause B. This is because the error is taking A occurring before B as adequate evidence that A caused B. The mistake is not that a person concludes A causes B when it does not; they could be right about that but would be right despite and not because of their poor reasoning.
Post Hoc resembles Hasty Generalization in that it involves making a leap to an unwarranted conclusion. In the case of the Post Hoc fallacy, that leap is to a causal claim instead of leaping from an inadequately sized sample to a generalization.
Not surprisingly, some superstitions might be based on Post Hoc reasoning. For example, suppose a person buys a good luck charm, does well on his exam, and then concludes that the good luck charm caused him to do well. This person would have fallen victim to the Post Hoc fallacy. This is not to say that all alleged superstitions have no basis at all. For example, some traditional cures do work and are not mere Post Hoc cases.
Post Hoc fallacies are often committed due to a lack of care in causal reasoning. Leaping to a causal conclusion is always easier and faster than thoroughly investigating a phenomenon. They can also be motivated by a form of Wishful Thinking; a person might really want something to work and thus fall victim to Post Hoc reasoning. Like most fallacies, the Post Hoc can be committed in good faith. In such cases the person does not realize they are committing a fallacy. They can also be committed in bad faith. For example, a person might use this fallacy to convince someone that a fake cure works so they can sell it to them.
Defense: Because Post Hoc fallacies are committed by drawing an unjustified causal conclusion, the key to avoiding them is careful investigation. While it is true that causes precede effects (outside of time travel, anyway), it is not true that precedence makes something a cause of something else. Because of this, a causal investigation should begin with finding what occurs before the effect in question, but it should not end there.
“I had been running so slow this track season. Then my girlfriend gave me these neon laces for my spikes, and I won my next three races. Those laces must be good luck…if I keep on wearing them, I can’t help but win!
Bill: “So, I got this new PC, and it has been working fine for months. Then I got this new game, and it keeps crashing.”
Ted: “You think the game is the cause?”
Bill: “Absolutely. I installed it and the next day, crash!”
Joan: “Helen, do you remember when your cat scratched me?”
Helen: “Yes. You poked her with a pencil, and she cut you.”
Joan: “Well, I have a fever now. I am sure that your filthy cat gave me cat scratch fever.”
Yancy: “So, the Republicans passed that tax law that benefited wealthy Americans. Then the economy tanked.”
Nancy: “So, should we blame the Republicans?”
Yancy: “Yes. Tax law then tank. It’s obvious.”
Yancy: “So, the Democrats passed that tax law that increased the taxes on wealthy Americans. Then the economy tanked.”
Nancy: “So, should we blame the Democrats?”
Yancy: “Yes. Tax law then tank. It’s obvious.”
Kevin: “The picture is all fuzzy on the TV.”
Jim: “Here, let me smack it.”
Kevin: “It cleared up! That did it!”
Jane: “I’ve got this nasty wart on my finger.”
Bob: “Yuck. I always suspected you were a witch.”
Jane: “Hah. But what should I do about the wart?”
Bob: “Cut a potato in half, rub it on the wart and then bury it under the light of a full moon. Let me know what happens.”
Jane, a month later: “It worked! My wart shrank and vanished!”
Bob: “Huh, I was just messing with you. I didn’t think you’d do that.”
Jane: “I did, and it worked!”