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post hoc fallacy

The post hoc ergo propter hoc (after this therefore because of this) fallacy is based upon the mistaken notion that simply because one thing happens after another, the first event was a cause of the second event. Post hoc reasoning is the basis for many superstitions and erroneous beliefs.

Many events follow sequential patterns without being causally related. For example, you have a cold, so you drink fluids and two weeks later your cold goes away. You have a headache so you stand on your head and six hours later your headache goes away. You put acne medication on a pimple and three weeks later the pimple goes away. You perform some task exceptionally well after forgetting to bathe, so the next time you have to perform the same task you don't bathe. A solar eclipse occurs so you beat your drums to make the gods spit back the sun. The sun returns, proving to you the efficacy of your action.

You use your dowsing stick and then you find water. You imagine heads coming up on a coin toss and heads comes up. You rub your lucky charm and what you wish for comes true. You lose your lucky charm and you strike out six times. You have a "vision" that a body is going to be found near water or in a field and later a body is found near water or in a field. You have a dream that an airplane crashes and an airplane crashes the next day or crashed the night before.

However, sequences don't establish a probability of causality any more than correlations do. Coincidences happen. Occurring after an event is not sufficient to establish that the prior event caused the later one. To establish the probability of a causal connection between two events, controls must be established to rule out other factors such as chance or some unknown causal factor. Anecdotes aren't sufficient because they rely on intuition and subjective interpretation. A controlled study is necessary to reduce the chance of error from self-deception.

See also ad hoc hypothesis, confirmation bias, control study, communal reinforcement, Occam's razor, placebo effect, regressive fallacy, selective thinking, self-deception, subjective validation, testimonials, and wishful thinking.

further reading

Browne, M. Neil & Stuart M. Keeley. Asking the Right Questions: A Guide to Critical Thinking (Prentice Hall, 1997).

Carroll, Robert Todd. Becoming a Critical Thinker - A Guide for the New Millennium (Boston: Pearson Custom Publishing, 2000).

Damer. T. Edward. Attacking Faulty Reasoning: A Practical Guide to Fallacy-Free Arguments 4th edition (Wadsworth Pub Co, 2001).

Giere, Ronald, Understanding Scientific Reasoning, 4th ed, (New York, Holt Rinehart, Winston: 1998).

Kahane, Howard. Logic and Contemporary Rhetoric: The Use of Reason in Everyday Life, 8th edition (Wadsworth, 1997).

Moore, Brooke Noel. Critical Thinking (Mayfield Publishing Company, 2000).


Mom: Son in 'extensive therapy' after viewing library book ... her son had removed the book unsupervised from the library’s general stacks last summer and put it in his backpack. She has kept it ever since.

“Now he’s in a home for extensive therapy,” she said.

Whoever said a little knowledge is a dangerous thing knew what she was talking about.

Desperate couple have miracle baby after acupuncture (This story is either a perfect example of post hoc reasoning or a lesson for men: we might be replaced by needles.)

Last updated 12-Sep-2014

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