Post Hoc Fallacy: Definition & Examples

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  • 0:01 The Cause of Illness
  • 0:49 Post Hoc Fallacy
  • 2:00 Correlation Rather…
  • 4:05 Lesson Summary
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Lesson Transcript
Instructor: Christine Serva

Christine has an M.A. in American Studies, the study of American history/society/culture. She is an instructional designer, educator, and writer.

In this lesson, identify a common error in thinking about cause and effect relationships. Learn about the post hoc fallacy and how tempting it can be to make this particular mistake.

The Cause of Illness

Tasha owns a pet rabbit. One day she changes the food that she gives her rabbit and the rabbit suddenly gets sick. She takes the rabbit to the vet and explains what happened. The vet examines the animal, looks at the old and new food, and tells Tasha that it's not likely that the food is causing the problem. Further examination will be needed, according to the vet. But Tasha was so sure it was the new food! She argues that her vet must be wrong and is just trying to take her money for more tests. She angrily takes her rabbit to another vet for a second opinion.

This lesson looks at an error in logic known as the post hoc fallacy and how to identify this tendency in your own arguments and those of others.

Post Hoc Fallacy

'There's nothing wrong with getting a second opinion,' Tasha's new vet says, 'but I'm afraid your old vet was correct. It's not the food.' Tasha is very surprised. The new vet has done an x-ray and found that her rabbit has ingested a piece of electrical wire from their home, which is likely the cause of her illness.

The error in Tasha's thought process is very common. Although it was logical to speculate that the new food and the illness could be linked, it was not logical to insist that they must be, without a doubt.

Her mistake can be described as a post hoc fallacy, or an argument that draws the conclusion that one event is directly caused by another event without evidence to prove this. For instance, when events happen close together in time, like the change in food and the rabbit getting sick, it's very easy to come to the conclusion that the food caused the illness.

The fallacy is sometimes known as a false cause fallacy because the argument is claiming a cause for an event that could turn out to be false. To remember the Latin phrase, post hoc, you can think of how 'post' can mean 'after' and how this fallacy is used when one thing happens after another.

Correlation Rather than Causation

Of course, two events that occur close together in time are related. Tasha could easily have been correct that the change in food was related to the illness. The two might have been correlated. In most cases, it's much more reasonable to suggest that events are correlated with one another, rather than having a cause and effect relationship. A correlation is a relationship between two things or events. It's common in science to find correlation between two events, without being able to claim that there is causation.

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