Post Hoc Fallacy | Definition & Examples

The post hoc fallacy is the assumption that because one event preceded another event, they must be causally related. In other words, the first event must have caused the second.

However, the chronological order of two events does not prove a cause-and-effect relationship between them.

Making erroneous assumptions about the cause of events can lead us to wrong decisions in many important areas of everyday life, including economics, policy, and health. The post hoc fallacy is also known as the false cause fallacy, fallacy of false cause, questionable cause, and faulty causation.

What is post hoc fallacy?

The post hoc fallacy occurs when we draw a causal conclusion without sufficient evidence to support it. “Post hoc” is a shortened version of the Latin phrase “post hoc ergo propter hoc,” meaning “after this, therefore because of this.”

Post hoc fallacies are committed when one argues that because B happened immediately after A, A must be the cause of B. The point is not that there can’t be a causal connection between A and B but rather that there isn’t adequate evidence for the conclusion.

In the example above, even if the software did cause the computer to crash, there isn’t enough evidence to prove this: the chronological order of the events alone does not justify the causal conclusion.

The post hoc fallacy is a form of logical fallacy because it is based on a false premise: the idea that if one event happens before another, then the first event must be the cause of the second.

More specifically, it belongs to the category of causal fallacies, where a causal connection is assumed without proof, merely on the basis of correlation or co-occurrence. In research, the same idea is expressed through the phrase “correlation doesn’t imply causation.”

Why is post hoc fallacy a problem?

The post hoc fallacy leads to misconceptions about the causes of various phenomena. It is at the heart of many pseudoscientific arguments, the most notable and persistent being the false belief that certain types of vaccines are responsible for autism in children.

The post hoc fallacy can also lead to magical thinking: the belief that unrelated events are causally connected despite the absence of any plausible link (e.g., believing that inner thoughts can influence the external world without action). Superstitions (e.g., attributing a misfortune to an “unlucky” event like walking under a ladder) follow the same pattern.

Sometimes, people make a logical leap to post hoc thinking when they believe that they can eliminate a problem by eliminating its (alleged) cause.

This often plays out in how politicians interpret unemployment statistics or crime reduction. In this case, post hoc fallacy (A caused B) is combined with denying the antecedent, another fallacy which takes it a step further by assuming that “if A caused B, avoiding A will prevent B.”

Why does post hoc fallacy occur?

People often commit the post hoc fallacy without realizing it, simply because of the way the human mind has evolved: it extracts causality from coincidences. Due to this, people  confuse the temporal order of events with an actual causal connection. Chronological order is a necessary but not sufficient condition of a cause-and-effect relationship.

A number of cognitive biases may also feed into people’s tendency to fall for the post hoc fallacy. For example, jumping-to-conclusions bias (making decisions without enough information) can misguide people into inferring causality.

Similarly, confirmation bias can lead people to search for or interpret information in a way that confirms their preexisting belief that a cause-and-effect relationship exists.

Post hoc fallacy examples

Sports fans also commit post hoc fallacy when they believe that following certain patterns of behavior will somehow influence the outcome of a game.

In research, post hoc fallacy occurs when researchers do not carefully consider the relationship between variables.

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Frequently asked questions about post hoc fallacy

What is an example of post hoc fallacy?

An example of post hoc fallacy is the following line of reasoning:

“Yesterday I had ice cream, and today I have a terrible stomachache. I’m sure the ice cream caused this.”

Although it is possible that the ice cream had something to do with the stomachache, there is no proof to justify the conclusion other than the order of events. Therefore, this line of reasoning is fallacious.

What is the difference between post hoc fallacy and hasty generalization fallacy?

Post hoc fallacy and hasty generalization fallacy are similar in that they both involve jumping to conclusions. However, there is a difference between the two:

• Post hoc fallacy is assuming a cause and effect relationship between two events, simply because one happened after the other.
• Hasty generalization fallacy is drawing a general conclusion from a small sample or little evidence.

In other words, post hoc fallacy involves a leap to a causal claim; hasty generalization fallacy involves a leap to a general proposition.

What is the difference between the post hoc fallacy and the non sequitur fallacy?

The difference between the post hoc fallacy and the non sequitur fallacy is that post hoc fallacy infers a causal connection between two events where none exists, whereas the non sequitur fallacy infers a conclusion that lacks a logical connection to the premise.

In other words, a post hoc fallacy occurs when there is a lack of a cause-and-effect relationship, while a non sequitur fallacy occurs when there is a lack of logical connection.

How can I identify a post hoc fallacy?

When someone uses an argument that claims there is a causal connection between two events, you can take the following steps to determine whether this is a post hoc fallacy:

• Is the causal connection presented as an absolute truth or as a possibility? Absolute statements often signal a leap in logic.
• Do they have evidence to back up their claim, other than the chronological order of events? If not, it is a case of post hoc fallacy.
• When in doubt, ask the other person to elaborate on their reasoning and offer supporting evidence.