Post Hoc Fallacy — Why “A” Didn’t Necessarily Cause “B”

Post hoc fallacy, or post hoc ergo propter hoc (“after this, therefore because of this”), is a logical fallacy that occurs when someone assumes that one event must have caused a later event simply because it happened after the other.

This type of reasoning is the basis for various kinds of beliefs, superstitions, and false findings in the search for causes of certain diseases.

In this article, we will explain in detail how this error in reasoning works, as well as show a variety of examples.

Definition

The post hoc fallacy is based on the false notion that since event B followed event A, event A must have caused event B. Such reasoning is logically fallacious because the fact that event A happened earlier doesn’t necessarily mean that it was the cause.

As Madsen Pirie explained in his book How to Win Every Argument: The Use and Abuse of Logic:

Although two events might be consecutive, we cannot simply assume that the one would not have occurred without the other. The second might have happened anyway.

For example, if you drink a glass of water and soon after you get a headache, it doesn’t follow that your headache must’ve been caused by the glass of water you had. The reason may be — and as in this case, is much more likely to be — something entirely else.

As such, events that occur in succession may well be causally related, but they may also be completely unrelated apart from the fact that the other happened after the other (“correlation does not imply causation”).

The logical form of the argument goes:

• Event A occurred, then event B occurred.
• Therefore, event A caused event B.

The post hoc fallacy is the foundation for an abundance of false beliefs. In particular, many superstitions are based on it: One may conclude that something bad must’ve happened to them because it’s Friday the 13'th, or because they broke a mirror or walked under a ladder earlier that day.

As humans, we naturally want to know and discover the causes for events that carry significance to us, which frequently leads us to fall victim to post hoc reasoning.

Examples

Real-Life Example

One real-life example comes from the world of football:

Pele, the Brazilian football legend, once gave his match shirt to his fan, and soon after his playing performance started deteriorating.

He believed it was because he lost his beloved shirt, so he instructed his friend to find it. And, as it turned out, his playing performance recovered quickly after the shirt was returned to him.

Based on these events, it would seem that this specific shirt was really behind Pele’s bad form. However, what his friend didn’t tell him was that they couldn’t find the original shirt, so they returned with just another shirt. (source: The Telegraph)

Other Examples

• “I sneezed exactly at the same time the power went off. My sneeze must’ve done something to make the power go off. “
• “I went camping last weekend but my trip was completely ruined by the rain. The rain must’ve been caused by me; it hasn’t been raining there for months before my arrival!”
• “The rooster crows always before the sun rises, therefore the crowing rooster causes the sun to rise.”
• “One month before I met my significant one I saw a shooting star, and I wished that I would find my soulmate. The legend is really true!”
• “The temperature has dropped this morning, and I also have a headache. Therefore, the cold weather must be causing my headache. “

Originally published at https://fallacyinlogic.com on January 16, 2020.