Lesson 11: Post Hoc Ergo Propter Hoc

Spot the Faulty Logic #

“I wore my lucky socks to the game and we won! My socks must have caused us to win.”

Discussion: Talk with your teacher about this example. What seems wrong with this reasoning?

How/Why It’s Often Used #

Our brains are wired to look for patterns and causes. When two things happen close together in time, it’s natural to assume they’re connected. This is actually useful sometimes - it helps us learn that touching a hot stove causes burns! But it can lead us astray when we see connections that aren’t really there.

People often use this reasoning to support superstitions, lucky charms, and rituals. It’s also common in advertising (“I used this product and then got better grades!”) and in everyday explanations of events.

Post Hoc in Action #

Did you spot the faulty logic? #

The team won for many possible reasons: good plays, team effort, the other team’s mistakes, or just luck. The socks happened to be worn on the same day, but there’s no logical connection between sock choice and game outcomes.

Second Example #

“I started taking this vitamin last week and my cold went away. The vitamin cured my cold!”

The Flaw #

Colds naturally go away on their own after about a week. The timing of the vitamin coincided with the natural end of the cold, but that doesn’t mean the vitamin caused the recovery.