AP® English Language and Composition · Logical Fallacies in Reasoning
Post Hoc and False Cause: After Is Not Because
Post hoc ergo propter hoc assumes that because one event followed another, the first caused the second. A sequence on a timeline is not a causal arrow until you rule out coincidence and other causes.
Brains are wired to hunt for causes, and the simplest story available is always the timeline. If B happens right after A, A becomes the suspect, because the mind would rather have a weak explanation than an open question. Post hoc ergo propter hoc, 'after this, therefore because of this,' takes that shortcut and presents it as proof. The fallacy converts sequence into causation by skipping the part where you check for a mechanism, a reason A could actually produce B, and by ignoring every other thing that happened to start around the same time.
Testing a causal claim means asking what else changed, and whether the two events move together for a reason that has nothing to do with either causing the other. Ice cream sales and drownings rise and fall together all summer, but ice cream does not drown anyone. Summer heat drives both: heat sends people to buy ice cream and heat sends people into the water, so the correlation is real while the causal arrow drawn directly between the two events is not. Once you find that third variable sitting underneath both lines, the original arrow collapses and gets redrawn pointing out from the hidden cause instead.
This matters most in cause-and-effect writing, where the whole structure can quietly turn into a post hoc claim if a writer offers only two dates and a straight line between them. A paragraph claiming the mill's closing caused a town's decline needs to name the actual mechanism, lost jobs, lost wages, families who moved away, not just report that closing came before declining.
Key terms
- Post hoc ergo propter hoc
- Latin for 'after this, therefore because of this,' the fallacy of treating sequence as proof of causation.
- Causal mechanism
- The actual process by which one event brings about another, required to support any causal claim.
- Confounding variable
- A hidden third factor that influences two events, making them rise and fall together without either causing the other.
- Correlation
- A measurable pattern where two variables tend to change together, which does not by itself establish cause.
Watch it explained
- How it worksHow does a post hoc claim convert a sequence on a timeline into a causal arrow?
- Worked exampleWalk me through testing 'Ice cream sales rose and drownings rose, so ice cream drowns people.'
- Common mistakeWhy isn't the order of two events enough to tell you which one caused the other?
- Going furtherWhat happens to a causal arrow when a third variable is drawn underneath both events?
- Ties togetherHow does post hoc reasoning sneak into a cause-and-effect method of development?
Check yourself
End of topic.