Logical Fallacies
Post Hoc Fallacy
Also known as: post hoc ergo propter hoc
The post hoc fallacy concludes that because one event followed another, the first caused the second. Sequence is treated as proof of causation. Since something must always precede any event, mere order is extremely weak evidence — coincidence, reverse causation, and common causes all produce the same pattern.
By Tajammal MaqboolFounder & Developer
Assuming that because one event followed another, the first caused the second. Sequence is mistaken for causation.
Example: You wore your lucky socks and your team won, so you decide the socks did it.
What it looks like
In a conversation about health
“I took the supplement and my cold cleared up in three days, so the supplement cured it.”
Why it fails: Colds resolve on their own in roughly that time. Recovery would have followed the supplement whether or not it did anything, so the sequence carries no information.
In a business review
“We redesigned the homepage in March and sales rose in April, so the redesign drove the increase.”
Why it fails: April may carry seasonal demand, a campaign, or a competitor's stumble. Without a comparison group that did not see the redesign, the rise has many candidate causes.
In a discussion about policy
“Unemployment fell after the law passed, so the law created those jobs.”
Why it fails: Economies move for many reasons at once, and the pre-existing trend may already have been downward. Attributing the change to one intervention requires ruling the others out.
Practice
5 questions. Answer each one, then read why the tempting wrong answers are wrong.
Frequently asked
- Does post hoc mean sequence is irrelevant to causation?
- No. Temporal order is a genuine necessary condition — a cause cannot follow its effect — so sequence does real work in ruling things out. The fallacy is treating it as sufficient. It narrows the candidates without identifying which one is responsible.
- How do I establish causation rather than just sequence?
- The strongest tool is a comparison group that differs only in the suspected cause, ideally assigned randomly. Short of that, look for a plausible mechanism, a dose-response relationship, replication across settings, and explicit consideration of confounders and reverse causation.
- Is post hoc the same as the correlation-causation error?
- It is a subtype. The general error treats any statistical association as causal; post hoc specifically treats temporal succession as causal. Every post hoc claim is a correlation-causation error, but not every correlation-causation error involves a sequence in time.