Post Hoc

What it means

Post hoc ergo propter hoc - “after this, therefore because of this” - is the fallacy of assuming that because event B followed event A, A must have caused B. It’s one of the most natural errors in human reasoning, because our brains are wired to detect patterns and construct causal stories. When two events occur in sequence, the mental leap from “after” to “because of” happens almost automatically.

The problem is that correlation - two things occurring together or in sequence - is not the same as causation. Ice cream sales and drowning deaths both increase in summer. Ice cream doesn’t cause drowning. Both are caused by a third factor: hot weather. This is obvious when spelled out, but the same logical structure underlies claims that are much harder to spot.

Post hoc reasoning is the engine behind superstitions, miracle cures, folk wisdom, and a surprising amount of bad policy. It feels like evidence because it has the structure of evidence - X happened, then Y happened. But without a plausible mechanism and proper controls, sequence tells you nothing about cause.

In the real world

“I started taking this supplement and my cold went away within a week.” Colds typically last about a week regardless. But the timing creates a powerful subjective experience of cause and effect. This is how the alternative medicine industry sustains itself - people recover (as they mostly would anyway) and attribute their recovery to whatever they were doing at the time.

In politics, post hoc reasoning is everywhere. “The economy improved after we took office, so our policies worked.” Maybe they did. Or maybe the economy was already recovering. Or maybe global conditions shifted. Or maybe the improvement would have been larger under different policies. Establishing actual causation requires careful analysis - but political narratives need only sequence.

Media coverage routinely commits this fallacy. “Violent crime rose after the new sentencing guidelines were introduced.” The implication is causal, but the guidelines might be irrelevant - crime might have risen for dozens of other reasons, or might have risen less than it would have without the guidelines. The temporal sequence creates a story, and the story substitutes for evidence.