fallacy

Post hoc ergo propter hoc

Also called: Post hoc, False cause

Post hoc reasoning assumes that because one event followed another, the first caused the second.

How it works

Time order is necessary for causation but nowhere near sufficient. Coincidence, a common cause or a trend already under way can all produce the same sequence.

Examples

"Crime fell the year after the new mayor took office, so her policies worked."

Crime may have been falling anyway, or fallen everywhere, for unrelated reasons.

"I wore my lucky shirt and we won, so the shirt helps the team."

The win following the shirt is coincidence, not evidence of an effect.

How to spot it

"After X, Y happened" offered as proof that X caused Y.
No comparison with what happened elsewhere or before.
A single sequence rather than a repeated pattern with controls.

How to respond

Ask what else changed at the same time.
Look for a comparison group or a longer trend.
Ask how the cause is supposed to produce the effect.