Correlation Is Not Causation

“Two things moving together does not mean one is causing the other.”

Classical logic fallacy (post hoc ergo propter hoc) ·
Philosophical foundation: David Hume, 1748 ·
Causal criteria formalized: Austin Bradford Hill, 1965 ·
Causal mathematics: Judea Pearl, 2000

Unlike most entries in this series, “Correlation Is Not Causation” belongs to no single person. It’s a foundational principle woven across statistics, logic, philosophy, and science — recognized so long ago that the formal name is Latin. Post hoc ergo propter hoc: “after this, therefore because of this.” The rooster crows, the sun rises. The rooster caused it. This is one of the oldest identified reasoning errors in recorded thought.

The principle is simple to state and extraordinarily easy to violate. Humans are pattern-recognition machines by nature — we are wired to find cause in sequence and coincidence. That wiring built civilization. It also generates enormous quantities of confident nonsense, most of which never gets corrected because it feels true.

The Three Reasons Any Correlation Exists

When A and B move together, there are exactly three structural explanations. Every legitimate causal investigation must rule out the other two before declaring the third:

Where You’ll See It

• Ice cream and drowning: Both spike in summer. Hot weather drives both. Banning ice cream would not save lives at the pool — but a naive reading of the correlation suggests it might.
• Nicolas Cage films and pool drownings: Statistician Tyler Vigen documented that the number of Nicolas Cage films released per year correlated at r=0.67 with pool drowning deaths from 1999–2009. This is the canonical illustration of spurious correlation — two trends moving in rough parallel for no causal reason whatsoever.
• Vaccine timing and developmental diagnoses: Autism symptoms often become apparent around 18–24 months — the same age children receive several vaccines. The temporal proximity generated decades of public fear. Multiple large-scale studies (covering millions of children) have found no causal link. The coincidence in timing was real; the causation was not.
• Shoe size and reading ability in children: Larger shoe size correlates with better reading ability in elementary school. The confounder: age. Older children have larger feet and can read better. Neither causes the other.
• Hospital admissions and mortality: Hospitals have higher death rates than the general population. This does not mean hospitals cause death. Sick people go to hospitals — the confounder is illness severity. Confusing the correlation would lead to the catastrophic conclusion that avoiding hospitals improves health.

Bradford Hill’s Criteria: When Correlation Actually Does Suggest Causation

In 1965, British epidemiologist Austin Bradford Hill — who had previously helped establish the link between smoking and lung cancer — published nine criteria for evaluating whether a correlation is likely causal. They remain the most respected practical framework in evidence-based medicine and public health. No single criterion is definitive; strength comes from satisfying several simultaneously.