The "Cum Hoc Ergo Propter Hoc" Fallacy

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"Cum hoc ergo propter hoc" is Latin for "with this, therefore because of this" or, in other words "A and B happen at the same time, therefore A must be causing B to happen". The "cum hoc ergo propter hoc" fallacy is a common mistake in the diagnosis and treatment of medical and psychological conditions.

"Cum hoc ergo propter hoc" is Latin for "with this, therefore because of this" or, in other words "events A and B happened at the same time, therefore A caused B"

Examples of the "cum hoc ergo propter hoc" fallacy:

  • The rooster crows every morning, therefore the rooster causes the sun to come up.
  • Hospitals are full of sick people. Therefore hospitals make people sick.
  • He sometimes behaves violently when I am around him. I don't know what it is that I am doing to make him become so violent.
  • When she spends money on herself she seems happier. I need to think of a way to earn more.

The “Cum hoc ergo propter hoc” fallacy is sometimes described as “Correlation is not causation”. In other words, when two events happen at the same time, one event did not necessarily cause the other event to happen. The two events may still be related, but a more thorough investigation is necessary to determine which causes which.

The “Cum hoc ergo propter hoc” fallacy occurs when a conclusion about cause is drawn without considering three possible alternate explanations for the correlation:

  1. A 3rd event is the real source of the correlation. Shoe sizes are positively correlated with handwriting skill. One could assume that increasing the size of your feet will improve your handwriting, but the real cause of the correlation is the general developmental growth of children.
  2. The direction of causation may be reversed.One could conclude that a railroad crossing bell causes the train to come, but the reverse is actually true.
  3. The correlation is purely a coincidence. Mark Twain was born in 1835 - and died in 1910 - both events occurring within days of Halley's comet passing over the Earth.

The fallacy is often referenced using the expression "correlation is not causation"