Third-cause fallacy

The third cause fallacy is a logical fallacy where a spurious relationship is confused for causation. It asserts that X causes Y when, in reality, X and Y are both caused by Z. It is a variation on the post hoc ergo propter hoc fallacy and a member of the questionable cause group of fallacies.

When third causes are ignored, it becomes possible to corral shocking statistical evidence in support of a nonexistent causality. For example:

"It seems that every time empty beer cans are piled up in a car, an accident occurs. It seems that the excess weight and shape of the cans must cause other cars to want to crash into the victim's car."

The fallacy in this situation would be the fact that the arguer focused on the first (beer cans) and second (car crashes) facts without looking for possible causes of both phenomena, such as drunk driving.

Other names

• Ignoring a common cause
• Questionable cause

See also

• Spurious relationship
• Questionable cause