The third cause fallacy is a logical fallacy that 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

Post hoc ergo propter hoc

Post hoc ergo propter hoc, Latin for "after this, therefore because of this," is a logical fallacy that states, "Since that event followed this one, that event must have been caused by this one." It is often shortened to simply post hoc and is also sometimes referred to as false cause,...

fallacy and a member of the questionable cause

Questionable cause

Fallacies of questionable cause, also known as causal fallacies, non causa pro causa or false cause, are informal fallacies where a cause is incorrectly identified...

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.