Ignoring a Common Cause




Also Known as: Questionable Cause


This fallacy occurs when it is assumed that because two things are regularly connected, one must be the cause of the other and the possibility of a common cause is not considered.  It has the following general structure:


Premise 1:  A and B are regularly connected.

Premise 2:  The possibility of a common cause is not considered.

Conclusion: Therefore, A is the cause of B.


This fallacy is committed when it is concluded that A is the cause of B simply because A and B are regularly connected. Further, the causal conclusion is drawn without considering the possibility that a third factor might be the cause of both A and B.

In many cases, the fallacy is evident. For example, if someone claimed a person’s sneezing was caused by her watery eyes and they ignored the fact that the woman was standing in a hay field, he would have committed this fallacy. This is because it would be reasonable to conclude that the woman’s sneezing and watering eyes were caused by a reaction to the plants.

In other cases, the fallacy can be more challenging to spot. For example, a doctor might find a growing population of bacteria in a patient and conclude it is the cause of the patient’s illness without considering that there might be a third factor causing both. It might be that a virus is making the patient ill and weakening their immune system, thus allowing the growth of the bacteria.

As with any fallacy of reasoning, the error is not that the conclusion must be false but that the evidence does not warrant the conclusion. A person could still commit this fallacy and be right about the cause. For example, if my video card drivers and a game keep crashing on my PC and I immediately infer that the drivers are causing the crash without considering a third factor is causing both, then I could be right but would still be making an error.


Defense: While causal reasoning is often difficult, this fallacy can be avoided by considering that other factors that might be the cause of both the suspected cause and the suspected effect. If a person fails to check for the possibility of a common cause, then they will commit this fallacy. Thus, it is always a good idea to always ask “could there be a third factor that is actually causing both A and B?”


Example #1:

One day Bill wakes up with a fever. A few hours later he finds that his muscles are sore. He concludes that the fever must have caused the soreness. His friend insists that the soreness and the fever are caused by some microbe. Bill laughs at this and insists that if he spends the day in a tub of cold water his soreness will go away.


Example #2:

Over the course of several weeks the leaves from trees along the Wombat River fell into the water. Shortly thereafter, many dead fish were floating in the river. When the EPA investigated, the owners of the Wombat River Chemical Company claimed that is it was obvious that the leaves killed the fish. Many local environmentalists claimed that the chemical plant’s toxic wastes caused both the trees and the fish to die, and the leaves had no effect on the fish.


Example #3:

A thunderstorm wakes Joe up in the middle of the night. He goes downstairs to get some milk to help him get back to sleep. On the way to the refrigerator, he notices that the barometer has fallen. Joe concludes that the storm caused the barometer to fall. In the morning he tells his wife about his conclusion. She tells him that it was a drop in atmospheric pressure that caused both the barometer to drop and the storm.

Originally appeared on A Philosopher’s Blog Read More