Joint effect is a logical fallacy of causation in which two phenomena that have a common cause are thought to be cause and effect themselves.

Consider the classic example: Ice cream consumption increases during the summer months. Murder rates also increase during the summer months. Therefore, ice cream consumption causes murder. Alternatively, committing murder causes (leads to) ice cream consumption.

Once this causation fallacy is believed, it may lead to the crafting of ideas to explain the 'causation'. For example, chemicals within the ice cream interact with the consumer's neurotransmitters, so as to lead to aggressive, sometimes-murderous behavior. Alternatively, the neurotransmitters released during the act of murder lead to cravings that can be satisfied by high-fat content, sweet foods.

At the root of this fallacy is that, indeed, ice cream consumption and murder rates are highly correlated. Now, does ice cream incite murder or does murder increase the demand for ice cream? Neither: they are joint effects of a common cause, namely, hot weather during the summer season.

Incidentally, if applied to regions of the former Soviet Union, where ice cream consumption increases in the winter months, even the correlative situation falls apart, thus highlighting the absolute lack of causation.

This is a special case of correlation implies causation. In statistics, the common cause is called a confounding factor and this fallacy is called a spurious relationship.

External link

• Stephen's Guide: Joint effect