The fallacy of the single cause, also known as joint effect or causal oversimplification, is a logical fallacy of causation that occurs when it is assumed that there is one, simple cause of an outcome when in reality it may have been caused by a number of only jointly sufficient causes.

Often after a tragedy it is asked, "What was the cause of this?" Such language implies that there is one cause, when instead there were probably a large number of contributing factors.

For instance, after a school shooting, editorialists debate whether it was caused by the shooter's parents, TV violence, our culture, stress on students, Hollywood or the accessibility of guns. In fact many different causes including some of those may all have necessarily contributed.

What caused the September 11, 2001 attacks is another prominent topic of discussion that may invoke this fallacy.

Causal oversimplification is a specific kind of false dilemma where conjoint possibilities are ignored. In other words, the possible causes are assumed to be "A or B or C" when "A and B and C" or "A and B not C" (etc.) are not taken into consideration.

See also: joint effect.