The fallacy of the single cause, also known as complex cause, causal oversimplification, causal reductionism, and reduction fallacy, is a fallacy of questionable cause that occurs when it is assumed that there is a single, simple cause of an outcome when in reality it may have been caused by a number of only jointly sufficient causes.
It can be logically reduced to: " X caused Y; therefore, X was the only cause of Y" (although A,B,C...etc. also contributed to Y.)
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 and not C" (etc.) are not taken into consideration.