The fallacy of the single cause, also known as complex cause, causal oversimplification,^[1] causal reductionism, root cause fallacy, and reduction fallacy,^[2] is an informal 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.

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Fallacy of the single cause 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.)^[2]

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 xor B xor C" when "A and B and C" or "A and B and not C" (etc.) are not taken into consideration; i.e. the "or" is not exclusive.

• Affirming a disjunct – Formal fallacy
• Essentialism – View that entities have identifying attributes
• Fallacy of composition – Fallacy of inferring on the whole from a part
• Formal fallacy, also known as non sequitur (logic) – Faulty deductive reasoning due to a logical flaw
• Jumping to conclusions – Psychological term
• Overdetermination – When a single effect has multiple sufficient causes
• Proximate and ultimate causation – Event which is closest to, or immediately responsible for causing, some observed result
• Spurious relationship – Apparent, but false, correlation between causally-independent variables