The duck test is a humorous term for a form of inductive reasoning. This is its usual expression:
If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.
The test implies that a person can identify an unknown subject by observing that subject's habitual characteristics. It is sometimes used to counter abstruse arguments that something is not what it appears to be.
Emil Mazey, the secretary-treasurer of the United Automobile Workers for 33 years, said at a labor meeting in 1946:
I can’t prove you are a Communist. But when I see a bird that quacks like a duck, walks like a duck, has feathers and webbed feet and associates with ducks—I’m certainly going to assume that he IS a duck.
- Sentinel, John (September 29, 1946). "Communist Expose The Case of the Duck". Milwaukee (WI) Sentinel.
- Christy, Howard Chandler; Ethel Franklin Betts (1982), The complete works of James Whitcomb Riley
- Denver, Joseph; Ethel Franklin Betts (1965), Cushing of Boston: A Candid Portrait
- Immerman, Richard H. (1982), The CIA in Guatemala: The Foreign Policy of Intervention, Austin, Texas: University of Texas Press