Duck test

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For the use of "the duck test" within the Wikipedia community, see Wikipedia:DUCK.
A mallard, correctly identified as a duck using the duck test.

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.


Indiana poet James Whitcomb Riley (1849–1916) may have coined the phrase when he wrote:

When I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck.[1]

The more common wording of the phrase may have originated much later with Emil Mazey, secretary-treasurer of the United Auto Workers, at a labor meeting in 1946 accusing a person of being a communist:

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.[2]

The term was later popularized in the United States by Richard Cunningham Patterson Jr., United States ambassador to Guatemala during the Cold War in 1950, who used the phrase when he accused the Jacobo Arbenz Guzmán government of being Communist. Patterson explained his reasoning as follows:

Suppose you see a bird walking around in a farm yard. This bird has no label that says 'duck'. But the bird certainly looks like a duck. Also, he goes to the pond and you notice that he swims like a duck. Then he opens his beak and quacks like a duck. Well, by this time you have probably reached the conclusion that the bird is a duck, whether he's wearing a label or not.[3]

Later references to the duck test include Cardinal Richard Cushing's, who used the phrase in 1964 in reference to Fidel Castro.[4][5]

Douglas Adams parodied this test in his book Dirk Gently's Holistic Detective Agency:

If it looks like a duck, and quacks like a duck, we have at least to consider the possibility that we have a small aquatic bird of the family Anatidae on our hands.[6]

The Liskov Substitution Principle in computer science is sometimes expressed as a counter-example to the duck test:

If it looks like a duck and quacks like a duck but it needs batteries, you probably have the wrong abstraction.[7]


  1. ^ Heim, Michael (2007). Exploring Indiana Highways. Exploring America's Highway. p. 68. ISBN 978-0-9744358-3-1. 
  2. ^ Sentinel, John (September 29, 1946). "Communist Expose The Case of the Duck". Milwaukee (WI) Sentinel. 
  3. ^ Immerman, 1982, p. 102
  4. ^ Denver, Joseph; Ethel Franklin Betts (1965), Cushing of Boston: A Candid Portrait 
  5. ^ Platt, Suzy. Respectfully quoted. Library of Congress, Congressional Research Service. ISBN 978-0-88029-768-4.  "Attributed to Richard Cardinal Cushing. Everett Dirksen and Herbert V. Prochnow, Quotation Finder, p. 55 (1971). Unverified."
  6. ^ Adams, Douglas (1987). Dirk Gently's Holistic Detective Agency. 
  7. ^ Bailey, Derick. "SOLID Development Principles – In Motivational Pictures". 
  • 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 

See also[edit]