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This looks like a duck, swims like a duck, and quacks like a duck. It must therefore be a duck.

The duck test is a common analogy in the United States. The duck test can be explained this way: If a bird looks like a duck, swims like a duck and quacks like a duck, then it is indeed a duck. The duck test is a form of inductive reasoning. A person can figure out the true nature of an unknown subject by observing this subject's visible traits.

It was coined in 1950 by Richard Cunningham Patterson Jr., United States Ambassador to Guatemala, during the Cold War. Patterson 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." (Immerman 1982, p. 102)

To Patterson and other United States officials, many traits of the Arbenz government showed that communists were infiltrating the Arbenz regime. In their view, the Arbenz government's censorship of the dissident press, discrimination against private capital investment, anti-imperialist rhetoric, and leftist reforms such as agrarian reform and legalization of labor unions were characteristics of communists. This behavior was evidence of the rising influence of communism in the Arbenz regime. U.S. officials did not need hard evidence of communist subversion to believe that the Arbenz government was communist, instead inferring the existence of communist subversion because agrarian reform (or agrarian revolution) was a significant factor in Mao Zedong's communist revolution in China leading communists attain power in 1949.

The United States Central Intelligence Agency (CIA) was eventually authorized by President Dwight D. Eisenhower to sponsor a coup of the Arbenz government to eliminate the supposed communist threat, ultimately causing a long civil war.

The term "duck test" is still frequently used in the United States to describe the process of attributing the identity of an unknown based on its traits, especially in certain forms of computing.

• Duck typing, a computer science term
• Digesting Duck
• Identity of indiscernibles
• Turing Test

• Immerman, Richard H. (1982), The CIA in Guatemala: The Foreign Policy of Intervention, Austin, Texas: University of Texas Press