The wug test is an experiment in linguistics, created by Jean Berko Gleason in 1958. It was designed as a way to investigate the acquisition of the plural and other inflectional morphemes in English-speaking children.

There are three plural allomorphs in English

/z/, the most general form (dogs)
/s/, which appears after voiceless consonants (cats)
/+z/, which appears after sibilants (horses).

The child is presented with some sort of pretend creature, and told, "This is a wug." Another wug is revealed, and the researcher says, "Now there are two of them. There are two...?". Children who have successfully acquired the allomorph /z/ of the plural morpheme will respond: wugs /w^gz/.

Very young children are baffled by the question and are unable to answer correctly, sometimes responding with "Two wug." Preschoolers aged 4 to 5 test best in dealing with /z/ after a voiced consonant, and generally say that there are two wugs, with a /z/; they do almost as well with the voiceless /s/. They do less well in dealing with /z/ in other environments such as after nasals, rhotics, and vowels. Children in grade 1 were almost fully competent with both /s/ and /z/. Both preschool and first-grade children dealt poorly with /+z/, giving the correct answer less than half the time, possibly because it occurs in the most restrictive context. Also, because the root of the test word often ended in /s/ in these cases, the children may have assumed that the word was already in its plural form. Even though the children were all able to produce the real plural "glasses" they generally responded two "tass" rather than two "tasses" when shown more than one nonsense creature called a "tass".

The Wug Test also includes questions that explore a child's understanding of verb conjugation and the possessive. Additional items were designed to investigate children's ability to handle common derivational morphemes such as the agentive -er (a man who "zibs" is a ....?). A final series of questions called on the children to explain common compound words in their vocabulary ("Why is a birthday called a birthday?") Very young children (preschoolers) form compounds rather than agentives with -er. They say that a man whose job is to "zib" is a "zibman". Young children also explain compound words in terms of their salient features: they respond that a birthday is called "birthday" because you get presents.

The major finding of the wug test was that even very young children have already internalized systematic aspects of the linguistic system that enable them to produce plurals, past tenses, possessives, and other forms of words that they have never heard before. The test has been replicated many times, and it has proven very robust. It was the first experimental proof that young children have extracted generalizable rules from the language around them.

The original Wug Test is reported in Gleason's article "The Child's Learning of English Morphology," Word 14:150-77 (1958).