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Abstract To efficiently recognize words, children learning an intonational language like English should avoid interpreting pitch‐contour variation as signaling lexical contrast, despite the relevance of pitch at other levels of structure. Thus far, the developmental time‐course with which English‐learning children rule out pitch as a contrastive feature has been incompletely characterized. Prior studies have tested diverse lexical contrasts and have not tested beyond 30 months. To specify the developmental trajectory over a broader age range, we extended a prior study (Quam & Swingley, 2010), in which 30‐month‐olds and adults disregarded pitch changes, but attended to vowel changes, in newly learned words. Using the same phonological contrasts, we tested 3‐ to 5‐year‐olds, 24‐month‐olds, and 18‐month‐olds. The older two groups were tested using the language‐guided‐looking method. The oldest group attended to vowels but not pitch. Surprisingly, 24‐month‐olds ignored not just pitch but sometimes vowels as well—conflicting with prior findings of phonological constraint at 24 months. The youngest group was tested using the Switch habituation method, half with additional phonetic variability in training. Eighteen‐month‐olds learned both pitch‐contrasted and vowel‐contrasted words, whether or not additional variability was present. Thus, native‐language phonological constraint was not evidenced prior to 30 months (Quam & Swingley, 2010). We contextualize our findings within other recent work in this area. 

Children are adept at learning their language’s speech-sound categories, but just how these categories function in their developing lexicon has not been mapped out in detail. Here, we addressed whether, in a language-guided looking procedure, 2-year-olds would respond to a mispronunciation of the voicing of the initial consonant of a newly learned word. First, to provide a baseline of mature native-speaker performance, adults were taught a new word under training conditions of low prosodic variability. In a second experiment, 24- and 30-month-olds were taught a new word under training conditions of high or low prosodic variability. Children and adults showed evidence of learning the taught word. Adults’ target looking was reduced when the novel word was realized at test with a change in the voicing of the initial consonant, but children did not show any such decrement in target fixation. For both children and adults, most learners did not treat the pho- nologically distinct variant as a different word. Acoustic-phonetic variability during teaching did not have consistent effects. Thus, under conditions of intensive short-term training, 24- and 30-month-olds did not differentiate a newly learned word from a variant differing only in consonant voicing. High task complexity during training could explain why mispronunciation detec- tion was weaker here than in some prior studies.