This study investigated how infants deal with cross-talker variability in the perception of native lexical tones, paying specific attention to developmental changes and the role of task demands. Using the habituation-based visual fixation procedures, we tested Cantonese-learning infants of different age groups on their ability to discriminate Cantonese Tone 1 (high level) and Tone 3 (mid level) produced by either multiple talkers or a single talker. Results demonstrated that the 12-month-old and 24-month-old groups showed reliable discrimination across talkers, whereas the 18-month-old group did not (Experiment 1), despite their ability to discriminate the same contrast when the talker was held constant (Experiment 2). In a task that included a novel object as a referent to the sound, the 18-month-olds discriminated the contrast across talkers from Tone 1 to Tone 3 (Experiment 3). These results revealed a U-shaped developmental path and perceptual asymmetry in native lexical tone discrimination across talkers.

This research investigated the impact of the number of talkers with whom children engage in daily conversation on their language development. Two surveys were conducted in 2020, targeting two-year-olds growing up in Japanese monolingual families. Caregivers reported the number of talkers in three age groups and children’s productive vocabulary via questionnaires. The results demonstrated significant effects of variability in talkers in fifth grade or above in Study 1 (N = 50; male = 23; r = .372) and in adult talkers in Study 2 (N = 175; non-nursery going; male = 76; r = .184) on children’s vocabulary development, after controlling for language exposure time and demographic variables. Possible mediating factors are discussed. This research extends previous findings from immigrant bilingual children to monolingual speakers in Japan, suggesting the potential contribution of available talkers other than caregivers in conversational environments.