Developmental process of the understanding of linguistic register in children: A comparison of typically developing children, autistic children, and children with Williams syndrome.

Although the developmental process of linguistic register-the appropriate manner of speech as determined by the listener and social situation-has been gradually clarified in typically developing (TD) children, research on the mechanism and developmental process of register acquisition in atypically developing children are insufficient. This study compared the developmental process of understanding linguistic register among TD children, autistic children, and those with Williams syndrome (WS), and examined the contributions of social cognition and motivation to the acquisition of linguistic register. Two experiments were designed to assess the recognition of which linguistic register to use when communicating with different listeners and of the listener's feelings according to the speakers' use of register. The results revealed that the process of understanding register-listener associations was nearly identical among all groups of children and their understanding improved with age. Conversely, their understanding of the effect of register selection on the listener's feelings varied. Importantly, as TD children mature, they become aware that adult listeners may feel negatively when spoken to in an inappropriate register, whereas autistic children and those with WS do not exhibit the same awareness. Thus, our results suggest that atypical social cognition and motivation do not disturb the understanding of register-listener associations. However, social cognition and motivation play important roles in understanding the effect of register selection on the listener's feelings. These findings provide a significant contribution to clarifying the mechanism of linguistic register acquisition.