A variable that may affect individual differences in the child-directed speech of Japanese women

Sixty-one single Japanese-speaking women between the ages of 18 and 26 years were recorded as they read aloud picture books to a young child and as they conversed with another Japanese-speaking woman. When their utterances were acoustically compared between the two settings with regard to prosodic features, both the average pitch and pitch excursions exhibited a significant increase when interacting with the child in 17 of the 61 women. In 36 of the remaining 44 subjects, neither of these parameters showed such changes. This individual variability was not related to the subjects' liking for picture books, previous experience with reading or being read them, or with baby-sitting. The only variable that could explain the results was whether the subjects had grown up with one or more siblings or as only children. If they were only children, the prosodic modification was significantly less likely to occur.