Language specificity of infant-directed speech: speaking rate and word position in word-learning contexts

Previous research indicates that infant-directed speech (IDS) is usually slower than adult-directed speech (ADS) and mothers prefer placing a focused word in isolation or utterance-final position in (English) IDS, which may benefit word learning. This study investigated the speaking rate and word position of IDS in two typologically-distinct languages: Dutch and Mandarin Chinese. We used a storybook-telling task to elicit semi-spontaneous ADS and IDS. The storybook contained target words that were familiar or unfamiliar to children. For each language, we asked: (1) whether IDS was slower than ADS; (2) whether mothers slowed down to highlight unfamiliar words; (3) whether the speaking rate of IDS changed between 18 and 24 months; and (4) whether mothers preferred placing unfamiliar words in isolation or utterance-final position in IDS. Results suggest that Dutch IDS, but not Mandarin Chinese IDS, was slower than ADS.Also, only Dutch mothers slowed down specifically when introducing unfamiliar words in IDS. In both languages, mothers placed target words in isolation (but not in utterance-final position) more frequently in IDS. These results suggest that the temporal modifications in IDS may vary across languages. Thus, language-specificity of IDS and its implications for language acquisition should be considered in future research.