Patterns of lexical and syntactic adjustment in early infant-directed speech related to language development in Hungarian

Infant-directed speech (IDS) is known to be characterised by phonetic and prosodic cues along with reduced vocabulary and syntax compared to adult-directed speech (ADS). However, there is considerable variation between mothers in the degree of lexical and syntactic reduction of their IDS. The present study aims to investigate the correspondences of the inter-individual variation of maternal IDS at 6 and 18 months with infants' language development at 18 months. 109 dyads of mothers and their firstborn infants participated in the study. Mothers' ID and AD storytelling based on standard picture stimuli were recorded at 6 and 18 months of their infants' age. We analysed measures of speech quantity (number of utterances and words), syntactic complexity (mean length of utterance), and lexical diversity (type-token ratio). Language growth was measured bimonthly using the Hungarian adaptation of the MacArthur-Bates CDI W&G form. The results did not reveal any association between characteristics of mothers' ID narratives and their infants' concurrent language skills at 18 months. However, we found a longitudinal link between a distinct pattern of linguistic simplification in maternal ID storytelling at 6 months and the development of expressive vocabulary in infants at 18 months. Infants whose mother tends to reduce both lexical and syntactic complexity of ID narratives the most are more likely to exhibit higher language outcomes. Further research is warranted to explore the background factors and longer-term effects of this maternal strategy.