Does child-directed speech facilitate language development in all domains? A study space analysis of the existing evidence

Because child-directed speech (CDS) is ubiquitous in some cultures and because positive associations between certain features of the language input and certain learning outcomes have been attested it has often been claimed that the function of CDS is to aid children's language development in general. We argue that for this claim to be generalisable, superior learning from CDS compared to non-CDS, such as adult-directed speech (ADS), must be demonstrated across multiple input domains and learning outcomes. To determine the availability of such evidence we performed a study space analysis of the research literature on CDS. A total of 942 relevant papers were coded with respect to (i) CDS features under consideration, (ii) learning outcomes and (iii) whether a comparison between CDS and ADS was reported. The results show that only 16.2% of peer-reviewed studies in this field compared learning outcomes between CDS and ADS, almost half of which focussed on the ability to discriminate between the two registers. Crucially, we found only 20 studies comparing learning outcomes between CDS and ADS for morphosyntactic and lexico-semantic features and none for pragmatic and extra-linguistic features. Although these 20 studies provided preliminary evidence for a facilitative effect of some specific morphosyntactic and lexico-semantic features, overall CDS-ADS comparison studies are very unevenly distributed across the space of CDS features and outcome measures. The disproportional emphasis on prosodic, phonetic, and phonological input features, and register discrimination as the outcome invites caution with respect to the generalisability of the claim that CDS facilitates language development across the breadth of input domains and learning outcomes. Future research ought to resolve the discrepancy between sweeping claims about the function of CDS as facilitating language development on the one hand and the narrow evidence base for such a claim on the other by conducting CDS-ADS comparisons across a wider range of input features and outcome measures.