Children steer the inflectional diversity of their parents: The role of word births and growing vocabulary

The inflectional diversity of parents’ speech directed to children acquiring Dutch was investigated. Inflectional diversity is defined as the number of inflected forms of a particular lemma (e.g. singular, plural of a noun) and measured by means of Mean Size of Paradigm (MSP). Changes in the inflectional diversity of infant directed speech (IDS) were analyzed as a function of children’s developing linguistic abilities. Two types of changes in the inflectional diversity of nouns and verbs were analyzed: (1) coarse tuning: changes relative to children’s growing vocabulary and (2) fine lexical tuning: changes relative to children’s use of specific lexical items. In addition, it was investigated if those changes were similar depending on particular characteristics of the children, namely, differences in their hearing abilities. Longitudinal recordings of spontaneous speech of 30 children (0;6-2;0) with normal hearing (NH) and 10 hearing-impaired children with a cochlear implant (CI) (0;6-2;6), and their parents were analyzed. As to coarse tuning, it was found that the inflectional diversity of IDS decreased at the beginning of the child’s lexical development but increased again parallel to infants’ growing cumulative vocabulary. As to fine lexical tuning, IDS showed less inflectional diversity before each child’s first use of a word and gradually more inflectional diversity afterward. In addition, parents of children with CI used less inflectionally diverse speech than parents of children with NH, which suggests an adaptation to specific characteristics of the children. In conclusion, inflectional morphology in IDS appears to be tuned to children’s hearing status and linguistic knowledge.