Who’s Talking to Whom and Does It Matter? The Impact of Multiple Speakers, Overheard Speech, and Child-Directed Speech on Infants’ Language Development

Infants who hear more child-directed speech (CDS) tend to have better language learning trajectories (Huttenlocher, Haight, Bryk, Seltzer, & Lyons, 1991; Pan, Rowe, Singer, & Snow, 2005). For 2-year-olds the amount of CDS from all individuals in their environment, but not the amount of overheard speech is related to their later vocabulary development (Shneidman, Arroyo, Levine, & Goldin-Meadow, 2013). However, it is unknown if younger infants similarly benefit from CDS from all individuals in their environment, or if the input from their primary caregiver is most relevant early in language learning. The features in the language input that are important for vocabulary development change over time (Rowe, 2012). Thus, it is likely that the features important for infants differ from those that are important for two-year-olds. Both CDS and overheard speech may be beneficial for younger infants’ language development. We tested the relationship between speech, both overheard and CDS, produced by infants’ primary caregiver and the other individuals in infants’ environment, and infants’ expressive vocabulary growth between 12- to 18-months of age. We also examined how the language input that infants receive at 12-months relates to their ability to learn novel word-object mappings in a laboratory-based word learning task at 18-months.

We collected recordings of language input in 12-month-olds’ (N = 50, Males = 26) environment and coded how much CDS they heard from their primary caregiver and from other individuals and the amount of overheard speech. At 12-, 15-, and 18-months we collected parent-report measures of productive vocabulary. At 18-month infants completed a laboratory-based novel word learning task.

Infants who heard more CDS tokens from all individuals had larger vocabularies. The number of CDS tokens produced by infants’ primary caregivers was related to their novel word learning ability; such that hearing more CDS tokens from the primary caregiver was related to higher levels of accuracy. The results suggest that by 12-months child-directed language input, but not overheard speech, is key for infants’ language development.