The home language environment predicts individual differences in language comprehension at 9 months of age.

By 18-24 months of age, infants whose caregivers talk to them more tend to recognize and comprehend common words relatively quickly and accurately. In turn, real-time language comprehension skill at this age is linked to language development later in childhood. Critically, infants begin to comprehend common words as early as 6-9 months of age, but it is unclear whether the origins of lexical comprehension skill are likewise influenced by hearing child-directed speech. Instead, ambient speech, including caregiver speech that is overheard by infants rather than directed to them, may play strong supportive role in very early language development. Thus, we tested how aspects of the home language environment (HLE) are related to performance on a lexical recognition task in 9-month-old American-English learning infants (N = 38). In our sample, 94.9% of infant participants were Caucasian, 8% Hispanic, 7.7% American Indian/Alaskan Native, and 5.1% Black. Families used the Language Environment Analysis (LENA) system to make audio recordings of the HLE across 2 days. We used LENA-generated measures, as well as measures of child-directed and overheard speech calculated from 2 hr of transcribed recordings, to determine which aspects of the HLE are related to lexical recognition. We found that LENA measures of the total amount of speech infants heard, and of their participation in conversational exchanges, predicted lexical recognition. However, hand-transcribed measures of child-directed speech were not related to lexical recognition. These results suggest that adult speech and speech heard within vocal interactions are important to early-emerging language comprehension abilities. (PsycInfo Database Record (c) 2025 APA, all rights reserved).