Developmental change in acquiring overhearable words from naturalistic contexts.

Overhearable speech constitutes over half of children's language exposure, but its role in language development remains underexplored. This study compares children's recognition of words that parents reported as primarily overhearable to children (i.e., "high-overheardness" words) with words that parents reported as primarily directed to children (i.e., "low-overheardness" words). We tested 79 predominantly White and mixed-race US English-learning children at 18, 24, and 30 months (data collection period: 2023.12-2024.04). We then re-tested the 18-month-olds at 24 months (N = 21). Results showed that children aged 24 and 30 months, but not 18 months, recognized the high-overheardness words. The gap between high- and low-overheardness word recognition at 18 months was narrower for children with larger vocabularies at 24 months. Vocabulary expansion via overhearing before age two may thus only be evident in more linguistically advanced children.