Infants' Knowledge of Individual Words: Investigating Links Between Parent Report and Looking Time.

Assessing early vocabulary development commonly involves parent report methods and behavioral tasks like looking-while-listening. While both yield reliable aggregate scores, findings are mixed regarding their reliability in measuring infants' knowledge of individual words. Using archival data from 126 monolingual and bilingual 14-31-month-olds, we further examined links across these methods at the word level, while controlling for potentially confounding child-level factors. When data were averaged at the child level, performance on the looking-while-listening task correlated well with parent-reported word production of the same words, as expected. However, mixed-effects model comparisons suggested that at the word level, looking-while-listening performance was significantly predicted by age and total productive vocabulary, but not by parent-reported knowledge of a word once these factors were controlled for. These findings invite careful consideration regarding the adequacy of these two popular methods for capturing children's idiosyncratic knowledge of individual words.