Infants' Lexical Processing Efficiency is Related to Vocabulary Size by One Year of Age

By 15-18months, infants' skill in interpreting familiar words, or lexical processing efficiency (LPE), improves substantially and is correlated with vocabulary size concurrently and several months later. Prior to this age LPE is quite poor, and to date there is little evidence that it is related to vocabulary size. If this relation only emerges once infants have relatively good LPE, and also know a substantial number of words, it could suggest that the processes that support the rapid growth in vocabulary commonly observed as infants approach age 2 may not yet be functional in the earlier stages of lexical development. However, using a modified LPE task we found that 12-month-olds with better LPE understood more words at that age and also produced more words several months later. These results suggest that meaningful individual differences in LPE are already emerging by 12months and may support lexical development across the second year.