Shared understanding and idiosyncratic expression in early vocabularies.

To what extent do toddlers have shared vocabularies? We examined CDI data collected from 14,607 infants and toddlers in five countries and measured the amount of variability between individual lexicons during development for both comprehension and production. Early lexicons are highly overlapping. However, beyond 100 words, toddlers share more words with other toddlers in comprehension than in production, even when matched for lexicon sizes. This finding points to a structural difference in early comprehension and production: Toddlers are generalists in comprehension but develop a unique, expressive voice. Variability in production decreases after two years of age, suggesting convergence to a common expressive core vocabulary. We discuss potential exogenous and endogenous contributions to the inverted U-shaped development observed in young children's expressive lexical variability.