A longitudinal study of the relation between language and theory-of-mind development.

Fifty-nine 3-year-olds were tested 3 times over a period of 7 months in order to assess the contribution of theory of mind to language development and of language to theory-of-mind development (including the independent contributions of syntax and semantics). Language competence was assessed with a standardized measure of reception and production of syntax and semantics (the Test of Early Language Development). Theory of mind was assessed with false-belief tasks and appearance-reality tasks. Earlier language abilities predicted later theory-of-mind test performance (controlling for earlier theory of mind), but earlier theory of mind did not predict later language test performance (controlling for earlier language). These findings are consistent with the argument that language is fundamental to theory-of-mind development.