"I know what's inside because you showed me": Training 33-month-old children to attribute knowledge and ignorance to themselves and others.

The present study investigated the role of language in two-year-old children's early understanding of knowledge and ignorance. An intense microgenetic training consisting of 12 to 14 training sessions within six to seven weeks was conducted between 33 and 36 months. One training group experienced and participated in discourse about epistemic states in theoretically relevant situations which highlighted, for instance, the relation between seeing and knowing or contrasts between different people's knowledge states. The other training group was trained on complement syntax using sentence repetition tasks. An age-matched control group received no training. The complement syntax training was not effective in improving complement syntax competence more than in the other two groups. In contrast, the mental state training led to higher improvements in the mental state training group than in the other two groups on tasks assessing comprehension of the targeted concepts (e.g., comprehension of the seeing-knowing relation). The mental state training also had an effect on children's metacognitive awareness of their own ignorance which was, however, not independent of complement syntax competence assessed at 33 months. No effect was obtained on epistemic perspective-taking skills. Our findings indicate that the use of mental state language in discourse promotes children's acquisition of epistemic concepts even before their third birthday.