Two- to three-year-old toddlers differentiate the epistemic verbs 'know' and 'think' in a preferential looking eye-tracking paradigm

The acquisition of mental language understanding is crucial for the social-cognitive development, especially for the development of Theory of Mind reasoning. While there is evidence for the production of epistemic terms in the third year of life, the comprehension of different degrees of speaker (un-)certainty has not yet been systematically investigated at this age. In the present study, we developed an eyetracking task and measured preferential looking as an indicator of an implicit understanding of the epistemic verbs ‘know’ and ‘think’ in children aged 27 (N = 199) and 36 months (N = 131). Toddlers were faced with two agents who indicated the location of a hidden object (right vs. left box), with a narrator attributing contrasting degrees of certainty to their statements (‘know’ vs. ‘think’) before asking the toddlers about the object’s location. We measured the extent to which children fixated the box associated with the agent described as knowing where the target was and found both at 27 and 36 months of age systematic differences in their looking behaviour to this box across the trial. Children appeared to display a spontaneous preference for the box associated with the agent who knew the target’s location, relative to the agent who only thought the target was in their box in the pre-questioning phase. Subsequently, their preference switched in the post-questioning phase; however, this effect was smaller. These results indicate that toddlers in their third year of life distinguish different degrees of speaker (un-)certainty, expressed by the verbs ‘know’ and 'think'.