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 social-cognitive development. 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 longitudinal study, we developed an eye-tracking task and measured preferential looking as an indicator of an implicit understanding of the epistemic verbs "know" and "think" in toddlers twice at the ages of 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 the target's location. At both 27 and 36 months of age, we observed systematic differences in their looking behavior toward 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 prequestioning phase. Subsequently, their preference switched in the postquestioning 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." (PsycInfo Database Record (c) 2025 APA, all rights reserved).