While it is well-known that language skills play an important role in children’s acquisition of a Theory of Mind (Milligan et al., 2007), there is an ongoing debate which aspect of language is particularly relevant. Some researchers argue that children’s acquisition of mental state terms (Olson, 1988) and their exposure to conversations highlighting different perspectives (Harris, 2005) drive their Theory of Mind development. Others emphasize complement syntax competence as a mechanism for understanding others’ mental states (de Villiers & de Villiers, 2009). Moreover, recent findings of early mental state reasoning abilities below the age of three (Harris et al., 2017; Setoh et al., 2016) have brought attention to the third year of life as an important time in children’s Theory of Mind development. Within the present study of N = 175 children, we contrasted the effectiveness of two different linguistic trainings – mental state language and complement syntax training – on 36-month-olds’ early Theory of Mind abilities in comparison with an untrained control group. Children’s participation in the mental state language training led to significantly stronger improvements in seeing-knowing tasks than in the control or complement syntax group. The mental state language training also affected improvement in a metacognition task but not in visual and epistemic perspective-taking tasks. The complement syntax training only tended to affect improvement in epistemic perspective-taking. The observed effects of a linguistic intervention on mental state reasoning abilities indicate that already young three-year-olds’ language abilities underlie and support the development of early Theory of Mind abilities.
