Both L1 and L2 proficiency impact ToM reasoning in children aged 4 to 6. Painting a more nuanced picture of the relation between bilingualism and ToM

Previous studies that contrasting bilinguals with monolinguals on Theory of Mind (ToM) have shown mixed results. We present a relatively large (N = 102) study comparing Polish–English sequential bilinguals living in the UK with Polish monolinguals living in Poland. Going beyond a simple group comparison, we explored the role of language proficiency and input in ToM abilities. A battery of eight tasks was used to measure ToM, and the groups were matched on age, gender, SES, IQ and L1 word comprehension. Although bilinguals did not differ from monolinguals in accuracy in ToM tasks, they demonstrated better reasoning abilities when providing justification for ToM responses. ToM accuracy scores were best predicted by L1 proficiency, but the justification scores were best predicted by both L1 and L2 proficiency. The findings suggest that the nuances of bilingual experience provide an important scaffolding context for ToM development.