Although numerous studies of preschoolers report robust associations between performance on tests of executive function (EF) and theory of mind (ToM), a lack of developmentally appropriate tasks so far has limited research on these cognitive skills in younger children. Here, we present new batteries of EF and ToM tasks that were administered to 140 two-year-olds from predominantly disadvantaged families, with analyses based on 129 children. Our results showed a strong association between EF and ToM, which remained significant when effects of verbal ability were controlled. Individual differences in EF and ToM were also examined in relation to both distal family factors (social disadvantage, number of siblings) and proximal family factors (quality of child's relationships with parents and siblings). Social disadvantage predicted significant variance in both EF and ToM but did not contribute to the association between these domains. Associations between positive parent-child relationships and both EF and ToM were nonsignificant when verbal ability was controlled. In contrast, positive sibling relationships predicted significant variance in ToM, even controlling for age, verbal ability, EF, social disadvantage, and parent-child relationships.
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