Older (but not younger) siblings facilitate false belief understanding.

Results from 4 experiments and an analysis in which all data from 444 English and Japanese children are pooled show (a) a linear increase in understanding false belief with the number of older siblings, (b) no such effect for children younger than 3 years 2 months, (c) no helpful effect of younger siblings at any age (despite the large sample), (d) no effect of siblings' gender, and (e) no helpful effect of siblings on a task measuring children's understanding of how they know something. Discussion involves speculation about how older siblings may assist children (e.g., through pretend play and mental state language) and how different aspects of a theory of mind may develop through different means.