This study investigated the relation between family size and ''theory
of mind.'' Results from an experiment with 3- and 4-year-olds showed t
hat children from larger families were better able than children from
smaller families to predict a story character's mistaken (false-belief
based) action. Results from a second experiment on children with exac
tly 1 sibling failed to show any superiority of older over younger sib
lings in promoting earlier understanding of belief. The data are inter
preted as suggesting that sibling interaction provides a rich ''data b
ase'' for building a theory of mind, and this is discussed in relation
to other studies showing that particular kinds of interaction between
sibling and child, or caregiver and child, have a beneficial effect o
n the understanding of false belief.