The purpose of the study was to determine whether the relationship between siblings and theory of mind might be mediated by executive abilities. The participants were 119 children from a lower income community, between the ages of 3 and 5, who completed an executive function battery and tests of an understanding of mind. The correlation data suggested a link between siblings and executive control and between theory of mind and executive control but, surprisingly, not between siblings and theory of mind. A further experiment with 71 children indirectly highlighted the potential contaminating influence of socio-economic status (SES) on the sibling advantage in theory of mind. Multiple regression analyses, controlling for age and verbal IQ, discriminated between the types of executive processes most likely to predict performance in mental understanding. The implications of this result are discussed in terms of a ‘primary’ and 'secondary’ executive deficit account.