Late, but not early, arriving younger siblings foster firstborns' understanding of second-order false belief

•Few studies have examined the influence of siblings on theory of mind in middle childhood.•A second-order false belief task enacted with Playmobil® figures was given to 229 7-year-olds.•Children with younger siblings outperformed those without.•However, children with early arriving younger siblings did not have the same advantage.•These findings demonstrate younger siblings can foster theory of mind beyond the preschool years.

This study examined the influence of younger siblings on children's understanding of second-order false belief. In a representative community sample of firstborn children (N = 229) with a mean age of 7 years (SD = 4.58), false belief was assessed during a home visit using an adaptation of a well-established second-order false belief narrative enacted with Playmobil figures. Children's responses were coded to establish performance on second-order false belief questions. When controlling for verbal IQ and age, the existence of a younger sibling predicted a twofold advantage in children's second-order false belief performance, yet this was the case only for firstborns who experienced the arrival of a sibling after their second birthday. These findings provide a foundation for future research on family influences on social cognition.