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.