We investigated longitudinal relations between siblings’ problem and prosocial behaviour, measured by the Strengths and Difficulties Questionnaire, among different sibship sizes in the UK’s Millennium Cohort Study. We identified 3436 families with two children and 1188 families with three children. All children (cohort members and their older siblings) had valid data on behaviour at two time-points (in 2004 and 2006). Using Structural Equation Modelling, we found that for internalising and externalising problems, older siblings (MOS1 = 6.3 years, MOS2 = 9.1 years at T1) exerted a dominant effect on younger siblings (Mage = 3.12 years at T1; 49.7% boys) across sibship sizes. For prosocial behaviour, there was older sibling dominance in two-child families and youngest sibling dominance in three-child families.

Zhaotian is a PhD student at the MRC Cognition and Brain Sciences Unit, University of Cambridge. Her research focuses on child and family mental health within the context of neurodevelopmental disorders. Prior to this, she obtained a bachelor’s degree in psychology from Beijing Normal University and a master’s degree in Child Development and Education from the University of Oxford.