Online Supplementary Materials for "What Siblings Share: How Family Background Shapes Early Childhood Socio-Emotional Skills Difficulties in the United Kingdom," published in Longitudinal and Life Course Studies (2026)

This is the online appendix for the peer reviewed article "What Siblings Share: How Family Background Shapes Early Childhood Socio-Emotional Skills Difficulties in the United Kingdom"AbstractFamily background is a recognised determinant of children’s socio-emotional outcomes, yet its measurement and the conceptualisation of family (dis)advantage remain inconsistent across research, policy, and practice. This study adopts a multidimensional inequality framework and a sibling design with random effects to i) estimate the overall impact of family of origin, both observed and unobserved factors, on children’s age 5 socio-emotional difficulties, ii) decompose this effect across five dimensions of family background observed at age 3: family demographics, emotional environment, parenting, educational environment, and socio-economic circumstances, and iii) assess the relative strength of family background indicators on socio-emotional difficulties. Using current data from the nationally representative United Kingdom Household Longitudinal Study (N = 2,204, including 916 siblings) and capturing socio-emotional difficulties with the Strengths and Difficulties Questionnaire (SDQ), family-of-origin accounted for 37% of the total variance in SDQ scores, with 38% of the family-of-origin influence explained by observed family dimensions. Parental occupational class, maternal psychological distress, and parenting behaviours related to schedules, praise, and punishment exhibited the strongest independent effects. Nonetheless, a substantial portion of the family-of-origin effect remains unexplained, underscoring the role of other unmeasured family factors. These results suggest that policy spaces should integrate a multidimensional approach to monitoring family (dis)advantage, and that holistic family support, whilst attending to children’s individual differences, offers the best chance of reducing early childhood socio-emotional difficulties, thus reducing barriers to opportunity.Key Messages:The way family background and family (dis)advantage shape children’s development is multifaceted and requires nuanced measurement. A multidimensional approach better captures the complex conditions influencing children's socio-emotional outcomes.In a UK sample of children born between 2007 and 2017, family-of-origin plays a substantial role, accounting for 37% of the variance in socio-emotional skills.Observed family background dimensions explain only part of the overall picture. Together, all measured domains account for 38% of the total variance in children’s socio-emotional difficulties, leaving 62% unexplained and suggesting the influence of unmeasured family shared factors.Of the observed family background indicators, socio-economic circumstances on their own explained most of the family-level variance of socio-emotional outcomes (about 18%); this was followed by maternal psychological health and parenting behaviour, which explained about 16% and 14%, respectively.