Sibship Size and Academic Achievement in the PISA 2022 Survey

The influence of siblinghood on psychosocial development has long been a topic of focus in developmental psychology. Prior research, however, has yielded conflicting conclusions: some studies argue that siblinghood dilutes family resources, thereby impeding individuals' academic success, while others hold that competition and cooperation among siblings can effectively boost the development of socio-cognitive abilities. Drawing on the conditional resource dilution model and ecological systems theory, this study globally examines the role of siblinghood and the moderating effects of family wealth, national economic conditions, and cultural contexts.

Data from PISA 2022 included 570,937 15-year-old adolescents (M_age = 15.79, SD_age = 0.29) from 77 countries. Analyses examined the relationship between sibship size and academic achievement, and the moderating effects of family wealth (student-level), GDP, and cultural individualism (national-level).

Adolescents with one sibling outperformed only-children and those with two or more siblings. Family wealth amplified the positive effect of having one sibling and buffered the negative effect of having two or more siblings. Multilevel modeling revealed that GDP positively moderated the interaction between sibship size and family wealth. Similarly, individualism positively moderated this effect in families with two or more children.

This study reveals a nonlinear relationship between sibship size and academic achievement. In two-child households, siblings may not significantly dilute resources but function as resources that facilitate academic development. The dilution effect increases significantly when sibship size exceeds two. Family wealth moderates these effects, which are further amplified by national wealth and individualism.