Background: One’s number of siblings is an important determinant of many life outcomes, such as educational attainment. In the last century the United States has experienced a ‘sibsize revolution’, in which sibship sizes declined, and which led to a convergence in family circumstances for children. Did this happen in other countries as well?

Objective: This study examines the development of sibship size and social disparities in sibship size in low-fertility countries across the 20th century.

Methods: We analyze sibship size data collected from 111 nationally representative surveys conducted in 26 low-fertility countries across the 20th century.

Results: Average sibship sizes have declined in virtually all countries. Average sibship sizes are socially stratified, with smaller sibship sizes among higher-educated parents. This social disparity in sibship size has declined over time, indicating convergence in most countries. This convergence applies to large families, but not to only-child families.

Contribution: Siblings are an understudied phenomenon in family demography, despite their growing importance in a time of increasingly complex family structures. Given the significance of sibship size for children’s educational outcomes and overall life chances, decreasing social disparities in sibship size suggest greater equality in the intergenerational transmission of advantage.