Understanding the development of social security systems constitutes the ultimate goal of social security research. This review traces and compares two schools of thought regarding social security development: the convergence and divergence schools. Using a thematic approach, this article first categorizes extant studies into one of these two schools and then identifies the broadly accepted mechanism of social security development by comparing them. After reviewing the extant research and its theoretical underpinnings, this article applies Mill’s methods of agreement and difference to show how the Chinese case contributes to and challenges our understanding of social security development. By discussing the assumptions of current research on social security development in light of the Chinese case, this article illuminates how political legitimacy serves as a common mechanism of social security development regardless of political context or structure.