There are many reasons for studying the idea of solidarity. Early social philosophers and sociologists in the nineteenth century observed that traditional feelings of togetherness and social bonds were torn apart in the process that gave birth to modern society, and they saw solidarity as a means for social cohesion and integration. The international labour movement made class solidarity a slogan and a weapon against social and political adversaries. The welfare state is often seen as the result of a struggle for solidarity and the institutional expression of solidarity. In Catholic social teaching and Protestant social ethics, solidarity gradually became more important than charity. Thus, solidarity is a key concept in the social theory and in the modern political discourse of two of the main political traditions within European politics –social democracy and Christian democracy. The key position of solidarity in social theory and modern political discourse is a compelling reason to make the concept an object of study.

In addition, solidarity is a key concept in social policy research. Predominant classifications of welfare states make the degree of solidarity in social benefits and structure a distinguishing criterion. In his pathbreaking book The Three Worlds of Welfare Capitalism Gösta Esping-Andersen links universalism to the socialist idea of solidarity (1990). The two kinds of parties studied in this book –social democratic and Christian democratic –were the political protagonists in the development of generous welfare states.