Introduction Social capital, defined as trust, norms, and networks that facilitate coordinated action for a mutual benefit, has drawn attention because it highlights the importance of participation, cooperation, and trust amongst economic agents in overcoming social dilemmas and producing public goods that maximize social welfare. It has been argued that generalized participation of individuals across social groups and organizations can offset the negative effects of special-interest groups and patron-client relations that promote particularist interests against public welfare, and thus inhibit wider social collaboration, reform, and development (for example, Putnam 1993; Olson 1996; Woolcock 1998). This paper empirically explores the determinants of social capital in the form of group membership in a sample of European countries. The aim of this paper is to contribute to the development of a consistent and integrated framework of social capital formation, which incorporates both individual and aggregate determinants of participatory behavior. Some studies tend to emphasize the role of individual characteristics and social determinants of income, education, gender, and employment (e.g., Glaeser et al. 2002; Oorschot etal. 2006; Halman and Luijkx 2006). Others give more weight to institutional or systemic factors, such as income inequality, confidence in government, social trust, human development, civil liberties, and regional differences (see Brehm and Rahn 1997; Costa and Kahn 2003; Bartkowski and Jasinska-Kania 2004). This study argues that social capital does not depend only on individual preferences, beliefs, interests, and strategies, but is also influenced by more institutional and systemic factors that characterize the social, political, and cultural context in which individuals interact. This hypothesis is tested by applying binary logistic regression models to regress an index of individuals' group membership on individual factors (income, education, employment, age, marriage, and gender), as well as aggregate socio-economic/socio-political factors (GDP per capita, unemployment, income inequality, social trust, trust in political institutions, corruption, and violation of political and civil rights). Data on individuals is derived from the European Community Household Panel (ECHP, hereafter), Wave 6, 1999, which covers a sample of European countries from the former EU-15. The benefit of this database is that the sample of individuals used in both pooled and country regression analyses is much larger compared to previous studies in the relevant literature. Additionally, it provides data on membership that differs from standard social surveys, in that it does not conform to a common list of social organizations, and thus might allow us to capture more locally-determined conceptualizations of networking and association across countries. The next section begins with an overview of the empirical literature on the determinants of group membership. The research in the third section conducts regression analyses for our sample of European countries and finds that both individual and aggregate factors have an important influence on group membership. Finally, based on empirical results, this paper discusses possible ways by which countries, such as Southern European countries, could change traditions of clientelism and nepotism, and expand opportunities for social participation and cooperation to promote social welfare and development. Earlier Work on Determinants of Social Capital Studies on the determinants of social capital focus on different sets of individual and aggregate factors, and view social participation as an outcome of investment decisions or contextual influences. Glaeser et al. (2002) adopt an individual-oriented approach based on a standard optimal investment model. They use pooled cross-sectional data from the General Social Survey (GSS) (1) for the 1972-1998 period to investigate the impact of individual and group characteristics on individuals' membership, measured as the number of social organizations to which individual respondents belong. …