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Interest groups struggle to build reputations as influential actors in the policy process and to discern the influence exercised by others. This study conceptualizes influence reputation as a relational variable that varies locally throughout a network. Drawing upon interviews with 168 interest group representatives in the United States health policy domain, this research examines the effects of multiplex networks of communication, coalitions, and issues on influence reputation. Using an exponential random graph model (ERGM), the analysis demonstrates that multiple roles of confidant, collaborator, and issue advocate affect how group representatives understand the influence of those with whom they are tied, after accounting for homophily among interest groups.

Investigations of American politics have increasingly turned to analyses of political networks to understand public opinion, voting behavior, the diffusion of policy ideas, bill sponsorship in the legislature, interest group coalitions and influence, party factions, institutional development, and other empirical phenomena. While the association between political networks and political behavior is well established, clear causal inferences are often difficult to make. This article consists of five independent essays that address practical problems in making causal inferences from studies of political networks. They consider egocentric studies of national probability samples, sociocentric studies of political communities, measurement error in elite surveys, field experiments on networks, and triangulating on causal processes.

Assuming a position as broker between disconnected interests is one way for an interest group to influence the making of federal health policy. This study demonstrates how groups use their connections with political parties and lobbying coalitions to augment their brokerage positions and enhance their influence over policy making. Evidence is drawn from statistical analysis of 263 interviews with health policy elites and a qualitative case study of the debate over the Medicare Prescription Drug, Improvement, and Modernization Act of 2003. The results explain, in part, how interest groups play their brokerage roles as dispersed actors in a decentralized system, rather than as central mediators that intervene in a wide range of policy disputes.

Antiwar movements mobilize in environments in which many of their potential supporters maintain loyalties to other social movements. Social movement organizations must find ways to attract supporters from these allied movements if peace demonstrations are to achieve critical mass. We argue that organizations with hybrid identities -those whose organizational identities span the boundaries of the antiwar movement and one or more other social movements -are vital to mobilizations for peace. Our analysis draws upon original data from surveys of 5,410 antiwar demonstrators conducted in 2007-2009, combined with publicly available data on 526 organizations that helped to mobilize them. Regression analysis using a two-stage mixed-process estimator shows that individuals with past involvement in non-antiwar movements are more likely to join hybrid organizations than are individuals without involvement in non-antiwar movements. Regression analyses using Tobit and Negative Binomial panel estimators establish that organizations with hybrid identities occupy relatively more central positions in interorganizational co-contact networks within the antiwar movement, and contact significantly more participants in demonstrations, than do their peer organizations. A movementlevel examination of the distribution of hybrid organizations helps to pinpoint some of the strengths and weaknesses in antiwar organizing. For example, numerous hybrid organizations form on the basis of gender and religious identities, but few organizations form to connect with environmental, African-American, and Latino activists. We conclude by proposing a broader research agenda on hybrid organizations, as well as on the role of individual biographies in social movements. Keywords