Studies of contextual effects on political behavior are plagued by concerns about internal validity. Perhaps of greatest concern are possible selection mechanisms that appear to present statistical support for contextual influence when social communication has no real effect. This paper presents an experimental framework for testing contextual effects that ameliorates these concerns through exogenous assignment to communication networks. This experiment allows for an analysis of the factors that make discussion partners influential. These factors can be divided into two categories:(1) characteristics of the dyad and its members; (2) characteristics of the residual discussion network. The most robust findings suggest that factors in this latter category play the greatest role in the likelihood that a discussion partner is influential. 1 1 The author wishes to thank Robert Huckfeldt, Matthew Buttice, Elizabeth Simas, and Benjamin Highton for their assistance with this project. This research was funded by a grant from the National Science Foundation (SES-0817082).It may not be polite to discuss politics, but many people constantly talk about politics at home, in their businesses, and while they have an evening out. Many people are talking, but some are more influential than others. Some individuals speak and change minds, while others speak and their arguments are rebuffed. This paper is about persuasion narrowly defined as when individuals convince others to support the same candidate they support.There are many factors that could influence whether or not an individual is persuasive. This paper evaluates the effects of the partisanship and expertise of a discussion dyad, as well as, the messages being sent by other discussion partners.There is a long and rich tradition of contextual research in political science and related fields (e.g., Berelson, Lazarsfeld and McPhee, 1954;Finifter, 1974;MacKuen and Brown, 1987;Huckfeldt and Sprague, 1995). In political science, the literature on contextual effects has been dominated by a single research strategy: the pairing of survey responses to contextual measures. Critics have pointed out that measurement error and endogeneity issues may lead researchers to find contextual effects when they do not actually exist (Achen and Shively, 1995;Johnson, Shively and Stein, 2002). These problems suggest the use of a laboratory experiment to overcome these internal validity concerns. This paper presents the results of a unique, group-based experiment in which subjects are placed in parties and contexts as they try to choose between two candidates. Subjects decide on a candidate using information provided by two sources: private information and messages from their fellow subjects. The private information is accurate on average, but any single piece of information may be inaccurate. The socially communicated information may come from subjects who are uninformed or biased in favor of one of the candidates.This means that subjects are often forced to make a choice. Do they believ...