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This dissertation extends the media effects literature into the realm of social media. Scholars have long known that partisan news contributes to political polarization, but claim that such effects are often limited to those who tune into politics. Social media, however, can filter political information to those typically uninterested in politics. Because social media feature entertainment and political news in the same space, entertainment-seekers may inadvertently see political news that they normally avoid in traditional media contexts. Through a combination of observational research, survey experiments, and field experiments, I demonstrate that social media facilitate personal influence, drawing new audiences to political news. This increased exposure to partisan media contributes to political polarization, regardless of the ideological congruence between source and receiver, or of news- or entertainment-seeking habits of the audience. But the most important contributions of this dissertation are how it demonstrates the need for scholars to use innovative methods that incorporate personal influence into social media studies, and that it draws scholarly attention to inadvertent media effects for entertainment-seeking audiences. Social media bring political news to new audiences numbering in the millions. Political communication scholars would be remiss not to investigate their influence. / Political Science

We have acquired, over the last several decades, a fairly rich understanding of the impact on voter behavior of political communication in general and of political advertising specifically. Yet much of this knowledge pertains to “traditional,” candidate-centered elections; comparatively very little is known with regards to ballot initiative races. In principle, these contests pit not people, but proposed policies, against each other. In practice, however, they not only feature ads discussing policy, but also frequently comprise ads highlighting a measure’s supporters and opponents, be they individuals, non-profit groups, media outlets, industries, or political parties. This, in turn, leads to a basic query: what types of advertising message carry the greatest weight with voters in initiative contests – and how do they differ (if at all) from the effects they have in similar ads run in candidate-centered elections? Through an original experiment, this dissertation aims to break new ground in the voter behavior, media effects, and direct democracy literature by tackling this question. / Political Science