Journal of Public Economics 143 (2016) 73–88 Contents lists available at ScienceDirect Journal of Public Economics journal homepage: www.elsevier.com/locate/jpube Homophily, group size, and the diffusion of political information in ଝ social networks: Evidence from Twitter Yosh Halberstama,*, Brian Knightb, c aUniversity of Toronto, Canada bBrown University, United States cNBER, United States ARTICLE INFO ABSTRACT Article history: We investigate the role of homophily – a tendency to interact with similar individuals–in the diffusion of Received 19 January 2016 political information in social networks. We develop a model predicting disproportionate exposure to like- Received in revised form 29 July 2016 minded information and that larger groups have more connections and are exposed to more information. Accepted 23 August 2016 To test these hypotheses, we use data on links and communications between politically-engaged Twitter Available online 3 September 2016 users. We find that users affiliated with majority political groups, relative to the minority group, have more connections, are exposed to more information, and are exposed to information more quickly. Likewise, we Keywords: find that users are disproportionately exposed to like-minded information and that information reaches Political knowledge like-minded users more quickly. Homophily © 2016 Elsevier B.V. All rights reserved. Social media Ideological segregation 1. Introduction providing voters with information. Given uncertainty over candidate quality and ideology, information helps voters to select candidates A long literature in political economy, including Black (1958), that are both of high quality and moderate ideology, reducing Downs (1957), Becker (1958), Putnam et al. (1994), Besley and adverse selection and increasing overall voter welfare. Moreover, Prat (2006),andBesley (2007), has highlighted the importance of information on the activities and accomplishments of incumbents is useful in terms of monitoring the behavior of politicians once in office and thus minimizing moral hazard. Traditional media outlets, such as radio, newspapers, and televi- ଝ We thank the co-editor, Francesco Trebbi, and anonymous referees for very help- sion, represent important sources of information for voters. Given ful feedback. We received many helpful comments from seminar participants at UC- this, a significant domain for government policy involves the reg- Berkeley, CU-Boulder, Hebrew U, Michigan State, Stanford, Toronto, UBC, Simon Fraser ulation of these media outlets. Policy objectives often involve pro- University, UQAM, the National University of Rosario, the Central Bank of Colombia, viding voters with a large number of independent outlets. That the 2014 Media and Communications Conference at Chicago-Booth, the 2015 Ameri- is, fixing the degree of independence across outlets, an increase can Politics Summer Conference at Yale and the 2016 Barcelona GSE Summer Forum and discussant Pablo Querubin. Ashwin Balamohan, Max Fowler, Kristopher Kivutha in the number of media outlets is associated with voters receiv- and Somang Nam jointly created the infrastructure to obtain the Twitter data used ing a larger volume of information. Moreover, fixing the number in this paper, and Michael Boutros helped design the MTurk surveys we used to ana- of outlets, a greater degree of variety across outlets (i.e., plural- lyze the content in tweets. Dylan Moore provided outstanding research assistance. ism) is associated with more independence in information across Special thanks to Darko Gavrilovic, the IT consultant at Toronto, who facilitated the data work for this project, and Pooya Saadatpanah for providing computing support. sources. These two goals can be supported, for example, by simulta- Finally, we are deeply indebted to Zack Hayat for helping get this project off the ground neously reducing entry costs and limiting cross-ownership of media and providing advice in the early stages of our research. We gratefully acknowledge outlets. financial support from the Social Sciences and Humanities Research Council of Canada Unlike mass media, information on social networking sites cir- (430130792). * Corresponding author. culates via nonmarket interactions among individuals. The growing E-mail addresses: [email protected] (Y. Halberstam), importance of social media platforms, such as Facebook and Twit- [email protected] (B. Knight). ter, in providing information to voters presents a new challenge http://dx.doi.org/10.1016/j.jpubeco.2016.08.011 0047-2727/© 2016 Elsevier B.V. All rights reserved. 74 Y. Halberstam, B. Knight / Journal of Public Economics 143 (2016) 73–88 to policymakers.1 While traditional media outlets tend to be hier- tweets as well as tweets that mention candidates. By combining archical in nature, with a small number of suppliers providing these data on links and communications, we are able to measure information to a large number of consumers, typical users of social whether or not users are exposed to a given candidate tweet or men- networking sites can both consume and produce information on tion via these political networks. Further, using the time associated these platforms. Moreover, users of the same social media platform with these retweets and the information on network connections, we may be exposed to significantly different content depending upon measure the speed of information flows through the network. the set of accounts that they follow. On the other hand, readers of Based upon these data, we find that the degree of homophily in newspapers and viewers of a television station are exposed to the the political network is similar to that documented in other social same information, at least to a first approximation. Given these networks, such as the offline high-school friendship networks ana- differences between traditional media outlets and social media plat- lyzed by Currarini et al. (2009). As predicted by our model, we next forms, practices in the regulation of traditional media outlets may show that members of larger groups have more connections and not translate well to social media platforms. are exposed to more tweets on a per-capita basis than members of To better understand voter exposure to political information on smaller groups. We also show that this information reaches majority social media platforms, we conceptualize social media as a social members more quickly. Turning to exposure to like-minded infor- network. If different types of individuals have different beliefs and mation, we first show that a key condition of the model–production tend to develop relationships with like-minded individuals, a phe- of like-minded information–is satisfied. Given this, we then show nomenon known as homophily, then users may be disproportion- that groups are indeed disproportionately exposed to like-minded ately exposed to like-minded political information. As noted above, tweets, and that retweets of candidate tweets flow through the a lack of independence across sources of information may reduce the national network more quickly to like-minded users than to users of quality of information, threatening to increase adverse selection and opposing ideology. Next, we examine the content of tweets, showing moral hazard in government. Moreover, due to homophily, minor- that the results regarding exposure and speed are stronger for politi- ity groups may be exposed to less information than majority groups, cal tweets by candidates than for non-political tweets. We also show potentially undermining the efficacy of democratic institutions via a that exposure to positive mentions of candidates, when compared to reduction in electoral competition. negative mentions of candidates, is more like-minded in nature. We begin by formalizing these ideas in a simple model of Taken together, our results suggest that homophily in social political information diffusion in a social network characterized media limits voter exposure to information. In particular, homophily by homophily and two ideological groups, majority and minority. generates a built-in advantage in knowledge for voters belonging to The model predicts that, with homophily, members of the majority the majority group and increases the correlation across information group, relative to the minority, have more network connections, are sources for all groups of voters, potentially undermining the speed exposed to more information, and are exposed to information more and likelihood of convergence to the truth. This may make it difficult quickly. We also use the model to show that, with homophily and for voters to select the best candidates and to monitor the behavior of a tendency of users to produce like-minded information, individuals politicians once in office. To the extent that such information is influ- are disproportionately exposed to like-minded information. Finally, ential, the rise of social media may lead to an increased polarization the model predicts that information reaches like-minded individuals of the electorate. more quickly than it reaches individuals of opposing ideology. The paper proceeds as follows: in Section 2, we discuss our con- The primary contribution of this paper involves an empirical tribution to the literature. Next, we provide a simple model that investigation of these hypotheses using data from Twitter. As the yields the key hypotheses for our empirical investigation. Section 4 theoretical model highlights, measuring exposure to information in describes the data, Section 5 develops the