  Homophily -- our tendency to surround ourselves with others who share our
perspectives and opinions about the world -- is both a part of human nature and
an organizing principle underpinning many of our digital social networks.
However, when it comes to politics or culture, homophily can amplify tribal
mindsets and produce "echo chambers" that degrade the quality, safety, and
diversity of discourse online. While several studies have empirically proven
this point, few have explored how making users aware of the extent and nature
of their political echo chambers influences their subsequent beliefs and
actions. In this paper, we introduce Social Mirror, a social network
visualization tool that enables a sample of Twitter users to explore the
politically-active parts of their social network. We use Social Mirror to
recruit Twitter users with a prior history of political discourse to a
randomized experiment where we evaluate the effects of different treatments on
participants' i) beliefs about their network connections, ii) the political
diversity of who they choose to follow, and iii) the political alignment of the
URLs they choose to share. While we see no effects on average political
alignment of shared URLs, we find that recommending accounts of the opposite
political ideology to follow reduces participants' beliefs in the political
homogeneity of their network connections but still enhances their connection
diversity one week after treatment. Conversely, participants who enhance their
belief in the political homogeneity of their Twitter connections have less
diverse network connections 2-3 weeks after treatment. We explore the
implications of these disconnects between beliefs and actions on future efforts
to promote healthier exchanges in our digital public spheres.
