Single-platform studies are, as noted, of limited value in identifying echo chambers, but there are several important studies that identify like-minded communities formed on individual social media platforms – whether through algorithmic selection, self-selection, or some combination thereof (Bakshy et al. 2015; Barberá et al. 2015; Kaiser and Rauchfleisch 2020; Vaccari et al. 2016). Even these, however, often conclude, like Barberá (2015, p. 28), that “most social media users receive information from a diversity of viewpoints.” And in the absence of evidence on what other media the individuals involved use in addition to the social media platform in question, these studies simply cannot establish whether people inhabit a bounded, enclosed media space where specific messages are magnified and insulated from rebuttal.

To understand why algorithmic selection is consistently found to lead to more diverse news diets, not narrower diets (let alone echo chambers), it is important to remember that the median number of different sources of news that people in the UK use on a weekly basis offline is two, and just one online (Newman et al. 2021). Search engines and social media do not vastly expand this number and it is not the case that people who use these platforms have very diverse and balanced news diets. Rather, they lead people to slightly more, and slightly more diverse, sources of news than what they seek out of their own volition. One recent study (Fletcher et al. 2021b), that includes the UK, used survey data from 2020 to assess the number of people in politically partisan online news echo chambers in Austria, Denmark, Germany, Norway, Spain, the UK, and the US by looking at how many people only use news sources with left- or right-leaning slants (measured in terms of the overall ideological slant of each outlet's audience). There is also some work on media, including media reporting about polarisation. US studies find that exposure to like-minded partisan media under experimental conditions can strengthen the views of already partisan individuals (Levendusky 2013). Panel survey work, which measures the same people's media use and attitudes at different points in time, has also found that using like-minded partisan media in the US can increase anger toward the ‘other side’ and make people more willing to share political information on social media (Hasell and Weeks 2016). At the same time, cross-cutting exposure, at least on social media, also seems to be able to increase polarisation, at least among political partisans (Bail et al. 2018). In Europe, public service media often help bridge these gaps, with differences between those with low interest and high interest being smaller in countries like the UK that have widely used public service media (Castro-Herrero et al. 2018). But many people self-select away from politics and news, choosing entertainment content instead (Prior 2005), and people with more limited levels of formal education and lower levels of income generally use less news than more privileged parts of the population (Kalogeropoulos and Nielsen 2018).This leaves distribution and demand as the main possible causes for the formation of echo chambers.

Hopkins, D. J., & Ladd, J. M. (2013). The consequences of broader media choice: Evidence from the expansion of Fox News. Rochester, NY: Social Science Research Network. Webster, J. G., & Ksiazek, T. B. (2012). The dynamics of audience fragmentation: Public attention in an age of digital media. Journal of Communication, 62(1), 39–56. First, a large number of empirical studies documenting that echo chambers are smaller than commonly assumed, and a growing amount of research rejecting the filter bubble hypothesis should not be confused with a Panglossian belief that we live in the best of all possible worlds or that our increasingly digital, mobile, and platform-dominated media environment does not come with any serious societal challenges. There are many, including the frequently overlooked fact of pronounced inequality in news and information use documented by many of the studies reviewed here, as well as a multitude of others, such as widespread online harassment and abuse, various kinds of misinformation, often invasive data collection by dominant platforms, a serious disruption of the established business of news and market concentration, and many more issues beyond the scope of this review.In this literature review, we examine evidence concerning the existence, causes, and effect of online echo chambers and consider what related research can tell us about scientific discussions online and how they might shape public understanding of science and the role of science in society. When it comes to media, there is limited research outside the US and this work does not always find the same patterns as those identified in the US but, at least in the specific context of the United States, it seems that exposure to like-minded political content can potentially polarise people or strengthen the attitudes of people with existing partisan attitudes, and that cross-cutting exposure can potentially do the same for political partisans. Digital media and public discussions around science ↑