  This systematic review synthesizes current research on echo chambers and
filter bubbles to highlight the reasons for the dissent in echo chamber
research on the existence, antecedents, and effects of the phenomenon. The
review of 112 studies reveals that the lack of consensus in echo chamber
research is based on different conceptualizations and operationalizations of
echo chambers. While studies that have conceptualized echo chambers with
homophily and utilized data-driven computational social science (CSS) methods
have confirmed the echo chamber hypothesis and polarization effects in social
media, content exposure studies and surveys that have explored the full
spectrum of media exposure have rejected it.
  Most of these studies have been conducted in the United States, and the
review emphasizes the need for a more comprehensive understanding of how echo
chambers work in systems with more than two parties and outside the Global
North. To advance our understanding of this phenomenon, future research should
prioritize conducting more cross-platform studies, considering algorithmic
filtering changes through continuous auditing, and examining the causal
direction of the association between polarization, fragmentation, and the
establishment of online echo chambers. The review also provides the advantages
and disadvantages of different operationalizations and makes recommendations
for studies in the European Union (EU), which will become possible with the
upcoming Digital Services Act (DSA). Overall, this systematic review
contributes to the ongoing scholarly discussion on the existence, antecedents,
and effects of echo chambers and filter bubbles.
