  We develop a rational inattention theory of echo chambers, where players
allocate limited attention across biased primary sources and other players to
gather information about an uncertain state. The resulting Poisson attention
network transmits information from the primary source to a player either
directly or indirectly through other players. Rational inattention creates
heterogeneous information demands among players who are biased toward different
decisions. In an echo-chamber equilibrium, each player focuses on his
own-biased source and like-minded friends, who attend to the same primary
source as his and can serve as secondary sources if the attention channel from
the primary source to him is disrupted. We establish conditions for the
emergence of echo-chamber equilibria, characterize the attention networks
within echo chambers, and offer insights for designing and regulating
information platforms.
