  Echo chambers and opinion polarization recently quantified in several
sociopolitical contexts and across different social media, raise concerns on
their potential impact on the spread of misinformation and on openness of
debates. Despite increasing efforts, the dynamics leading to the emergence of
these phenomena stay unclear. We propose a model that introduces the dynamics
of radicalization, as a reinforcing mechanism driving the evolution to extreme
opinions from moderate initial conditions. Inspired by empirical findings on
social interaction dynamics, we consider agents characterized by heterogeneous
activities and homophily. We show that the transition between a global
consensus and emerging radicalized states is mostly governed by social
influence and by the controversialness of the topic discussed. Compared with
empirical data of polarized debates on Twitter, the model qualitatively
reproduces the observed relation between users' engagement and opinions, as
well as opinion segregation in the interaction network. Our findings shed light
on the mechanisms that may lie at the core of the emergence of echo chambers
and polarization in social media.
