  The evolution of opinions in a population of individuals who constantly
interact with a common source of user-generated content (i.e. the internet) and
are also subject to propaganda is analyzed using computer simulations. The
model is based on the bounded confidence approach. In the absence of
propaganda, computer simulations show that the online population as a whole is
either fragmented, polarized or in perfect harmony on a certain issue or
ideology depending on the uncertainty of individuals in accepting opinions not
closer to theirs. On applying the model to simulate radicalization, a
proportion of the online population, subject to extremist propaganda radicalize
depending on their pre-conceived opinions and opinion uncertainty. It is found
that an optimal counter propaganda that prevents radicalization is not
necessarily centrist.
