  Are we as a society getting more polarized, and if so, why? We try to answer
this question through a model of opinion formation. Empirical studies have
shown that homophily results in polarization. However, we show that DeGroot's
well-known model of opinion formation based on repeated averaging can never be
polarizing, even if individuals are arbitrarily homophilous. We generalize
DeGroot's model to account for a phenomenon well-known in social psychology as
biased assimilation: when presented with mixed or inconclusive evidence on a
complex issue, individuals draw undue support for their initial position
thereby arriving at a more extreme opinion. We show that in a simple model of
homophilous networks, our biased opinion formation process results in either
polarization, persistent disagreement or consensus depending on how biased
individuals are. In other words, homophily alone, without biased assimilation,
is not sufficient to polarize society. Quite interestingly, biased assimilation
also provides insight into the following related question: do internet based
recommender algorithms that show us personalized content contribute to
polarization? We make a connection between biased assimilation and the
polarizing effects of some random-walk based recommender algorithms that are
similar in spirit to some commonly used recommender algorithms.
