  Since the 1960s, Democrats and Republicans in U.S. Congress have taken
increasingly polarized positions, while the public's policy positions have
remained centrist and moderate. We explain this apparent contradiction by
developing a dynamical model that predicts ideological positions of political
parties. Our approach tackles the challenge of incorporating bounded
rationality into mathematical models and integrates the empirical finding of
satisficing decision making---voters settle for candidates who are "good
enough" when deciding for whom to vote. We test the model using data from the
U.S. Congress over the past 150 years, and find that our predictions are
consistent with the two major political parties' historical trajectory. In
particular, the model explains how polarization between the Democrats and
Republicans since the 1960s could be a consequence of increasing ideological
homogeneity within the parties.
