A predictive model of a polarized group, similar to the current U.S. Senate, demonstrates that when an outside threat—like war or a pandemic—fails to unite the group, the divide may be irreversible through democratic means. Published in the Proceedings of the National Academy of Sciences as part of a Dynamics of Political Polarization Special Feature, the model identifies such atypical behavior among the political elite as a powerful symptom of dangerously high levels of polarization. "We see this very disturbing pattern in which a shock brings people a little bit closer initially, but if polarization is too extreme, eventually the...