Political polarization is on the rise in many democratic societies. In a recent work , we analyzed the increasing ideological distance between Republicans and Democrats in the USA between 1994 and 2017. We found that political polarization was mainly driven by opinions of Democrats moving increasingly to the left of the political spectrum, while Republican supporter opinions moved to the right to a lesser degree. Policies and policy proposals introduced by the Democratic Party such as the Affordable Care Act and same-sex marriage may have contributed to these changes in opinion.
In the USA the differences between the political values of Republicans and Democrats in many areas have been increasing in recent decades. However, the reasons for these increasing divisions are not well understood.
We used survey data on the political ideologies of the American public collected between 1994 and 2017 by the non-partisan Pew Research Center to create a mathematical model of political change. Approximately 1,500 people participated in each of the seven surveys conducted by the Pew Research Center. In the modeled scenarios, individuals changed their ideologies over time in response to social interactions with people of differing ideologies and after being exposed to information and initiatives by politicians, interest groups, and activists.
We found that between 1999 and 2004 self-identified Republican supporters seemed to have ideologically moved towards the center of the political spectrum, while from 2011 to 2017 they moved to the right. Conversely, with each subsequent survey between 1994 and 2017 self-identified Democrat supporters moved further towards the left of the political spectrum. Overall the political ideologies of Republican supporters seem to have remained relatively unchanged between 1994 and 2017, whereas those of Democrat supporters shifted to the left.
We observed that polarization seemed to be greater among the politically engaged than among non-politically-engaged individuals. The increased difference between the ideologies of Republicans and Democrats coincided with legal acts and policy proposals by the Democratic Party including the Dodd-Frank Financial Reform Act and the Affordable Care Act and activities by the Republican Party opposing these measures. We found that as the impact of these initiatives became more pronounced, the political ideologies of Democrats moved further to the left and polarization between Republican and Democrats increased.
Our findings suggest that Democrat supporters moved towards the left of the political spectrum in response to Democratic Party policies, policy proposals, and campaigns to a greater extent than Republican supporters moved to the right of the political spectrum in response to Republican policies, policy proposals, campaigns and Republican opposition to Democrat policies over the period 1994-2017. The data we used in our study covers the period from 1994 to 2017. Future research utilizing later surveys may provide further insight into the impact of policies and proposals made since 2017 on the opinions of Democrats and Republicans.
This article was co-written by Prof. Dr. Hans Gersbach.
 Böttcher, Lucas, and Hans Gersbach. “The great divide: drivers of polarization in the US public.” EPJ Data Science 9.1 (2020): 1-13.