US Political Polarisation

A few minutes browsing online and we’ll encounter hundreds of people voicing their strong views on a wide range of topics such as immigration, religion, and the never ending free market versus interventionist state” debate. This reality speaks to a continuing polarisation of world views amplified nowadays by social media platforms such as twitter and facebook.

In the US, the last few decades have seen political polarisation on the rise with the phenomenon becoming more prevalent in recent years. Several causes have been identified for this growing trend. These include, growing racial and ethnic diversity, new digital media outlets, the rise of identity politics, among others (see The Top 14 Causes of Political Polarization).

Academics have also weighed into this issue as early as 1984 with empirical measures aimed to quantify this polarisation. For instance, the political scientists Keith Poole and Howard Rosenthal developed a statistical method called NOMINATE to determine the ideological position of US congress members based on their voting records. The first dimension of the NOMINATE score measures where every congress member fits on the economic Liberal-Conservative spectrum. In this sense, the score represents each congress member ideological view with respect to the fundamental role of government in the economy.  The values of this ideology score range from -1 for the most liberal legislators to 1 for the most conservative ones.


The figure here builds on the methodology developed by Poole and Rosenthal and shows the distribution of their ideology score for all US congress members from the 81st congress (1950) to the 115th (2018). As expected, democrats have predominantly negative scores and republican positive ones. Also, by construction, values close to zero represent centrist legislators. That is, those who are more likely to “cross the aisle” and support bills sponsored by the opposite party. Given the lack of overlap of the two distributions in recent years, these centrist legislators are now non-existent.

The recent absence of more moderate congress members can be interpreted as another symptom of a widespread polarisation in US politics.  A major problem of a polarised political landscape is that it lessens the probability of bipartisan solutions for the most pressing (and perhaps even existential) problems of today’s world such as climate change.

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