Networks of Political Division
The way in which politicians vote in Congress can be visualized on a graph of nodes.
Blue nodes represent Democrats, and red nodes represent Republicans. The edges of the nodes represent two members with similar values on roll call votes, and the thickness of the edge represents the strength of that connection. Over time, there has become a strikingly increasing amount of separation between the two parties in addition to stronger clustering within parties.
First, let’s look at the graph from 2007. The two partisan clusters are nearly entirely disconnected, with one local bridge (the red node in the center) being the primary connector between the parties. The red and blue dots are also tightly clustered together within each party.
This is a sharp comparison to the earlier graphs of the 1950s, for instance, that are much less tightly clustered and have many clear lines between party members. It indicates that, over time, the amount of agreement within parties has increased (i.e. – Democrats will agree more with other Democrats than they used to, creating more central and homogenous policy beliefs within each party) and the amount of agreement across parties has decreased (i.e. – Democrats will be less likely to agree on a policy with a Republican than they used to, decreasing the amount of ties the members of the parties have to each other). Together, these effects have created polarization, with the parties each so standardized and clustered together within themselves that there is little room for inter-party dialogue or agreements.
Source:
Andris C, Lee D, Hamilton MJ, Martino M, Gunning CE, Selden JA (2015) The Rise of Partisanship and Super-Cooperators in the U.S. House of Representatives. PLoS ONE 10(4): e0123507. https://doi.org/10.1371/journal.pone.0123507