“Hostile feelings for the opposing party are ingrained or automatic in voters’ minds,” Shanto Iyengar, a political scientist at Stanford, and Sean Westwood, a post-doctoral researcher at Princeton, wrote in a July 2014 paper “Fear and Loathing Across Party Lines.” Partisans now discriminate against their adversaries “to a degree that exceeds discrimination based on race.” The authors find that this discrimination pervades decision making of all kinds, from hiring to marriage choices.

In a separate 2012 study, “Affect, Not Ideology,” Iyengar and two other colleagues used a polling method known as a “thermometer rating” to measure how Democrats and Republicans feel about each other. The temperature scale goes from 1 to 100. One means the respondent feels cold toward the group; 100 implies that the respondent has warm feelings. Iyengar and his colleagues found in 2008 that Democrat and Republican ratings of the opposition party had dropped to just below 32 degrees. In comparison, Protestants gave Catholics a 66 rating, Democrats gave “big business” a 51, and Republicans rated “people on welfare” at 50.

One of the most striking findings of Iyengar’s 2012 paper is the dramatic increase in the percentages of members of both parties who would be upset if their children married someone in the opposition party (shown in figure 1).