The American public’s response to the Covid-19 pandemic illustrates vividly the phenomenon of extreme party polarization: simple preventative measures recommended by public health experts to reduce the spread of the virus have become political statements. Republicans are significantly less likely to wear facial coverings and to practice social distancing than Democrats, a difference attributable to the mixed messages emanating from Donald Trump’s administration. Unlike other major national crises, Covid-19 has not instilled in people a sense of common purpose or unity. The failure to implement an overarching national response has resulted in, at the time of this writing, nearly two hundred thousand deaths and incalculable economic loss.
Political scientists have typically treated polarization as a matter of ideology, proposing the ideological distance between party platforms as the appropriate yardstick for measuring our differences. By this standard, it is clear that elected officials representing the two major American parties have indeed become polarized over the past half century. It remains unclear, however, whether rank-and-file party members have followed suit along the same ideological lines.
Taking extreme positions on political issues is one way of defining partisan polarization. An alternative definition considers polarization as the extent to which partisans view each other as a stigmatized out-group. In the US two-party system, partisanship is about identifying with the Democrat group or the Republican group. Psychologists have demonstrated that any form of group identity, even one based on the most trivial of shared characteristics, triggers both positive feelings for the in-group and negative evaluations of the out-group. In the case of political identity, animus toward the out-group is especially virulent.
Affective Polarization: The Evidence
A large body of evidence is now documenting that partisans on each side treat one another as disliked out-groups. Researchers have tracked Americans’ feelings toward the opposite party since the 1970s. Over time, but especially after 1990, people who identified with one of the two major parties have increasingly expressed hostility toward their opponents. Since 2000, the share of partisans expressing intense negativity for the out-party has climbed to nearly 25 percent. Out-groups defined on the basis of religious or racial identity are treated with far more respect. Intense dislike for the out-group is most apparent in the domain of politics.
Another metric for assessing group polarization is social distance, the extent to which individuals feel comfortable interacting with out-group members in a variety of settings. In recent years, the country has witnessed several high-profile instances of political “shunning.” As Harvard professor Alan Dershowitz, a prominent supporter of President Trump, complained to the New York Times, “My liberal friends have stopped inviting me for dinner.” More generally, the argument is that partisans have become averse to entering into close interpersonal relationships with their political opponents. The most vivid evidence of increased social distance across the party divide concerns dating and marriage. In a longitudinal analysis spanning the past five decades, scholars found that spousal agreement on partisanship among recently married couples increased from just over 50 percent to 75 percent. Moreover, researchers concluded, this level of agreement among newlyweds reflects the deliberate selection of mates based on political identity.
Dating and marriage both entail long-term and intimate relationships. Does political partisanship also impede the initiation of more casual friendships? Surveys by the Pew Research Center suggest it does. About 64 percent of Democrats and 55 percent of Republicans say they have “just a few” or “no” close friends who are from the other political party. Similarly, a recent study demonstrates that discordant partisanship decreases the likelihood that people will establish friendships with others even when they are not seeking a romantic relationship. This research suggests that most Americans are embedded in homogeneous social networks.
In summary, the scholarly evidence is overwhelming: Americans are deeply divided on the basis of their politics and are more polarized today than at any other period since the advent of modern survey research. How did this happen?
The period over which mass polarization has intensified (1980–today) coincides with multiple changes in American society and politics, including greater differentiation between Democrats and Republicans, increased network homogeneity, and a fundamentally altered media environment. Each of these factors has the tendency to reinforce the others, further contributing to the rise of affective polarization.
Looking first at the differences between Democrats and Republicans, in the last fifty years the percentage of “sorted” partisans—those who identify with the party most closely reflecting their ideology—has steadily increased. When most Democrats are also liberals and most Republicans conservatives, individuals in each group are less likely to encounter conflicting political ideas and identities and more likely to see non-identifiers as socially distant. As political party and ideological identities have converged, other salient social identities, including race and religion, have also come into alignment with partisan identities. Today, Democrats are the party of women, non-whites, professionals, and residents of urban areas, while Republicans represent older white men, evangelical Christians, and residents of rural areas. In essence, the reinforcement of political and social divides makes it much easier for partisans to make generalized inferences about the opposing side.
A second potential cause of hyper-polarization is network homogeneity. When family members identify with the same party, as noted above, they also express more extreme positions on political issues and harbor hostile views toward their opponents. In a 2015 national survey of married couples, respondents evaluated the presidential candidates Hillary Clinton and Donald Trump on a 100-point “feeling” thermometer. Among spouses who agreed on their party identification, the average difference between the in- and out-party candidate thermometer score was 59 points. Among the few couples with divergent loyalties (Democrat-Republican pairings), this margin of difference fell by more than 30 degrees. Clearly, partisan agreement within the family strengthens polarization.
A third potential contributor to affective polarization is the technological progress that has brought us into a fundamentally altered media environment. The revolution in information technology has empowered consumers to encounter news on their own terms. The availability of twenty-four-hour cable news channels provided partisans with their first real opportunity to get their news from like-minded sources—first, Fox News for Republicans, and later, MSNBC for Democrats. The development of the internet provided a much wider range of media choices, which greatly facilitated partisans’ ability to obtain ideologically slanted political information. A growing number of outlets, motivated in part by the commercial success of the Fox News network, offered “news” reporting tinged in varying degrees with partisan commentary. Many of these online outlets depict the opposing party in harsh terms, focusing disproportionately on out-party scandals, real or imagined. The creation of vast online social networks permitted extensive recirculation of news reports, even to those not particularly interested in seeking out news.
While there are good reasons to believe that “new media” have contributed to the growth in partisan animus, it is possible that enhanced consumer choice also sets in motion processes that weaken polarization. As media platforms have multiplied, consumers gain access not only to more news providers, but also to more entertainment providers. The availability of entertainment programming on demand enables some to drop out of the political arena entirely. Thus, the net impact of the vastly increased availability of choice on consumers is unclear.
In fact, despite myriad changes in the media environment, the evidence, to date, demonstrating that news consumption exacerbates polarization is certainly open to question. While small-scale experimental studies of browsing behavior confirm the tendency of partisans to self-select into distinct audiences, more generalizable real-world studies find only slight traces of audience segregation. In their pioneering analysis of Americans’ web browsing behavior, conducted in 2008, Matthew Gentzkow and Jesse M. Shapiro found that online audiences were only slightly more segregated than audiences for network or cable news. They concluded that “internet news consumers with homogeneous news diets are rare. These findings may mitigate concerns . . . that the internet will increase ideological polarization and threaten democracy.”
More recent large-scale tracking of online browsing behavior suggests that the segregation of news audiences is increasing. A 2013 study showed that although most people relied on ideologically diverse online sources, audience segregation increased among individuals who used search engines to locate news stories and when social media users encountered links in their news feeds. Both these pathways to news exposure feature personalized algorithms, making it more likely that individuals encounter information consistent with their political loyalties. In the case of Facebook, now a major source of news, most individuals find themselves in politically homogeneous networks, increasing the likelihood of exposure to polarizing messages.
To the extent partisans do gravitate to like-minded news providers, has the diffusion of high-speed internet facilitated this behavior? Here, too, the evidence is mixed. In those parts of the country where broadband is more available, traffic on partisan news sites is greater and broadband diffusion has strengthened partisan animus. On the other hand, affective polarization has increased the most among those least likely to use social media and the internet. Given these inconsistent results, it is too early to conclude that internet usage plays a causal role in the growth of affective polarization.
The willingness of Democrats and Republicans to treat each other with disdain has far-reaching consequences for the body politic. For one thing, it creates incentives for politicians to use inflammatory rhetoric and demonize their opponents. The chair of the House Budget Committee, Democrat John Yarmuth, for instance, recently invited a psychologist to address the committee on the state of President Trump’s mental health. The most frequent and enthusiastic chant at 2016 Trump rallies was “lock her up.” Yet another example is the president’s use of the terms “rapists and drug dealers” to describe illegal immigrants. And, earlier, during the debate over the passage of the Affordable Care Act, some Republicans likened the mandatory insurance requirement in the law to the forced deportation of Jews by the Nazis. In response, liberal commentator Keith Olbermann declared that Republicans’ opposition to the law was tantamount to racism.
At the level of electoral politics, heightened polarization has made it almost impossible for partisans to abandon their party’s candidates, no matter their liabilities. The release of the Access Hollywood tape—in which Trump was heard making crude references to his willingness and ability to grope women—would surely have ended the candidacy of a presidential candidate in any election cycle from the 1980s or 1990s. Yet the impact on Trump’s poll numbers was miniscule. And in Alabama, in the 2017 Senate election, evidence of Republican candidate Roy Moore’s inappropriate relations with women and girls hardly raised eyebrows among Republican voters, a mere seven percent of whom defected.
Partisans have become so committed to their parties that scholars have had to update the standard finding of public opinion research—that voters are utterly ignorant of current events. Today, partisans are not merely uninformed, but also misinformed and deliberately misled. Partisan voters have become such reliable team players that politicians now enjoy considerable leeway in their efforts to influence public opinion. Well before he became a presidential candidate, Trump was the principal sponsor of the conspiracy-oriented “birther” theory concerning former president Barack Obama’s place of birth and citizenship. Since taking office, Trump has continued to show little respect for facts and evidence. He claimed that extensive voter fraud caused his deficit in the popular vote and that charges of possible collusion between his campaign and the Russian government amounted to a “hoax.” Trump’s rhetoric has proved persuasive for Republicans, many of whom believe Trump’s false claims.
What, if anything, can be done to ameliorate polarization? Some suggest that it is a matter of reestablishing partisanship as a civic role to be played out within a broader democratic framework. To the extent we think of ourselves more as Americans and less as partisans, political animus is likely to recede. Another possibility is to foster bipartisanship and compromise among elites in Washington with the hope that voters will take the cue. A key difficulty with both of these ideas, of course, is that few politicians today perceive that moderation of positions and civility will be rewarded.
All told, intensified affective polarization portends serious repercussions, especially during times of political turmoil. There are multiple parallels between Watergate and the current era, yet polarization has fundamentally altered the political dynamics of scandal. Investigative news reports that brought to light the cover-up in the Richard Nixon White House became widely accepted as credible evidence of official wrongdoing. The media spotlight resulted in a significant erosion of Nixon’s approval among both Democrats and Republicans. By way of contrast, the multiple investigations swirling around the Trump administration have, to date, done little to undermine his standing among Republicans. Partisans’ willingness to ignore information that challenges their sense of political identity is disturbing and undermines the ability of the press to act as the “fourth branch of government.” Even more troubling is the possibility that hyper-partisanship may erode fundamental democratic norms. In the aftermath of a closely contested election, the losing candidate may choose to question the validity of the vote count rather than concede defeat, a practice that would call into question the very foundations of representative democracy.
President Trump famously claimed that he could stand in the middle of Fifth Avenue and shoot somebody at no cost to his electoral support. We can only hope that he is mistaken.
Shanto Iyengar is the William Robertson Coe Professor of American Studies at Stanford University, where he is also professor of political science and co-principal investigator for the American National Election Studies. Iyengar’s research has been supported by grants from the National Science Foundation, the National Institutes of Health, the Ford Foundation, the Pew Charitable Trusts, and the Hewlett Foundation. Iyengar is author or co-author of several books, including News That Matters, Is Anyone Responsible?, Explorations in Political Psychology, Going Negative, and Media Politics: A Citizen’s Guide
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 Morris P. Fiorina, Samuel A. Abrams, and Jeremy C. Pope, “Polarization in the American Public: Misconceptions and Misreadings,” Journal of Politics 70, no. 2 (April 2008): 556–60.
 Donald Green, Bradley Palmquist, and Eric Schickler, Partisan Hearts and Minds: Political Parties and the Social Identities of Voters (New Haven, CT: Yale University Press, 2002); Leonie Huddy, Liliana Mason, and Lene Aaroe, “Expressive Partisanship: Campaign Involvement, Political Emotion, and Partisan Identity,” American Political Science Review 109, no. 1 (2015): 1–17.
 See, for instance, M. G. Billig and Henri Tajfel, “Social Categorization and Similarity in Intergroup Behavior,” European Journal of Social Psychology 3, no. 1 (1973): 27–52.
 Shanto Iyengar, Tobias Konitzer, and Kent L. Tedin, “The Home as a Political Fortress: Family in an Era of Polarization,” Journal of Politics 80, no. 4 (2018): 1326–38.
 Gregory A. Huber and Neil Malhotra, “Political Homophily in Social Relationships: Evidence from Online Dating Behavior,” Journal of Politics 79, no. 1 (January 2017): 269–83.
 Matthew S. Levendusky, The Partisan Sort: How Liberals Became Democrats and Conservatives Became Republicans (Chicago: University of Chicago Press, 2009).
 Sonia Roccas and Marilynn B. Brewer, “Social Identity Complexity,” Personality and Social Psychology Review 6, no. 2 (2002): 88–106.
 Iyengar, Konitzer, and Tedin, “The Home as a Political Fortress,” 1326–38.
 Jeffrey M. Berry and Sarah Sobieraj, The Outrage Industry: Political Opinion Media and the New Incivility (Oxford: Oxford University Press, 2014).
 Markus Prior, Post-Broadcast Democracy: How Media Choice Increases Inequality in Political Involvement and Polarizes Elections (New York: Cambridge University Press, 2007).
 See Shanto Iyengar and Kyu Hahn, “Red Media, Blue Media: Evidence of Ideological Selectivity in Media Use,” Journal of Communication 59, no. 1 (2009): 19–39; and Natalie J. Stroud, Niche News: The Politics of News Choice (New York: Oxford University Press, 2009.
 Matthew Gentzkow and Jesse M. Shapiro, “Ideological Segregation Online and Offline,” Quarterly Journal of Economics 126, no. 4 (2011): 1799–1839.
 Seth Flaxman, Sharad Goel, and Justin M. Rao, “Filter Bubbles, Echo Chambers, and Online News Consumption,” Public Opinion Quarterly 80, special issue (2016): 298–320.
 Eytan Bakshy, Solomon Messing, and Lada A. Adamic, “Exposure to Ideologically Diverse News and Opinion on Facebook,” Science 348, no. 6239 (2015): 1130–32.
 Yphtach Lelkes, Gaurav Sood, and Shanto Iyengar, “The Hostile Audience: The Effect of Access to Broadband Internet on Partisan Affect,” American Journal of Political Science 61, no. 1 (2017): 5–20.
 Levi Boxell, Matthew Gentzkow, and Jesse M. Shapiro, “Greater Internet Use Is Not Associated with Faster Growth in Political Polarization among US Demographic Groups,” Proceedings of the National Academy of Science,114, no. 40 (2017): 10612–17.
 Adam J. Berinsky, “Rumors and Health Care Reform: Experiments in Political Misinformation,” British Journal of Political Science 47, no. 2 (2017): 241–62; and Donald. J. Flynn, Brendan Nyhan, and Jason Reifler, “The Nature and Origins of Misperceptions: Understanding False and Unsupported Beliefs about Politics,” Political Psychology 38, supplement 1 (2017): 127–150.
 Erik Peterson and Shanto Iyengar, “Partisan Gaps in Political Information and Information-Seeking Behavior: Motivated Reasoning or Cheerleading?,” American Journal of Political Science 64, no. 4 (2020).