If most voters have not become dramatically more extreme in their views, why has hostility grown so much? One of the most influential answers is that polarization is less about ideas than about identity: who people think they are, and who they think the other side is.
The great realignment
For much of the twentieth century, American parties were ideological mixtures. The Democratic Party held both Northern liberals and conservative white Southerners; the Republican Party held both business conservatives and liberal Northeasterners. The Civil Rights Act of 1964 and the Voting Rights Act of 1965, passed with Democratic leadership and broad Republican support in Congress, began a long realignment in which conservative white Southerners gradually moved to the Republican Party and liberal Republicans gradually disappeared. The process took decades and also reflected economic change, religion and migration. By the 1990s, the parties had become far more internally consistent.
Sorting versus polarization
The political scientist Morris Fiorina, in the book Culture War? (2005, with Samuel Abrams and Jeremy Pope), argued that most Americans had sorted rather than polarized. Ordinary voters, he said, were still largely moderate and ambivalent; what had changed was that the choices on offer were more extreme and the labels more consistent. Alan Abramowitz, in The Disappearing Center (2010), replied that engaged citizens really had moved apart, and that engaged citizens are the ones who vote in primaries, give money and shape the parties. Both sides of this debate have good evidence; the answer partly depends on whom you count.
Mega-identities
Lilliana Mason's book Uncivil Agreement (2018) connected sorting to hostility. Her key idea is that when many social identities line up behind one party, partisanship becomes a "mega-identity". Fifty years ago, a typical Democrat might be a churchgoing rural union member, and a typical Republican might be a secular suburban professional. Each person's identities pulled in different directions. Today party, religion, race, region, and even taste in food and television line up much more closely. When all of a person's identities point the same way, a political loss feels like a threat to everything they are.
Mason drew on social identity theory, developed by the psychologist Henri Tajfel in the 1970s, which found that people favor their own group even when groups are formed on trivial grounds. The new element is the stacking. Cross-cutting identities act like shock absorbers; when they disappear, conflict travels straight through.
Negative partisanship
Alan Abramowitz and Steven Webster added the idea of negative partisanship: voting mainly against the party one dislikes rather than for one's own. Many Americans who call themselves independents vote reliably for one party, and their main motive is often opposition to the other. This can produce very stable voting patterns even when people feel little enthusiasm for their own side.
The geography debate
The journalist Bill Bishop, in The Big Sort (2008), argued that Americans were increasingly choosing to live among like-minded people, so that more counties gave landslide victories to one party. Critics, including Abrams and Fiorina, said that people choose neighborhoods mainly for jobs, schools, prices and lifestyle, and that the political clustering is mostly a side effect. A large 2021 study by Jacob Brown and Ryan Enos, which mapped the addresses of about 180 million registered voters, found that many Americans live in neighborhoods with few members of the other party, whatever the cause. Most of the separation follows the urban to rural divide: dense cities lean left and rural areas lean right in many democracies, not just the United States.
Geography matters because contact matters. If you rarely meet members of the other party in daily life, your picture of them comes from news, social media and their loudest representatives, which is exactly the recipe for false polarization described in Level 1.
A fair caution
Identity explanations can be overstated. They can make voters sound purely tribal, as if policy did not matter. Yet voters do respond to economic conditions, scandals and candidates, and some switch parties. The identity account explains why hostility is intense; it does not mean that ideas are irrelevant.
Worked example: one voter, two eras
Consider a woman in rural Ohio in 1975. She is Catholic, a union member and a Democrat, but she is socially conservative and attends church with Republicans. When her party loses an election, she is disappointed but not alarmed: her church friends are happy, her union is still there, and the winners are people she knows.
Now picture her granddaughter in the same county today. The granddaughter's party, religion, neighborhood and favorite news sources all line up. Almost no one in her social circle votes the other way. When her party loses, the loss is not just about policy; it feels like a defeat for her whole community and way of life, inflicted by people she does not know and imagines as hostile. Nothing about her policy views needs to be more extreme than her grandmother's for her reaction to be far stronger. This is Mason's argument in miniature.