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About 10 years ago, political scientist James Adams saw something troubling. New polls asked Americans whether they agreed or disagreed that people in the opposing political party weren’t simply wrong but evil. Nearly half of people from both political parties agreed.

The responses didn’t surprise him. Adams studies political polarization measured by how people feel about others in their own and opposition political parties. By those measures, sharpening negativity had been growing since at least the 1990s and continues today.

Since then, Adams has been at the frontier of more detailed measures of polarization that show not only how people feel about those in opposition parties but what drives those feelings. His research gives some insights on what might change the course of the nation.

“If you use your eyes and your ears you can't miss just how toxic this political environment is today,” said Adams, a professor of political science in the College of Letters and Science at �°������ϲ���Ļ��Ϣ Davis. “We really need to be careful in the same way that if someone is barreling down the highway at 100 miles an hour at midnight in a rainstorm with one of the headlights out.”

Polarization in the U.S. is not what it used to be

The term “polarization" used to be a way to describe political disagreements that take place without undermining people’s basic respect for each other. “Affective polarization” is different. Affective polarization is how political scientists describe anger, distrust, contempt, hostility and even hatred across party lines.

“Affective polarization is about feelings, and in particular it's about very negative, frankly nasty, feelings that people have for the other side,” said Adams.

The first measures of affective polarization that date back to the 1980s are based on surveys that asked people how they felt about their own and opposing political parties. This type of measure is called a “feeling thermometer scale.”

Adams said that negative feelings have always existed in American politics, but thermometer scores today are the lowest they have ever been in the nearly 50 years since the measure existed.

Adams wondered, are Americans unique in their political division?

American political polarization in an international context

In their book, American Affective Polarization in Comparative Perspective (), Adams and his co-authors Noam Gidron, from the Hebrew University of Jerusalem, and Will Horne, from Princeton University, compared affective polarization in the U.S. against 19 other western democracies. Their analysis covers over 80 national election surveys between 1996 and 2017.

The analysis found that affective polarization in the U.S. is not a complete outlier compared with other Western democracies. Out of the 20 countries in this study, the U.S. ranked eighth in affective polarization but fourth in opposition-party dislike.

The analysis also showed that Adams’ personal experience from a decade ago mirrored the data. Since the mid-1990s, affective polarization intensified more sharply in the U.S. than in most other Western countries. This rise was driven by Americans’ growing dislike of people in opposition parties.

The causes of American political hostility

This comparative approach also makes it possible to pinpoint key causes for affective polarization. Whether the cause is income inequality, economic recession, immigration or a winner-take-all voting system, these same causes will drive hostility and division in other nations.

The analysis showed that countries with a winner-take-all voting system do tend to display higher levels of anger and hostility between parties, and so do countries with bigger gaps between rich and poor. The U.S. has some of the highest inequality in the world, for its score, which is a measure of income inequality.

In the U.S., cultural debates — such as over immigration, LGBTQ rights and abortion — have particularly fueled cross-party anger. Out of the 20 countries in the analysis, the U.S. showed the highest increase in polarization over these kinds of cultural debates across the 20-year period.

“Economic debates are over who gets what and you can compromise over who gets what,” said Adams. “Cultural debates are over who we are and it's hard to compromise over who we are.”

How political polarization ends

So how does this extreme polarization Adams has identified usually end?

“We don't know,” said Adams. “We don't have good enough data over a long enough period of time to really know, unfortunately.”

At the same time, two of his recent studies show what political circumstances might reduce partisan hostility.

A Adams published with his co-authors Horne and Gidron analyzed how government power-sharing between parties affects political hostility. They compared 19 countries over two decades that ended in 2017, and in each of them different political parties were forced to work together in a coalition government.

The analysis found that this kind of interparty cooperation did, in fact, reduce hostility in broader society. This improvement also lingered for years after the coalition government dissolved.

In another , Adams and his co-authors showed that a higher number of women in government is associated with lower levels of partisan hostility. This study also spanned 20 countries for over two decades ending in 2017.

Adams said that he personally believes that what is missing from our politics is a simple respect from both sides, at a minimum because people on both sides of a debate can be right. He said that even as a political scientist who has spent a lifetime studying and publishing research on politics, he himself has got things wrong before.

“I try to remember that you over there, who sees the world totally differently than I do, you're surely right about some things and I'm likely wrong,” said Adams. “What right do I have to look down on you simply because you don’t see the world the way that I do when my own understanding is so imperfect?”