One potential explanation for the intensity of partisan political hatred in the US comes from a 2022 study published in the British Journal of Political Science. Examining data from 20 Western democracies since the mid-1990s, it found that people’s anger over political disagreements on issues like migration, religion and LGBTQ+ rights has intensified more than disagreement over economic issues.

The findings hint at what other scholars have proposed in the US context: that polarisation today has much to do with identity and emotion. The most central topics in politics, such as abortion rights or racial policy, are essentially identity-based issues, says political psychologist Lilliana Mason of Johns Hopkins University, co-author of the 2022 book Radical American Partisanship: Mapping Violent Hostility, Its Causes and the Consequences for Democracy.

“And when the issue is about a group, about their very existence or status, or whether they are respected equally as Americans, or whether they have the same rights as other Americans, it doesn’t feel like an issue for that person. It feels like an existential threat,” Mason points out.

“That change, from debates about the economy to debates about cultural issues, based on our research, has been most dramatic in the United States,” says James Adams, a political scientist at the University of California, Davis, and a coauthor of the British Journal of Political Science study. “That may be one of the reasons why affective polarisation has intensified dramatically in the United States over the last 25 years or so.”

Parties across Europe have also homed in on cultural issues, especially populist leaders who leverage explosive topics to rally voter blocs. But while most experts see the rise of extremist parties as a key driver of polarisation, studies may fail to fully capture this. Measuring affective polarisation in the multiparty context seen in many European countries is challenging because traditional metrics, such as the feeling thermometer, are designed for two-party systems. In multiparty systems, voters may intensely hate one party but feel neutral toward others, diluting the final score.

The US has experienced the most rapid growth in affective polarisation among the general electorate since 1980 among the 12 OECD countries considered in this 2024 study, with five other countries experiencing smaller increases in polarisation, and six experiencing declines in polarisation.

Despite this, there’s a clear polarizing trend to be seen, with these intensely disliked populist parties growing stronger and disliking the mainstream in return, says political scientist Markus Wagner of the University of Vienna, who specialises in research on party competition and political behaviour.

Research suggests that affective polarisation is also fuelled by the negative campaigning that has become a staple of modern electoral communication. A 2024 paper analysing surveys and 17 elections between 2016 and 2020 found that political leaders’ attacks – on opponents’ policies, records or character – exacerbate affective polarisation. This effect is more pronounced among those with strong partisan leanings.

Study coauthor Alessandro Nai, who specialises in political communication at the University of Amsterdam, explains that the whole system is part of a toxic spiral, with increased aggressiveness between politicians radicalizing the public, and the public – which is now more radicalised – demanding more aggressiveness from politicians. Nai also has evidence that more extreme, populist followers are more likely to appreciate negative campaign messages, finding them more amusing and fairer than moderate citizens do, and are more apt to respond to such messaging with increased affective polarisation. People on the populist extremes score higher in traits such as aggressiveness, narcissism and callousness in studies, Nai notes, adding that a “cold” person is therefore more likely to buy the aggressive playbook from politicians.

Importantly, some of Nai’s preliminary research also suggests that negative messaging from one’s own camp stokes more dislike than do attacks from adversaries. In other words, if one’s own preferred politician attacks an adversary, this boosts affective polarisation much more than if an adversary attacks your preferred politician.

That might be especially true in today’s siloed, targeted and viral media environment. In their 2022 Journal of Politics paper on how Americans overestimate each other’s political extremeness, Matthew Levendusky, a political scientist at the University of Pennsylvania who coauthored the studyand coauthors suggest that the caricatured image of “the other side” as extreme ideologues is fuelled by social media, where much of the political content is created by people who are disproportionately committed to politics.

As a result, what comes to mind when people think of those from the other party, the study argues, are “fervent partisans pleading their cases” rather than neighbours or colleagues who rarely discuss politics. The mass media’s focus on political conflict has further enforced this partisan stereotyping.

The negative impacts of severe polarisation are clear. When democratic institutions are viewed as battlegrounds for existential issues rather than arenas for reasoned policymaking, it leaves legislators in gridlocks, judiciaries stacked with loyalists and erodes democratic norms. A crisis of interpersonal trust is added to one of institutional trust: Fellow citizens are reduced to loathed enemies with malicious aims; civility is replaced by hostility as public discourse deteriorates – all while the doorway widens for populist leaders who exploit our emotions by peddling divisive rhetoric and extremist ideologies.

Experts worldwide are now thinking hard behind the scenes about how to pull our societies back from the brink. It’s a mammoth task, especially if considering broader underlying issues. For example, Adams’ research highlights that inequality and unemployment drive affective polarisation, and that winner-takes-all electoral systems – such as those in Canada, Britain and the United States – also tend to exacerbate the animosity. These are systemic, structural issues that can’t be solved by focusing on polarisation alone. However, researchers have found that some – surprisingly simple – interventions can nonetheless be very effective.

An analysis by a research consortium studying anti-democratic sentiment identified 25 ways to reduce partisan animosity. Out of those, three stood out as particularly promising.

One successful intervention involved participants watching a commercial where pairs of individuals with opposing political views formed bonds despite their differences. The video highlighted disagreements on topics like climate change, feminism and transgender identity, yet showed the two people collaborating and ultimately choosing to socialise together. This intervention yielded a 10.47 per cent decrease in animosity, based on survey questions conducted before and after the intervention.

In another intervention, participants read book quotes arguing that the news media creates polarisation to maximize its audience and that most Democrats and Republicans are part of an “exhausted majority” that rejects polarisation. Participants also saw data suggesting that increased consumption of news media correlates with more distorted perceptions of opposing views. Participants were then guided on retaking control from media influence and asked to advise others on achieving this. It resulted in a 10.22 percent decrease in animosity.

A third intervention had participants read about the pivotal role of democracy in America’s leadership in technology and culture, and how extreme partisanship threatens this. They learned that research shows that the vast majority of Americans support democracy and that, contrary to popular belief, most members of both parties support democratic rules, disapprove of violence and like one another. Participants were then asked to write about their two favourite things about being American. The intervention resulted in a 9.20 per cent decrease in animosity.

By appealing to people’s sense of camaraderie, common nationality and resistance to exploitative media, these simple methods proved capable of reducing partisan animosity, at least in study settings. In her 2022 book on American partisanship, Mason similarly notes how minimal interventions can have significant impacts.

“We found that just reading a quote from either Joe Biden or Donald Trump that says violence is not OK makes people less approving of violence,” she says.

Adams emphasises the need to dial down the aggressive rhetoric among leaders, where Democrats and Republicans publicly demonise the other side. He references a working paper on the 2022 US elections that found a reduction in Twitter toxicity with this unusual method: Researchers informed politicians that they were monitoring their accounts and that their tweet toxicity scores would be sent to a non-governmental organization for possible publication just before the election.

Another effective method, Adams says, might be demonstrating to politicians that toxic rhetoric doesn’t necessarily benefit them. An additional working paper found that although politicians who frequently use insults gain more media attention, they generally perform worse than politicians who focus on policy: They’re assigned to less powerful committees, don’t perform better in elections and don’t raise more campaign funds.

Researchers are only starting to understand the prevalence and drivers of polarisation around the world. And while there is no silver bullet to close the yawning social and political gaps dividing our societies, it’s a hopeful sign that the issue has now entered the public consciousness. Only five years ago, Adams says, few academic journals paid any mind to research about affective polarisation.

“I think a lot of political scientists just thought, ‘Well, it’s only about people’s feelings,’” he says. Then came January 6 and the storming of the US Capitol. “Suddenly, now, everyone is interested in affective polarisation.”

Reiljan tells a similar story – that when he started his research as a PhD student in 2015, there was barely any cross-country scholarship on the topic available. Some months back, he hosted a conference on polarisation where scholars from across Europe and North America gathered to brainstorm strategies to reduce it.

• A Knowable Magazine report