America, it is often said, has never been more divided. Gone are the days when politicians routinely brokered deals across the aisle, and Republican and Democratic voters could disagree on policy yet still get along over the dinner table. Instead, polarization has transformed the democratic process, which once thrived on compromise and respectful dialogue, into a winner-takes-all battleground of dysfunction and animosity.

Congress, mired in perpetual gridlock, struggles to pass even the most essential legislation, undermining the principles of democracy. This legislative stalemate reflects a broader societal trend where Americans increasingly see those with opposing views not merely as adversaries, but as enemies.

This revelation has caused scholars to reevaluate the historical assumptions about what divides us as citizens: Is it anti-immigrant nationalism that pushes us towards the poles or is it the widening chasm of inequality? Could the culprit be unchecked online vitriol and the diminished gatekeeping role of traditional media? Or perhaps it’s the allure of strongman leadership in uncertain times?

These deep divisions, which have intensified since the 1980s, were once deemed a largely American saga, with many experts blaming the country’s unusual two-party system. But in the past decade, political scientists have uncovered a similar trend across the globe – not just in countries with two invariably dominating political parties but in nations with diverse societal compositions, governing structures and levels of economic development.

 Such divisions have been reported in well-established multiparty democracies like Switzerland, Denmark and New Zealand, and in emerging economies such as Bulgaria, Turkey, the Czech Republic and Poland.

While there is no one explanation for why countries worldwide are fraying into extremes, recent findings suggest a common denominator: namely, that the key driver of polarisation is more about emotion and identity than policy positions – a phenomenon known as “affective polarisation.”

Researchers are increasingly focusing on how people’s emotional and identity-based attachments to their political groups fuel hostility toward the opposition.

This shift in understanding emphasises that the divide isn’t merely about differing opinions on issues. It is deeply rooted in the perception of political opponents as threats to one’s way of life and core values.

The US wasn’t always so polarised. In fact, there was a time when some experts argued that American politics wasn’t polarised enough. In the 1950s, the American Political Science Association published a study concluding that the parties needed to become more distinct and coherent.

“What the political scientists were saying in the 1950s was, ‘Nobody can tell the difference between our parties, nobody knows who to vote for, because we’re not making it clear, and so we need to be more different,’” 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.

That ’50s report was widely criticized by the political science community at the time, but the decades that followed would set in motion sociopolitical changes that, eventually, more than fulfilled its vision.

The 1960s were the starting point of a sharper alignment in American politics, as civil rights legislation prompted many Southern Democrats to switch to the Republican Party, initiating a sorting that eventually grouped economic and racial conservatives together. Today, the increasing party distinctiveness is unmistakable in Congress. Between the 1950s and 1970s, typical members of Congress voted with their party on divisive questions just more than 60 per cent of the time; in the 1990s, this figure rose past 80 per cent, and since 2000, it has exceeded 87 per cent.

In 2023, the average Democratic senator voted with President Biden 99 per cent of the time on issues where Biden had a clear stance, according to the ABC News-owned polling website 538. In the House, that figure was 93 per cent, while the average Republican voted with the president only 5 per cent of the time.

However, that doesn’t mean politicians are farther apart on policy issues than in the past, says Frances Lee, a political scientist now at Princeton University who wrote about the effect of party polarization on governance in the 2015 issue of the Annual Review of Political Science. What is often missed, Lee explains, are changes in the scope of policy issues over time. For example, even though measurements show increasing party division over what are categorized as civil rights issues, many of today’s votes are narrower in their effects than the 1960s civil rights legislation. This makes historical comparisons difficult.

In fact, Lee adds, while some measurements suggest more division, what they actually show is simply that the voting behaviour of politicians has become more predictable. Unlike the bipartisan support for the Civil Rights Act of 1964, for example, today’s politicians tend to vote strictly with their party on divisive topics. Voting with one’s team may have become more important than policy.

For example, a 2022 study published in The Journal of Politics shows that Americans severely misestimate the extent of ideological extremism of the opposing party’s voters. The authors surveyed over 13,000 Americans selected to reflect the demographics of the US population. Forty-nine per cent of respondents said that voters belonging to the opposite party are extreme in their policy positions and frequently discuss politics; in reality, only 14 per cent of voters behave that way.

“One thing we know is that it’s a very small slice of the public that has genuine, deep, ideological roots,” says Matthew Levendusky, a political scientist at the University of Pennsylvania who coauthored the study. “If you could give an essay quiz and ask people to explain liberalism and conservatism, that’s the domain of a small set of political activists and political hobbyists.”

Two bar graphs clearly show that people overestimate the political extremeness of those who support the other party, and believe they talk about politics far more than they do.

Americans regularly misperceive people with different political loyalties. Participants in this survey reported their perceptions of the “typical” member of the rival party – how ideologically extreme their positions were as well as how frequently they engaged in political discussions. These ratings were then compared with actual behaviours among survey respondents.

The rest of us couldn’t explain the underlying philosophy of our policy positions. Rather, Levendusky says, people look to their party for guidance on what positions they should take, such as being in favour of more stringent gun control and climate policy if you’re a Democrat and being pro-gun rights and anti-tax if you’re a Republican.

In other words, as politicians increasingly prioritised party loyalty over policy deliberation, the electorate followed suit. By the 1990s and early 2000s, the Republican Party had become the camp of white Christian America, while the Democratic Party became a mix of everyone else, Mason says. Geographically, the divide grew: While members of different parties used to meet at church or the grocery store, Democrats increasingly became identified with urban and coastal areas and Republicans with rural and heartland regions.

As a result, party affiliation evolved beyond political preference and became linked to racial, religious, cultural and geographical identities, Mason and coauthor Julie Wronski wrote in a 2018 paper. This alignment, they noted, intensified loyalty to one’s own party and diminished tolerance for opposing partisans. The growing focus on identity in American politics has been evident in the past decade, reflected in policy priorities, campaign strategies, media coverage and the actions of social movements.

Interestingly, Mason and Wronski’s research suggests – despite what some commentators posit – that Republicans are more susceptible to identity-based politics than those on the left, due to their party’s social homogeneity.

Mason also notes that even when Republicans and Democrats agree on policy, these identities can still foster animosity. To her, rather than carefully weighing the policies of a given party, supporters are becoming just that – supporters. In essence, we’re acting more like fans of a football team going to a game than a banker carefully choosing investments, she says.

The Democratic and Republican parties have become increasingly distinct in their demographic and ideological compositions. The Democratic Party has seen a growing representation of Blacks, Hispanics, seculars and liberals, shifting from a predominantly white coalition to one characterised by diversity and inclusion of non-traditional groups.

In contrast, the Republican Party has solidified its base among whites, Christians and conservatives, reinforcing a homogeneous profile. This clear demographic sorting has led to a widely accepted view of Democrats as champions of racial diversity, religious secularism and liberalism, while Republicans are generally identified as white Christian conservatives.

Other researchers have also concluded that mass polarisation in the United States is less about policy-based division and more about people liking their own political camp and disliking the opposing one. A 2019 article in the Annual Review of Political Science, coauthored by Levendusky, shows that such affective polarisation has increased drastically in the last four decades. Using survey data from the American National Election Study (ANES), the review measured feelings of warmth or coldness toward one’s own party and the opposing party on a “feeling thermometer” scale from 0 to 100.

Affective polarisation, calculated as the difference between these ratings, rose from 22.64 degrees in 1978 to 40.87 degrees in 2016. While warm feelings toward one’s own party remained stable at around 70, sentiment toward the opposing party worsened significantly, dropping from the high 40s to just above 25.

More recent ANES data show that between 2016 and 2020, both warm feelings toward one’s own party and cold feelings toward the opposition increased even further, with affective polarisation jumping from 41 to 52.2 degrees.

Negative feelings toward the rival party have worsened significantly, while positive feelings toward one’s own party have remained relatively stable. The trend illustrates rising affective polarisation, with partisan sentiments becoming increasingly negative toward the opposition.

Researchers have found evidence of a similar dynamic playing out in other countries. For instance, one 2019 study – using the same feelings-thermometer score system – assessed supporters’ feelings toward their preferred and opposing parties in 22 European democracies and the United States between 2005 and 2016. It found that affective polarisation was acutely present in European party systems – especially in Central Eastern Europe and Southern Europe.

As in the United States, hatred between voters in Europe isn’t necessarily connected to policy-based division, the study showed. But the link between the two appears to be stronger in structured, Western democracies with clear left-right spectrums, such as Germany and Sweden, notes political scientist Andres Reiljan of the European University Institute near Florence, who authored the study. Conversely, in poorer and less stable countries like Bulgaria and Montenegro, issues such as corruption and perceived incompetence might be more likely to drive hatred than policy divisions, the study suggests.

A 2021 study across 51 countries reported similar results, with countries like Bulgaria, Slovakia and the Czech Republic among the most polarised nations, while Iceland, Finland and the Netherlands appeared significantly less divided. Interestingly, these two studies showed the US as only moderately polarised when compared with other countries, but Reiljan, for one, notes that in his study, the categorisation was based on figures from 2016. “Now, with our more recent data, I will say that, yes, it is comparatively also very highly polarized,” he says. “And it’s increasing faster, I would say, than most other countries.”

Reiljan’s suspicion received support from a study measuring trends in 12 OECD countries over the last 40 years. Of the countries included, the US experienced the largest increase in polarisation. A 2023 study using the latest available data showed similar results, with the Unites States leading the pack in polarisation if measuring the whole electorate, while affective polarization increased most noticeably in the United States and Germany among those who report a partisan identity.

• A Knowable Magazine report