James Tilley

Political tolerance of others’ civil liberties is an essential and everyday condition of democratic politics without which citizens cannot engage constructively with those of different views. In this paper, we combine insights from political theory and political behaviour to develop and test the concept of ‘partisan intolerance’. We conceptualize partisan intolerance as the gap between a person’s willingness to interfere with contentious activities by in-partisans versus the same activities by out-partisans. Using two pre-registered experiments, we find high levels of partisan intolerance in Britain. Moreover, while partisan intolerance is not associated with abstract measures of political tolerance, we find a strong association with affective partisan polarization. Our findings thus suggest that increasing affective polarization among partisans is accompanied by a high degree of intolerance towards their opponents’ basic civil liberties such as freedom of speech and the right to protest.

This chapter develops a theoretical framework for understanding the creation of issue-based identities and the way in which they generate tribal politics. Building on social identity theory and self-categorization theory, we argue that political identities can form not only around entrenched social categories or party affiliations, but also around specific political issues. We define issue-based identities as social identities based on shared stances on a specific political issue and outline three key conditions for their formation: issue contestation (intense, high-stakes public conflict), issue expression (individual commitment through voting and political engagement), and issue alignment (the issue cuts across traditional party lines). These identities have all the standard hallmarks of social identities: group awareness, emotional attachment to the ingroup, and adherence to group norms. Moreover, we also highlight how issue-based identities, once established, give rise to outgroup prejudice, discrimination, and cognitive bias. These dynamics underpin the emergence of tribal politics, through which political divisions become identity-based and emotionally charged. The framework is then used to understand the formation of Brexit identities, previewing the analyses in the following chapters. This approach sheds light on how issue-based groups can emerge rapidly in response to political events and reshape political behaviour and perceptions in democracies.

This chapter draws on the case of the 2016 Brexit referendum to introduce the idea of tribal politics grounded in issue-based identities. We argue that the referendum did not merely expose pre-existing divisions in public opinion but actively generated new political identities: Leavers and Remainers. These identities divided the nation and came to shape social and political behaviour in the years that followed. This introductory chapter sets out the book’s central argument: that three specific conditions—issue contestation, issue expression, and issue alignment—combined to transform disagreement on a political issue into a deep social identity divide. It introduces the theoretical framework for understanding both the formation, and persistence, of issue-based identities, but also the important consequences of those identities. We also emphasize the broader relevance of the Brexit case for understanding identity-based politics in other established democracies.

In this chapter, we look at the most concerning consequence of tribal politics: group bias. Not bias towards fellow and rival identifiers, but bias in how people perceive reality. A combination of two psychological processes—confirmation bias and rationalization—means that people adjust their perceptions and interpretations of the world to better fit their political identities. We explore how Brexit identities bias people’s views in four ways. First, we look at the broader consequences of Brexit and examine, in people’s own words, what effects they thought Brexit has had and will have. Second, we examine people’s economic perceptions and how they attribute responsibility for those economic outcomes. Third, we turn to people’s policy attitudes and show how these attitudes were brought into line with Brexit identities. Finally, we turn to the legitimacy of the political system and how Remainers, as losers of the referendum, interpreted the outcome as a product of a flawed democratic system. In all cases, we find ample evidence of tribal bias via the Brexit lens: people’s identities shape both their perceptions of the world and their own political opinions.

This chapter examines three core consequences of tribal politics, using Brexit identities as a case study. First, we explore ingroup affect, showing that individuals consistently felt more emotionally attached to their Brexit identity than to their party identity in the years following the referendum. Second, we highlight outgroup animosity and prejudice, a key feature of tribal divisions, where people not only dislike those on the opposing side, but also view them as less intelligent, open-minded, and honest. These emotional attachments and hostilities constitute a form of affective polarization. Evidence shows that Leavers and Remainers express high levels of prejudice against each other, reinforcing identity-based divisions. Third, we examine discrimination as a behavioural outcome of tribal politics. Through experimental evidence, we demonstrate that people are less willing to engage with, or accommodate, those from the opposing Brexit group, even in non-political settings. Our findings reveal how Brexit identities not only shaped political opinion but also influenced interpersonal relations and social cohesion.

This chapter examines how Brexit identities shaped electoral politics in the UK in the years following the referendum. We show that although the Leave-Remain divide cut across traditional party loyalties, political parties gradually adjusted their rhetoric and positioning to appeal to the new Brexit tribes. The Conservative Party, in particular, shifted decisively towards a pro-Brexit stance, while Labour, constrained by its Remain-leaning base and Leave-leaning constituencies, struggled to offer a clear alternative. Voters responded accordingly. Across the 2017, 2019, and 2024 general elections, Leavers consistently favoured the Leave bloc of parties (Conservatives, UKIP, Brexit Party, and Reform UK), while Remainers largely supported the Remain bloc (Labour, Liberal Democrats, SNP, Plaid Cymru, and the Greens). The 2019 election marked the high point of Brexit-driven realignment, with party choices sharply structured by identity-based divisions. By 2024, Brexit had faded as a salient issue, yet the underlying tribal alignment persisted: voters continued to sort into broad identity-based blocs, even as party fortunes shifted. The chapter concludes that while the explicit politics of Brexit receded, its legacy endured in reshaping the structure of British electoral competition.

This chapter examines the 2016 Brexit referendum as the critical moment that transformed an issue of low public salience into the basis for enduring political identities. We outline how the referendum met three key conditions for the emergence of issue-based identities: high levels of issue contestation, widespread opportunities for issue expression, and a lack of alignment with traditional party divisions. Drawing on evidence from the campaign and its aftermath in Parliament and beyond, we show how the binary nature of the referendum increased the salience of EU membership, forced individuals to express a clear stance, and presented ambiguous partisan cues due to divided party leadership. This context facilitated the creation of two new identity groups: Leavers and Remainers. The chapter argues that other specific features of the Brexit referendum, including the continued partisan divisions and public debate following the vote, sustained the salience of the issue and reinforced the emerging tribal divide. We thus highlight how certain political events, such as referendums, at certain times, such as 2016, can act as catalysts for identity formation.

This chapter traces the evolution of elite and public attitudes towards European integration in Britain, highlighting the contrast between the high salience of the issue to political elites and public indifference. We argue that Britain’s ambivalent relationship with the EU was driven not only by historical contingency, but also by deep and recurring internal divisions within the major political parties. As European countries came together to form the European Communities in the 1950s, British politicians were hesitant about membership. Initially, the Labour Party was most sceptical about membership but also most internally split over the issue of Europe. But by the 1990s, the Conservative Party had become not only the party most opposed to European integration, but also the party most divided over the issue. Drawing on party manifesto data, expert surveys, and public opinion polling, we show how the EU issue repeatedly fractured party unity even as it remained of little interest to most voters. While British political elites were more divided and sceptical about EU membership than their continental counterparts, we argue that the public also expressed a distinctive mix of indifference and latent Euroscepticism. Ultimately, this meant that it was elite-level contestation—rather than public demand—that created the conditions for the 2016 referendum. The referendum then transformed a previously marginal issue for voters into a central axis of political identity and conflict in Britain.