On 23 June 2016, the British voter changed. Before that day, they picked a party, usually red or blue. By that morning, only two tribes mattered: remain or leave. And they kept mattering long, long after the result was declared. Rather than bin those short-lived and now stale allegiances, voters made them their personas. No longer a “Labour man” or a “Conservative family”, they became instead “remoaners” or “Brexiters”. Even today, 60% of Britons still identify themselves by where they scrawled a single cross in a one-off poll 10 years ago.

Ask about the difference Brexit has made and the answer normally concerns policy or high politics: how our economic trajectory has become bumpier, or how the Tories keep getting into punch-ups with each other. But it became so much bigger than Boris v Dave. The civil war blazed through the country, and recruited nearly all of us to one side or the other. The effects still ripple through our elections and media today.

Before the murder of George Floyd or the arrival of the Covid vaccine, contemporary Britain’s most powerful form of identity politics was Brexit. Before Gaza, it was the event that radicalised a generation of voters. Without the referendum, you have no GB News and definitely no The Rest Is Politics. There are neither “centrist dads”, nor any “gammon” heckling at the panel on Question Time. Nigel Farage and Zack Polanski aren’t topping the polls or preparing for triumph in next month’s elections. And racism Tommy Robinson-style remains a fringe pursuit. The history of each of these aspects of today’s Britain runs through the summer of 2016.

Our evidence comes from a new book by politics professors Sara Hobolt and James Tilley. In Tribal Politics: How Brexit Divided Britain, they conducted and analysed surveys of large numbers of voters over many years. Put together, the story is both simple and very different from the one told by the likes of Farage.

Listen to the co-founder of the company trading as Reform, and Brexit was a desire clutched to the breast of all right-thinking Britons. The truth is that, until the referendum, the British public hardly gave any thought to the EU. If polled, most would express some form of Euroscepticism, but no overwhelming desire for exit. When David Cameron instructed his party in 2006 to “stop banging on about Europe”, it was because the subject left voters cold. But that was years before the Tory leader capitulated to his backbenchers.

At that point, an obsession of one small fraction of the Westminster elite was made a public concern, given months of airtime and front pages. The rest of us picked one of two sides, talked about it down the pub or at family dinners. Anyone who has read a recent self-help book knows what happens next. The author of the bestseller Atomic Habits (25m copies and counting), James Clear, writes: “To change your behaviour for good, you need to start believing new things about yourself. You need to build identity-based habits.”

Your position on Brexit became an identity-based habit, reiterated over and over. Crucially, none of this stopped on polling day. The narrowness of the result, the shock it caused at Westminster and the scale of the change ahead for British politics, businesses and households meant the argument continued, became even more public. There were street stalls, special EU berets sold and marches through central London. When I went to the Last Night of the Proms in 2017, activists were handing out EU flags at the door, and I watched as a mini flag-off broke out between remainers in the stalls and union jack-wielding traditionalists down by the conductor.

In forging these new identities, the aftermath mattered more than the campaign. Among Hobolt and Tilley’s graphs is one on “emotional attachment to Brexit identity” before and after polling day. A month beforehand, a modest attachment is clearly visible, which gets stronger as the big vote looms. But the biggest jump comes after the results are out. Once the match is over, the fans keep shouting and they get a lot louder.

The tribalism doesn’t fade with time; it remains strong. Whether you were in or out shapes your view of whether Brexit is going well or badly, which is no surprise. But it also shapes how you view the other side: remainers see leavers as selfish, hypocritical and closed-minded, and vice versa. By 2025, only about 40% of leavers can bear even to talk politics with remainers; according to the data, the feeling on the other side is mutual. Numbers like this don’t signify something as mild as opposition; they are evidence of discrimination. Those on one side don’t want those on the other to share their house or marry their children.

“Remainers and leavers did not just disagree over Brexit,” write the authors. “They increasingly disagreed over reality itself.” They show how, as late as 2024, the sides argued over how well the economy is doing.

A spectre is haunting this new politics: the spectre of class. The 20th century was the era of class politics. Two words changed that: Tony Blair. A previous study co-authored by Tilley shows that the working class were staunch voters until the 1990s, when the party of labour declared “we are all middle-class now”. The study concludes: “The decline of class-based voting was driven by Labour’s shift to the political centre-ground.” Keir Starmer boasts of his working-class origins and team, yet it is in large part gestural, a matter of putting out union jacks (very post-Brexit, that), but not delivering meaningful change.

When class is banished from politics, all you’re left with is culture wars. The UK’s withdrawal from the EU was fundamentally about changing our economic and trading relationship with 27 other countries, but the leave campaign had no clear ideas on what the new terms of trade should be – one large reason why the aftermath of the vote was so messy for so long. For the Brexiters, immigration was the killer “baseball bat”, as Dominic Cummings termed it, to be swung very hard against the opposition.

Perhaps the most dismal chart in Hobolt and Tilley’s book is the one that summarises where remainers and leavers differ on policy. Top comes immigration, obviously, followed by foreign aid and the death penalty. The two sides have little to say on whether Britain should be more equal, should treat workers better or have more public ownership. In other words, nothing that will make much of a difference to how much money you earn, pay in bills or have after taxes. The people who prosper from that kind of empty politics are those who are prosperous enough already. And they prosper even when they lose. By the end of 2016, Cameron no longer lived in No 10 and he was reportedly getting almost as much for a 60-minute talk as he used to earn in a year as prime minister. Members of his team scored honours and lucrative consultancies advising businesses on how to deal with the consequences of a referendum they’d botched.

We live in an era of polarisation and grift, of blatant lying and blaming institutions. But the British only got here by passing such milestones as the Brexit vote of 2016, in which yet another elite debacle turned into a long and bloody national breakdown that set neighbours and workmates and families against each other. And for what?

• Aditya Chakrabortty is a Guardian columnist