There are two theories about racial prejudice. Most people talk as if there is a fixed block of people ‘the racists’: always white and extreme right wing, and usually covered in tattoos. They are ugly to be sure, but they are just a few irreconcilables in the otherwise merrily diverse land of multi-faith, multi-cultural Britain.

The alternative is less cheering. Prejudice can overcome all or most of us in the right circumstances. It just lies there, like a virus waiting to be triggered. We may not know we have it, but we are capable of succumbing in the right circumstances.

The rotten apple theory of racism has taken a battering in 2016. As I have probably written too much on the subject, I will allow others to speak. Sadiq Khan, the mayor of London, talks of using the full force of the Metropolitan police against the extraordinary rise in racist attacks since the European referendum. He at least has no doubt what authorised the abuse of foreigners or anyone who looks remotely foreign.

“ We cannot let the level of debate during the referendum campaign normalise behaviour that is completely unacceptable.

Fraser Nelson, the editor of The Spectator, is equally stunned by Theresa May’s refusal to guarantee that EU migrants in Britain should have the right to remain. May, the supposedly moderate and sensible candidate for PM, does not understand that

“ Britain needs these people; our NHS needs these people. We don’t keep them as a favour to Poland and nor should we ever dream of bargaining their residency in some game of diplomatic hardball. The EU may threaten deportation of Brits: it’s a corrupt and undemocratic institution which is why the 52pc of us voted to leave. But no British government should ever consider kicking out any of the two million EU nationals who are already with us.

He sees her playing with the lives of decent people as a betrayal of the leave campaign that never once threatened the security of migrants already in Britain, somewhat naively, I fear.

Over on the left, Martin Robbins, a science writer for the Guardian, writes of his friends in the 'skeptic' movement, supposedly rational believers in evidence-based politics, falling for the oldest irrational hatred of all.

“ I'm genuinely staggered by the number of people I've seen on Facebook in the ranting about 'Zionism' or 'Zionists'. I never expected to find myself watching Jewish friends on here have terms like 'militant Zionist' chucked at them, or references to conspiracies mounted by 'the moneyed'. It's like turning a rock over and finding a whole, weird new layer of social media I didn't know about before. And what's disconcerting is how close this stuff is to me. I've had to unfriend a number of skeptics spouting this stuff. How did these seemingly reasonable people get so obsessed with this shit?

The history of totalitarianism shows that scientists and technicians are not immune from the seductions of unreason, but Robbins' question remains a good one. The fact that they fall for it suggests that the darkness within theory is clearly true. But it has its difficulties. Most people most of the time don’t abuse immigrants or bait Jews. They need authorisation from above to release feelings they may never have known they possessed.

I am not laying into Fraser Nelson and other liberal Brexiters when I say that, unlike Sadiq Khan, they wholly misunderstand how and why Vote Leave won, and the consequences of its victory.

It began by saying it wanted nothing to do with Ukip. It was to fight a campaign based on sovereignty and the supposed economic benefits of Brexit. (Where are they, by the way?) 'We don't major on immigration in the campaign per se,' a Vote Leave spokesman said in October as he explained why the issue barely featured on its website.

“ Immigration is a major issue for lots of people whether they support in or out. But the biggest issue is whether they believe the in campaign scares about losing their jobs if we leave the EU. That's why we are focusing mainly on economic arguments.

Then Johnson, Gove and the rest decided that the only way to win was to forget about economic arguments they knew they would lose. Instead of focusing on our place in the world and sovereignty, we had, to quote from one of hundreds of immigration scare stories, Boris Johnson claiming that 'uncontrolled' immigration from the European Union was driving down wages and putting pressure on schools and the NHS. No mention of the work immigrants do in keeping schools open and the NHS working. No mention of the economists who find that migration does not affect wages. Just a demand to take back control and stop the foreigners, which chimed with 60 per cent of leave voters who said immigration, rather than the lack of democracy in the EU was their prime concern.

Racism, when it goes beyond its normal limits, requires authorisation from on high. Political or religious leaders need to give the impression that feelings, which are normally sublimated, are free to flourish. I am not saying that Vote Leave wanted violent assaults. But by making the referendum a vote about immigration above all other issues, they created a climate where a victory for leave was a victory for people who wanted immigrants to go.

What applies to the left, applies to conservatives. The Labour leadership has so lowered defences against anti-Semitism, it would have no right to be shocked if Jews were targeted, Equally, liberal conservative have no right to be shocked by the wave of abuse against foreigners or by May's determination to throw EU citizens lawfully living and working here into confusion.

She is cruel and stupid. She is not just causing fear among millions of good people, and terrifying businessmen and women, who depend on EU labour to serve their customers, stack their shelves and pick their crops. But May understands the public mood better than her liberal Conservative critics. In particular, I suspect, she understands the mood of the pro-leave members of the Conservative Party, she hopes will propel her Downing Street.