Racist jokes have a long and sad existence. In the 70s and 80s they provided comic fodder for standups such as Jim Davidson and Bernard Manning and were delivered to an audience of the predominantly converted. More recently they have been adopted by the "hipster racist", who can, at the expense of black people, "ironically" don blackface for cheap laughs yet remain genuinely bemused at being labelled a racist. It has reinvented itself yet again. Now it is acceptable to crack a racist joke in the presence of those it is dehumanising.

It is a Friday night and I have just boarded the train from a village in West Yorkshire to Leeds. A group of youngsters, out to celebrate their A-level successes, are sitting next to me, huffing and puffing, congratulating themselves on making the 19.18 on time. Passing around the wine they have stowed away in their water bottles, they laugh easily until one member of the group declares that she is "sweating like a Jew in a shower". Perhaps I am a little slow because at first I am not fully aware of the meaning behind the statement. Her friends glance around the carriage, giggly but visibly uncomfortable; one of them tells her to "shush" and asks whether she has had "too much". The penny finally drops. The depravity of the statement is breathtaking.

While the friends quickly forget the moment, and go back to enjoying their Friday night, disbelief makes me replay her words in my head. Sickened, I wonder if any of the group of friends are themselves Jewish and what they must be feeling if they are. One friend is south Asian, the look in her eyes particularly restless.

The group on the train were aware their friend had crossed a line yet they felt unable to challenge her. But in not confronting your friends when they make racist jokes, you unwittingly collude in their racism. As a society, we have failed in teaching our young about the ingenuity and flexibility of racism; it is why such jokes remain. Many adults themselves find it difficult to grasp. Racism is not simply a cardboard cut-out of the extreme right – it is more fluid and complex than we give it credit for.

I am reminded of another incident told to me by my dual heritage 18-year–old neighbour. Out at a pub in Hertfordshire, the only person from an ethnic minority within the group, friendly banter leads a good friend to label her "a tree ornament". She too doesn't fully get the joke until she recounts it to her white mother, whose anger and pain makes her explain to her daughter that her friends are dangerous. Through tears, the daughter loyally insists that her friend is not a racist.

The anachronistic nature of these jokes – one about the terror of lynching, the other about the horror of the Holocaust – seemingly makes their delivery somehow much more acceptable in the eyes of the joke teller. This joke really isn't about you; it is about dead people, dead people from a long time ago, so you don't have to make such a big deal about it. This is especially difficult for young people, black and white and in diverse friendship groups, to tackle.

"I am not a racist but…" has become the prelude to saying something deeply racist. I have heard that phrase far too often. Uttered in my presence, it furtively tells me that I have been given special dispensation as a black person to hear the "truth" about another ethnic minority. That it is not specifically about me is somehow proof enough that the speaker isn't a racist who can and should reconsider what they are saying. It asks me to collude.

Racist jokes, made publicly and in the company of black and minority ethnic friends, operate in much the same way. I am not a racist because you (insert ethnic minority label) are my friend, so how can I be? I am not a racist, so I can share this brutal and brutalising joke with you. I am simply engaging in audacious wordplay, taking part in the postmodern irony that is a sign of our ambivalent times.

Our society is postracial – or so we are frequently told when racism rears its ugly head. It's a society in which the prejudice many ethnic minorities experience is often countered with incredulity, minimised and diminished. It is a society with a predominantly two-dimensional understanding of how bigotry operates: racists exist on the margins and are evil; they do not look like you or me but are somehow physically marked as monsters, rather than everyday people. This line of argument does no favours to any of us, least of all people from ethnic minorities. What it serves to do is to create the false idea that racist acts are infrequent, extraordinary and, crucially, committed only by those who self-identify as such.