One of 2010's most feted books has been The Warmth of Other Suns by Isabel Wilkerson, a powerful history of the 20th-century migration of African-American families from America's deeply segregated south. Epic in the scale of its hurt and hope, it tells the largely untold story of a people fleeing a society of segregation and lynchings to start more fulfilling lives in Chicago, New York and the west coast. In these anonymous sprawls, racial prejudice may have been alive and well in people's heads but the crucial difference was that it wasn't sanctioned by policy and pulpit.

I watched the Pulitzer prize-winning Wilkerson talk on television about her book and was struck by the number of times and the deliberation with which she used the term "caste system" to describe the purist infrastructure of Jim Crow country, whose codes were as repugnant as the pollution laws of untouchable India. I phoned her at Boston University and she confirmed something that, strangely enough, has escaped reviewer attention: nowhere in the 622-page book does the word "racism" occur. Wilkerson even did a word search to make sure it wasn't there.

"Racism is such a divisive, loaded word that it has become shorthand for all kinds of things," she explained. "Using the term 'caste system' not only forces readers to challenge notions of how race and class play out in the US, it also places an ethnic equation in a larger historical structure. I prefer caste system because I believe it better characterises the larger forces at work. It focuses on structure rather than emotion, it answers so many questions about the behaviour of people at all levels of the caste system and explains why those perceived to benefit from it will work so hard to maintain it, and why those at the bottom of it would be driven to do whatever it takes to escape it."

In a timely coincidence, her decision shines the searchlight on that contentious human-rights nettle of whether caste can be included under race law. An issue of political significance now, with Britain on its way to being the first western country to do so with the Equality Act 2010. Last week, the National Institute of Economic and Social Research (NIESR) published its long-awaited study, which states that caste discrimination does exist in the UK among people of Indian origin across religions, who comprise 5% of the population.

Several British-Indian forums have opposed listing caste as an aspect of race (as, historically, have the Indian government and several distinguished scholars) on the academic point that a caste-race conflation is scientifically false and that education, not legislation, will change mindsets.

Britain's Dalits, who have lobbied hard for the law, say that the debate is wrongly framed. Even if caste and race are not the same, the experience of being inferior meted out by casteism and racism is unique and like no other. Which is why civil rights leaders like Martin Luther King Jr have compared the status of African-Americans to India's untouchables and Dalit literature (Dalit means "broken people", the earlier term was "untouchable") has drawn inspiration from revolutionary black literature and the civil rights movement.

The Warmth of Other Suns helps reframe the debate. Wilkerson does not glibly seek to substitute race with caste – she readily points out that it is a false equivalence. Instead, she uses terminology to show that the two are complementary systems of oppression that feed off and belong with each other. As fellow-travellers of feudalism, they are "bound up inextricably in society's DNA" with race being used to justify a caste system whose imperatives are essentially economic – to perpetuate a workforce to carry out the odious and difficult tasks of society (whether it is scavenging or cotton picking) for little or no pay.

For the African-Americans in Wilkerson's narrative and for many of the UK and US's Dalits, migration was an act of secession from an oppressive social order. Of course, these families soon found that prejudice is a nifty migrant, too, and that old customs quickly take root in unaccustomed earth. One well-known Dalit writer from Mumbai recalled how he had dropped into a pub in Southall in 1992 and found scribbled on the wall: "Chamars (untouchables) and dogs not allowed."

And while it may be impossible simply to legislate caste discrimination out of people's memories – India has progressive laws but prejudice persists – parliamentary cognisance will not only provide protection in the workplace and schoolroom, it has effectively pulled the shroud off the elephant in the room. The Equality Act's specific listing of caste is a brave step toward acknowledging the persistence of an ancient affliction and an affirmation to Britain's Dalits that their new country, however watery the warmth of its fitful sun, cares about their freedom.