How many black men have white partners?

Did you know that 40 per cent of young black men in Britain are married to, or live with, a white partner? The trend is less common on the other side of the sexual divide. Even so, one in five young black women has a partner who is white.

These figures emerge from the enthralling new results of the 1991 census, which for the first time asked people to classify themselves ethnically. The statistics, reported in Ethnicity in the 1991 Census, are a landmark in our understanding of contemporary British society - in particular, the tug-of-war between separation and assimilation. The numbers matter.

The census gives weight to the view that much public comment on racial matters is skewed by the "London factor" - what national journalists and politicians in the capital see around them gives a false picture of the country as a whole. About 45 per cent of all people from ethnic minority groups live in London, and one Londoner in five belongs to an ethnic minority. Yet Britain as a whole remains very white indeed; there is nothing "multicultural" about it. At the census in 1991, ethnic minorities came to about 5.5 per cent of the population: that is, just over three million in a total population of almost 55 million.

Britain's West Indian population is almost half a million. Six out of 10 live in London. So do eight out of 10 black Africans. As a result, London-based commentators may fail to realise that the largest minority group in the country is one of the quietest - the 840,000 people whose families came from India, either directly or via Africa.

There should be compulsory bus tours to Leicester, a strongly-Indian city full of little businesses, all striving to get bigger. Mr and Mrs Patel are willing to work longer hours than Mr and Mrs Smith. They have high ambitions for their children. Research shows that Indian families in Britain are now, on average, at least as well-off as their white counterparts. Their children are better educated.

The West Indians were the first of the great post-war immigrations, but their numbers are falling fast. At the present rate, you could even foresee them vanishing as an identifiable group. In the 1980s, the West Indian population dropped by 14 per cent, partly because of old age and death; marginally because of people going back to Jamaica; and on top of this, that extraordinarily high rate of "marrying (or cohabiting) out," which seems to be accelerating.

Among Britain's ethnic minorities, according to the census, one pre-school child in five is now of mixed race. The proportion is rising. This throws a spanner into the bizarre social work policy of ethnically-correct adoption and fostering.

The new study should help speed up the process of ceasing to regard ethnic minorities as a single undifferentiated mass - the "blacks", as both left and right combine to call them. If there is multiculturalism in Britain, it lies within these very different new populations. All are having to come to terms with a dominant white society, each in their own way.

Consider the evolving patterns of marriage. Among Indians, for example, the traditional marriage rules are being re-jigged. In Britain, two out of three Indian women work: the same proportion as white women. (By contrast, only one Bangladeshi woman in 10 works, and only one in five of Pakistani women.) So the old Indian idea of a dowry is changing.

It is now important to find a bride who can bring earning power with her, not just a cash lump-sum. And, if there is a dowry, young women who work can chip in. They may then demand some control over what happens to it. As these trends evolve, Indian marriage is becoming less an old- style, pre-arranged alliance between families, and more of a new-style partnership between individuals. Divorce remains uncommon, in contrast to the trend among the white majority, the census found. Though it isn't forbidden in Islamic, Hindu or Sikh culture, it is strongly discouraged. A divorced woman may find she can't rejoin her own family.

But the explosive subject is, of course, West Indian marriage - or, rather, non-marriage. The explosiveness is, no doubt, why (as the census report acknowledges) even the academics have shied away from studying it. It makes the new findings all the more valuable.

Half of all men and women from black ethnic groups (which includes Africans) are unmarried. Forty per cent of black children are in lone-parent families. This contrasts strikingly with 15 per cent of white children, and 10 per cent of Asian children.

The census study notes that explanations, in both the United States and Britain, focus on the constraints faced by black women in finding a suitable spouse. There are the scars of colonialism and slavery; and currently the higher levels of unemployment and lower levels of education among black men. In the Caribbean, couples may traditionally live together in a common-law marriage, or they may have a "visiting union" - a regular sexual partnership, but without living together. Because of the taboo on research, no one knows how frequent this is in Britain.

One possible historical explanation seems to have slipped through the net. Cultural patterns are enormously durable. Everyone acknowledges that, on the one hand, jazz and, on the other hand, voodoo, have West African origins. What about family life? Intriguingly, the census shows that all the black minority groups - African as well as West Indian - have sharply higher figures for lone parenthood. If you focus on lone parents with dependent children, almost a quarter of the black population lives in such a family. No other group, including whites, exceeds 7 per cent.

We are, by the way, talking about swelling numbers of Africans. There are already more Africans in Britain than Bangladeshis. Walk along the Old Kent Road, in south-east London, and you see African evangelical churches all around you. In the local borough, Southwark, Africans now outnumber West Indians.

Immigration produces clusters like this. Once upon a time, the supposed experts thought that the best thing was to encourage new arrivals to spread out across Britain. When Indians were expelled from East Africa, officialdom tried to stop them concentrating in Leicester. It failed. The pull of family and mutual aid was too strong.

All the ethnic minorities are concentrated in towns and cities. This concentration is increasing. After London, the biggest area of settlement is the West Midlands. Next come the former mill towns of South Lancashire and West Yorkshire. The least visible group is the Chinese, who are scattered, family by family, chippie by chippie. But on Sundays, they shut up shop and drive into Manchester's or London's Chinatowns to celebrate together.

Most Pakistanis live away from London, because that is where the factories used to be, which drew them in. Bangladeshis live mainly in the London borough of Tower Hamlets, and in Luton and Oldham. But they are gradually drifting south. Whatever the hazards of living in east London, it offers more chances of doing something for yourself and your family than Oldham.

Britain often follows American social patterns after a time-lag. The growing concentration of minority families in certain towns and cities may be evidence of "white flight". It is not just that some people arrive. Other people also leave.

A young white couple may set up house in, say, Stoke Newington, in inner London. But when their children arrive, they start to fret about schools where many pupils may come from homes in which English is seldom spoken. They move house to St Albans. Or Essex. Or Kent. But could this be a class effect, not a race effect? The argument here would be that, as people, of whatever background, get more prosperous, they will move out. So far, at best, the verdict is "non-proven".

David Coleman, Lecturer in Demography at Oxford and joint editor of the new study, pours gentle scorn on those who, for the best of reasons, try to establish a historical lineage for blacks in Britain. Admittedly, in the 18th century, there was a fashion for black serving-men. The highest estimate is 15,000. They disappeared almost without trace in the 19th century. Some left. Most of the rest died childless. (There were few black women.) Even in the early 1950s, as post-war migration began, there were only about 40,000 non-whites in Britain.

On any reasonable reckoning - and this study is the best reckoning so far - Britain has been admirably successful in absorbing such a novel upsurge in its non-European population. Immigrants have classically been a source of strength to a nation. This study shows each group going about its own business, but also in its own way beginning to assimilate. To Britain's benefit.

The writer is Senior Fellow of the Institute of Community Studies