According to a report to which the Guardian has had exclusive access, the ethnic group most likely to uphold the "old-fashioned" structure of British family life is that from the Pakistani and Bangladeshi communities. On the other hand, white and particularly Caribbean relationships are increasingly characterised by divorce, cohabitation and single parenthood.

"The Asian patterns can clearly be labelled 'old-fashioned' in the sense that many of their characteristics now could have been observed among white families in the past," writes Richard Berthoud, a professor at the institute for social and economic research at the University of Essex.

"The key feature of family life in south Asian communities is the very high rate of marriage. Around three-quarters of Pakistani and Bangladeshi women are in partnerships by the age of 25, compared with just over half of white women, [and] virtually all south Asians with a partner are in a formal marriage."

The research paper to be released today, Family Formation in Multicultural Britain: Three Patterns of Diversity, is based on information from the fourth national survey of ethnic minorities and the Labour Force survey and concentrates on three ethnic groups: Caribbeans, south Asians (Pakistanis, Bangladeshis and Indians), and whites.

Its findings reveal the widely divergent experiences of minorities in Britain, which make it increasingly problematic to refer to a "common black and Asian experience" beyond discrimination.

"All non-white minorities may face a common experience of racism and harassment. But in most other spheres, it is the diversity between minority groups that is their most striking characteristic," the research claims.

The difference between those of Caribbean extraction and other ethnic groups remains startling. Only 39% of Caribbean adults under the age of 60 are in a formal marriage, compared with 60% of white adults.

With half of the British-born Caribbean men and one third of the women who are in relationships having chosen white partners, they are also far more likely to intermarry than any other group.

"The number of mixed partnerships among Caribbeans combined with the low rate of partnership and marriage in the first place, means that very few Caribbean men and women are married to each other... Only one quarter of 'Caribbean' children live with two black parents."

While there is a long tradition of women living independently of their children's father in the Caribbean, figures from the report suggest that this practice has become more, not less, common in Britain than it is in the Caribbean. Half of Caribbean families with children are now single parents.

The cultural shift in gender roles, greater expectations of sexual equality, and the attendant educational and employment opportunities for women that come with them, mean that all ethnic groups have seen a decline in the traditional family structure of two-parent households with only the father working outside the home.

In 1973 more than two-thirds of all British women in their late 20s were married with children; by 1996 the proportion had fallen to less than a third. As a result there are far more options open to Britons of all ethnicities about the kind of family life they wish to pursue .

But these trends are far less marked in the Pakistani and Bangladeshi communities than elsewhere: "A clear majority of Bangladeshi and Pakistani women report their primary activity to be looking after the house and family."

"The objective fact is that white families are moving in a particular direction," claims Prof Berthoud. "Pakistanis and Bangladeshis, and to a lesser extent Indians, are behind that trend. Caribbeans are in front. In fact, all groups studied are moving in the same direction."

The findings have ramifications which go beyond race to issues of education, social security and child poverty. The majority of Pakistani and Bangladeshi women look after their home and family full-time, although that proportion declines considerably among those with more qualifications. The combination of large families and non-working wives mean half Pakistani and Bangladeshi households have a standard of living below half the national average, even if the father is working.

While Caribbean lone parents are far more likely to work than their white counterparts, more than half still depend on income support.

Prof Berthoud writes: "It is difficult to avoid the conclusion that it is the number of lone parents in the Caribbean community, rather than high levels of unemployment, which confines more than a third of all families with children to the safety net."

However, the temptation to draw simplistic ethnocentric conclusions from the study should be avoided, he argues. "This provides plenty of scope for moralising, from a white perspective, about the subordination of women to men and family (in one case) and irresponsible parenting (in the other). These issues are better sorted out within the relevant communities."

• Further information is available from the institute for social and economic research, tel 07768-661095, website www.iser.essex.ac.uk

Domestic differences

The married couple

Kashif and Sabeen Ashraf are a model nuclear family: a young, happily married couple with a one-year-old son. Sabeen, from Pakistan, joined Kashif in Oldham after their arranged marriage in 1996, and says she would never have dreamed of living with him out of wedlock.

"You hear of people who've cohabited, but none of my friends have," says Kashif, 32, a careers officer. "It tends to be people in interracial relationships. Maybe a generation down the line it might happen."

He adds: "I think marriage is more binding. And also, once I graduated and got a job, the next thing in life was to have a family to complete the circle. I was born in Oldham and have been to university, but I still chose the traditional route."

He says that divorce is still stigmatised, although he knows a few Asian divorcees.

Sabeen, 25, who runs a boutique, is similarly pragmatic about the rising divorce rate. "Nobody wants to get divorced, but if they're not happy then there's no point living together," she says.

The unmarried couple

Janet McLean and her partner Aled Murphy are a typical example of a modern-day white couple. After two years living together in Fife they say marriage is "not an issue", although they would like to wed if they have children.

The couple see their bond of trust and their practical commitments - a mortgage and joint bank account - as more important than a piece of paper. Forty years ago their families might have disagreed. But what was once condemned as living in sin is now a normal way of life for millions of Britons.

"Nobody in my family is married," says Janet, 27, a research psychologist. "My brother and mum and dad all cohabit with partners, and my dad keeps pointing out to me that it doesn't matter any more because under Scottish law we are more or less man and wife."

Aled, 30, an electronics engineer, agrees that families are now much more understanding: "A lot of people's parents did it themselves in the 60s and 70s, although my mum didn't; I don't think she would have been allowed to."

In fact, in a reversal of the old norms, the couple say they would never contemplate marrying someone without living with them first.

The single mother

Yasmin Ferguson spent 13 years with her childhood sweetheart, but moved out of their shared house in Bristol after their son was born eight years ago.

"I didn't want my son growing up with shouting and I knew it was the fact we were living together that was causing problems," she explains. She continued to see her partner until a year ago.

Yasmin, 29, is a qualified social worker studying for a degree in health and community studies. She believes her self-reliance is partly due to an immigrant work ethic inherited from her parents.

"My mum and dad came over from Jamaica to find a better life for themselves. When we were growing up we were taught we should be independent and self-sufficient," she says. "Women in general tend to be told you have to go out and find prince charming."

None of her seven elder brothers and sisters are married, but she still hopes to find a lifelong partner. "I think that's why I'm single," she says. "I contribute a lot to relationships and I want my expectations to be met. I do hope one day I will meet someone for ever.

"But I see the disappearance of the image of a nuclear family as a good thing. It will never be seen negatively, but let it be seen as another way of life rather than the way."

Tania Brannigan