Unmarried couples are more likely to suffer ill health, be out of work, and their children are more likely to do less well at school or be the victims of abuse, according to a new book.

In Marriage-Lite, author Patricia Morgan, a sociologist specialising in family policy, reviews the research into outcomes of cohabitation, compared to couples who tie the knot, and argues that marriage is much more than 'just a piece of paper'.

Ms Morgan found the married and unmarried couples were very different in a number of ways and that married couples were more likely to be faithful and stay together than unmarried people.

The average unmarried relationship lasted less than two years with less than four per cent lasting ten years or more.

Cohabitants also have more health problems, probably, suggests Ms Morgan, because cohabitants tolerate behaviour in their partners which husbands and wives would not such as smoking, excessive drinking and drug abuse. Cohabitants are also much more likely to suffer from depression than married people.

Ms Morgan also found that married men work harder - especially when they have children - while unmarried men behave more like single men in their work-patterns, even when they have children.

Other research shows that unmarried women are much more likely to be victims of domestic violence.

The children of cohabiting couples do less well at school and are more likely to suffer from mental illness than children of married couples, says Ms Morgan.

They are also at significantly higher risk of severe child abuse or murder from 'live-in' or visiting boyfriends.

But despite the poor outsome of unmarried relationships, the Government increasingly treats unmarried couples as though they were married - something that should stop, argues Ms Morgan.

She added: "Strengthening the alternatives to marriage has the effect of weakening marriage itself, which appears increasingly pointless. This undermines the only institution ever shown to be capable of raising children successfully."

"People should have more information," said Ms Morgan, who told Adoption-net that she had been surprised by some of the findings of the research.

She argued that there should be an end to tax and benefit advantages to unmarried people saying that married people presented a wealth and health liability for the state.

"Governments are treating unmarried relationships as just as stable as married ones and it just isn't true," she said.

Ms Morgan added that she did not support the Government's recent move to make it easier for single and unmarried couples to adopt children. "I would ask them why don't you make that commitment?"

Her book, Marriage-Lite: The Rise of Cohabitation and its Consequences, is available from the Institute for the Study of Civil Society (CIVITAS, 2 Lord North Street, London, price £6.80.