- guardian.co.uk, Wednesday November 22 2000 02.27 GMT
It found 70% of children born within marriage will live until their 16th birthday with both natural parents, compared with 36% of children born to cohabiting couples.
The figures were based on an analysis of a government sample survey of 10,000 British households and the statisticians concluded that the birth of a child made it 60% less likely that a cohabiting couple would marry.
"The temporary nature of many cohabiting relationships means they should not be considered simply as marriage without the vows.
"They are a new and relatively unstable family structure which is leading to a dramatic increase in childbearing outside marriage and significant growth in the time children spend living in one parent families," said the au thors, John Ermisch and Marco Francesconi.
The study found 70% of first partnerships were cohabitations outside marriage. Typically they lasted two years before the couple either married or separated.
About 60% of first cohabitations became marriages and 35% dissolved within 10 years.
This pattern had contributed to postponement of the average age of marriage and motherhood. But more women were having children while cohabiting, rising from 2% in 1980 to 22% in 1997.
On average, children born to cohabiting couples spent 4.3 years of their childhood with just one parent, compared with 1.7 years for children born within marriages and 6.6 years for children born outside a live-in partnership of any kind.
Robert Whelan, of Family and Youth Concern, said: "Children need to have stable lives and be born within marriage."
The findings, based on the British Household Panel Survey, are published today in a book, Seven Years in the Lives of British Families.
