Australian supermodel Elle McPherson, dubbed "The Body" for her gorgeous figure, is to marry the father of her three-year-old son, Flynn. Arpad "Arki" Busson, a French financier, has been trying to get her to marry him for the past five years.

With one failed marriage behind her, McPherson has resisted, describing marriage as "old-fashioned". Whatever kind of marriage she manages to forge with her Frenchman, one thing is sure: by sticking with the father of her child, she's bucking the trend.

Parents who are cohabiting, rather than married, when they have a child are less likely to stay together after the birth of a baby, according to Broken Hearts, Family Decline and the Consequences for Society, a report by the Centre for Policy Studies in Britain.

More than half of cohabiting couples split up within five years of their first child's birth, the report shows.

Only 8 per cent of married couples separate within five years of becoming parents together.

For children, this means that being born to unmarried, cohabiting parents gives you a six times greater chance of living with only one of your biological parents at the age of five.

We know that one in three children in the Republic is born outside marriage, but we don't know what happens to their family structures after that.

One theory is that the parents marry after the birth, but the British research suggests that such marriages are more likely to be unstable. We don't know this for a fact, however, so we need Irish research into changing family structures and their effects on children.