Children raised by cohabiting parents are at a greater risk of suffering psychological stress, physical abuse and of causing trouble at school, according to a new study.
The report, Why Marriage Matters, 3rd edition, published by the National Marriage Project in the US, says that cohabitation is now a bigger problem for children than divorce.
Brad Wilcox, a report co-author and head of the National Marriage Project, says divorce rates have steadily dropped since their peak in 1979-80, while rates of out-of-wedlock childbearing have soared.
Forty-one percent of all births are now to unwed mothers, many of them living with - but not married to - the child's father.
Wilcox says the children of the divorce revolution grew up to be understandably gun-shy about marriage. Many are putting it off, even after they have kids. But research shows such couples are twice as likely to split.
"Ironically," he says, "they're likely to experience even more instability than they would [have] if they had taken the time and effort to move forward slowly and get married before starting a family."
According to the British Millennium Cohort Study, only 10pc of married couples will have broken up by the time their child is five, compared with 25pc of cohabiting couples.
Only 35pc of British children born into a cohabiting union will live with both parents throughout their childhood, compared with 70pc born to married couples.
According to the only piece of Irish research on cohabitation, only one in four cohabiting relationships in Ireland last seven years or more. The rest end in marriage or breakup.
Couples who cohabit before marriage are more likely to divorce than those who do not cohabit first.
Another recent study finds that a quarter of American women with multiple children conceived them with more than one man.
Psychologist John Gottman, who helped co-author Why Marriage Matters, says that the instability produced by such situations can have a range of negative impacts on children.
Children of cohabiting couples are at greater risk of both increased levels of aggression and increased levels of depression compared to the children of married couples, she says.
Gottman says the evidence for marriage is strong. The institution's wide-ranging benefits - better health, longevity, greater wealth - are not conferred on those who cohabit.
"Because," he says, "they're basically saying, 'If you get into trouble, baby, you're on your own; I'm not there for you.' I think that's the big problem."