Children of unmarried parents, in other words, are born into relationships with a high likelihood of instability
Part of the explanation for the increase in children experiencing their parents living apart is a growth in the proportion of children born to lone mothers. In 2005, the latest data we have, 13% were born to lone mothers. This compares with 3.8% of children born between 1976 and 1980, demonstrating the extent and rapidity of social change, notwithstanding the widespread availability of contraception and abortion.
The odds of a de facto couple with children breaking up is more than seven times as high as a married couple who had not lived together before marriage, and more than four times as high as those who had lived together but went on to marry.4 Marriage matters. It has a profoundly stabilising influence on relationships.
This is consistent with the overseas evidence. Data from the Fragile Families study in the US found that parental separation by the time the child was 3, was five times greater for children born to cohabiting than married parents. Differences in financial wellbeing and family characteristics between cohabiting and married parents explained some of the difference, but after controlling for race, ethnicity, education, economic factors, family characteristics and an extensive set of other covariates, parents who were cohabiting at their child’s birth still had over two and a half times the risk of separating compared with parents who were married at their child’s birth.
Findings from the Millennium Cohort Study in Britain, initially comprising a cohort of 18,000 mothers who gave birth during 2000 or 2001, are similar. Again, after controlling for confounding factors, cohabiting couples are consistently 2 to 2.5 times more likely to split up compared to their married counterparts, across all income groups, by their child’s fifth birthday.5
More recently, Bzostek and Berger, analysing data from the Fragile Families study, have found that “children born to unily structure transitions by age 9, with approximately one-half experiencing two or more transitions and approximately one-quarter experiencing three or more transitions by that time”.
In summary, the explanation for the massive increase in the proportions of Australian children who do not reach the age of 15 in an intact family with both of their biological parents is two-fold. First 
, more and more children are being born to single mothers who are neither married nor living in a cohabiting de facto relationship. Secondly, as the proportion of the population who live together outside marriage has increased relative to the married population, and as more and more de facto couples have children outside marriage, so the chances of the parents still being together by the time the child reaches 15 have substantially diminished.6
Mental health
There is now a lot of evidence that the instability of family life is having an adverse effect on the mental health of children and young people. While adverse trends are not observable in every study, there is some compelling evidence now that adolescent mental health problems have been increasing significantly over time. Yes, teenagers have always suffered from depression, other forms of psychological distress, and illnesses such as anorexia; but just not in the numbers we are seeing now.
Why? Of course there are many reasons. This is a complex subject and I don’t want to be simplistic at all; but the evidence suggests that family breakdown is a significant contributing factor.
One important study was published in 2010. In this US study, researchers conducted the most comprehensive analysis to date of data collected through the widely used Minnesota Multiphasic Personality Inventory (MMPI) test. The researchers examined data collected from college students between 1938 and 2007. They found that each generation had experienced poorer mental health than the previous one. At least five times as many college students in 2007 as in 1938 had measures indicating psychopathology on the various elements included in that test. These included measures of hypomania, schizophrenia, psychopathic deviation, paranoia and depression. On many of these measures, by 2007, the increases were dramatic. This substantial increase in mental disorders was clear even after controlling for a greater willingness over time to acknowledge the presence of mental health issues. The researchers noted that their findings might actually underestimate the increase in psychopathology, given the numbers of Americans taking antidepressants.
