Sophia Worringer is a parliamentary researcher for Iain Duncan Smith, and was formerly a researcher at the Centre for Social Justice.

It’s the middle classes’ best-kept secret. One that improves children’s school performance, supports their physical and mental health and reduces the chances of exclusion, joining a gang or ending up NEET.

Among the top quintile of couples by income, 84 per cent tie the knot. Among the bottom quintile only 45 per cent do.

Why the difference? The Centre for Social Justice’s new report, Family Structure Still Matters, provides a close analysis of the correlation between family structure and various outcomes. It has found that even after controlling for income and education, the gap in outcomes between children whose parents are married and those who cohabit clearly persists. Robust academic research has shown that family structure has a greater impact on the presence of externalising behaviours – linked to cognitive development, physical and mental health, school attainment, criminal justice involvement and social and emotional development – than education or poverty.

This is about more than money. There is a significant difference in the family breakdown rates between the children with cohabiting parents and those with married parents. A child of cohabiting parents is more than twice as likely to have experienced their parents separating, with all the accompanying disruption that the loss of a parent from the home entails. After income controls were applied, 88 per cent of married couples were still together when their child was aged 5, compared to 67 per cent for cohabitees.

Marriage has a powerful social meaning that conditions the behaviour of its participants. It provides clarity for the future of a relationship, forces ambiguity into the open and requires a public commitment in front of family and friends. Cohabiting relationships are less likely to have that specific moment of an articulated promise.

This is a social justice issue. Already half of all children are no longer living with both their parents by the time they sit their GCSEs but for children in our poorest communities this is true by the time they start primary school. 6% of those aged 5–10 with married parents had a mental health disorder compared to 12% of the same age with cohabiting parents. Rates of cohabitation are on the rise, but largely in the low income bracket.

High income couples continue to marry at consistently high levels. Just look at our politicians: they choose to marry to bring up children, knowing the benefits that stability and commitment bring, and yet they are wary of making any distinction between these two very different types of relationships when enacting policy.

Having married parents can be a step out of poverty. An American study found that amongst families in equal poverty levels children with married parents had a 30 percentage point lead – an 80 per cent chance – of moving out of poverty compared to those with cohabiting parents.

Not only are married couples more likely to stay together, they are more likely to enjoy better mental and physical health, to engage meaningful in their wider community or belong to a voluntary association, have higher relationship satisfaction and income and lower rates of domestic abuse and conflict with their children.

Given the clear advantages of marriage, government should make every effort to distinguish between marriage and cohabitation in data collection and in policy recommendations. It does not. By ignoring this distinction, the government is leading many people to believe that married and cohabiting relationships are interchangeable. This risks robbing couples of making an informed choice about what kind of relationship they should embark on.

We owe it to the most disadvantaged in society to not be silent on the benefits of marriage.