Samantha Callan is an Associate Director of the Centre for Social Justice and a former adviser to David Cameron on family policy.

Many politicians agree that we need to do more to support families – but add the disclaimer that this shouldn’t mean promoting one family form over any other or treating the prevention of family breakdown as a policy goal. They tiptoe around the elephant in the room of marriage and stability. Yet children fare best when their parents made a solid commitment to each other before embarking on the white-knuckle ride of child-rearing and, typically, when they stay together through all the thrills and spills.

Often when I am among Conservative MPs someone will say they are ‘fiscally conservative and socially liberal’. While I appreciate this expresses a tolerant and non-judgemental approach, on the subject of family structure – specifically whether one or two parents are ideally needed to raise children – this is, frankly, a somewhat incoherent position to take.

Monday’s opener to this ConservativeHome series by Paul Goodman quoted estimations that the annual bill for family breakdown – whether divorce or separation, fatherlessness or dysfunction – stands at between £47 and 100 billion, depending on whether direct or indirect costs are included. The smaller – yet still gargantuan – figure is built up from an additional £16.5 billion benefits bill, £5.5 billion in extra housing costs, £15.5 billion more for health and social care, £7 billion for criminal justice and child maintenance and £2.5 billion for education – all of which ultimately have to be paid for through higher taxation.

Self-evidently, families come in all shapes and sizes, and children can flourish in a wide range of family types. However, when looking at trends, family structure cannot be treated as a neutral bystander, but as an influential determinant of whether or not families will be a cost or contributor to the public purse. Typically, no second parent in the home means much more dependence on the state. Yesterday David Burrowes wrote on this site that children now have a 50:50 chance of still living with both their parents by the time they sit their GCSEs but in poorest areas these odds fall sharply.

A major reason is that marriage and parenthood are becoming increasingly disconnected further down the income scale: around half of the poorest parents are married when they have children, compared with 85 per cent on those on incomes a little over £50k. This matters because unmarried parenthood is a massive risk factor for a subsequent split. The Institute of Fiscal Studies has found that parents who cohabit are approximately three times more likely than those parents who are married to have separated by the time the child reaches the age of five.

This divergence in stability holds regardless of education and income, and widens even further when parents were not living together when their child was born. A staggering 60 per cent of these ‘closely involved’ parents are no longer an item by the time their child is five. Taking the longer view, Understanding Society data reveals that among parents who remain together by the time their child is aged 15, 93 per cent are married.

Researchers trying to understand why marriage is more stable agree ‘selection effects’ play a part: those with the soft skills and other attributes that make them more able to sustain a relationship are also more likely to get married. However, in addition, those who decide to go through a ‘commitment ritual’ such as marriage, with all the orientation towards the future and explicit ‘sinking one’s lot in’ with someone else this entails, are buying into the behaviours and attitudes which can help them stick together during the inevitable tough times. Conversely, those who slide into a less formal relationship can then feel ‘stuck’ or even trapped in it when babies come along. They didn’t choose permanence but had permanence thrust upon them.

It is for this reason that we urgently need a change in culture so that people will increasingly choose to have children within a committed relationship and do all they can to mend rather than end those relationships if they hit major difficulties, not least because of the effect of break up on their children. This will require easy access to relationship support that has mainstream appeal, and policies that incentivise and reward couple parenting.

First and foremost, this requires political will. In 2006 ,I wrote: ‘successive governments have neglected to consider adequately the distinct possibility that much breakdown might be preventable and that many marriages and partnerships might be worth saving, in financial as well as emotional terms.’ Encouragingly, the Department for Work and Pensions has made family stability a key priority for tackling child poverty, and there is wider governmental recognition that children need safe, stable and nurturing relationships in order to thrive. Hence the family test – now carried out by the department.

Second, that political will needs to translate into policies that can reduce the likelihood of children experiencing family breakdown – and a cabinet-level champion to drive and demand progress across all government departments. Most importantly, we need to make sure that existing infrastructure such as Children’s Centres helps whole families by supporting couple as well as parenting relationships, fathers as well as mothers, parents of teenagers as well as toddlers. I have written extensively about how they should evolve into Family Hubs.

Also, although the new recognition of marriage in the tax system is very welcome, the level of financial benefit needs to send a stronger signal than it is currently doing. Doubling the amount that can be transferred if parents have children under three would be a good place to start as this is the point in the family life cycle when finances can be most tight.

Those who dismiss this as an attempt at social engineering should consider that while people on low incomes aspire to marriage, the financial and cultural barriers they have to overcome to realise those aspirations are significantly greater than those who are affluent. Promoting marriage and preventing family breakdown are not moral crusades but key to advancing social justice.