Does the left "get" family values? No. And if it means believing that two-parent families are the moral foundation of a good society, nor should it. A more progressive view is to recognise that many kinds of family structures can provide love, care and support. This is confirmed by research which shows that the quality of family relationships matters far more than whether there are two parents.

What has confused the picture and turned the matter into a political football is the very damaging effect on children of living in families in relative poverty - and the reality that so many single parents are poor. The UK, which has a high proportion of single-parent families, does badly in international rankings of child well-being; hence the worry that "broken families" cause antisocial behaviour, including the recent riots, and what David Cameron calls our "broken society".

However, in societies with similarly high percentages of single-parent families, such as the Scandinavian countries, levels of child well-being and social cohesion are high and levels of violence are low. The explanation is that these are far more equal countries, with strong welfare systems that keep the vast majority of single parents out of relative poverty. Good government makes a difference.

Inequality heightens status competition, increasing the stigma of poverty and making money even more important. So, those in less equal societies work longer hours, save less and are more likely to get into debt. The result is toxic to family life. Tensions increase as parents are tired, stressed and often out, or are unemployed and depressed, struggling on inadequate incomes. There is also some very plausible evidence that inequality increases divorce and the break-up of families.

Inequality has both direct and indirect effects on families, particularly for those at the bottom of the income-distribution ladder. Mental illness is much more common in less equal societies. Twenty-three per cent of adults in the UK experience some kind of mental illness each year. That means many families have one or more caregivers suffering from depression, anxiety and other emotional constraints on their ability to be parents. Parental depression is a powerful risk for children's developmental and behavioural problems.

Research has repeatedly shown that inequality is divisive and damages the social fabric. It weakens social cohesion, trust and the sense of community and increases crime and violence. Add to that the effect of the national unemploy­ment rate of 40 per cent for 16-to-17-year-olds. Then add the increased downward prejudice and the lack of educational qualifications, which bars most of the looters from upward social mobility and hope. Now, surround them with shops displaying all that seems to separate their lives from those of the celebrities. The results are hardly surprising.

More surprising is why very well-heeled MPs and bankers should have behaved in much the same way, with their expenses and their bonuses. Why did they, too, help themselves to as much as they thought they could get away with? If we're talking of moral leadership, they set a poor example.

Kate Pickett and Richard Wilkinson are the authors of "The Spirit Level: Why Equality Is Better for Everyone" (Penguin, £10.99)

Next: Marc Stears: Blaming everything on inequality is a cop-out.