Since the late 1970s, the UK has become one of the most unequal countries in the developed world. As the rich have got richer, the rest of us have been left behind. Research released today by Oxfam shows that just five families now have as much wealth as the poorest 20 per cent of the population. It’s a frightening statistic, and for most people, one that offends their basic sense of decency. Can anyone really be "worth" this much? Can so many people be worth so little?

Worryingly, for some the answer appears to be "yes". For these people, an elite but small group of "wealth creators" are delivering jobs and driving economic growth. If five of these people happen to have the same wealth as a fifth of the population, well that’s just reward for their hard work and, no doubt, superior intellect. Perhaps even more concerning is the argument that inequality is not just acceptable, but desirable - driving the competitiveness vital to entrepreneurialism. But does seeing someone paid hundreds or even thousands of times more than you act as encouragement, or as a painful reminder of how little society values you?

The reality is that the consequences of the UK’s extraordinarily high levels of inequality are far-reaching and catastrophic. Of the developed OECD countries, the UK is ranked 17th out of 23 for life expectancy, 19th out of 22 on obesity, 17th out of 21 on teenage births, and 17th out of 23 for imprisonment. More equal societies, meanwhile, top the table on almost every measure.

Inequality shapes how we see others, our levels of trust in strangers, our sense of community. It erodes the bonds between individuals. But might it go even further? The IMF and others have pointed towards the damaging effects that inequality may have on economic growth. It makes perfect sense; if wages stagnate or fall for the majority of people, a consumer-led recovery becomes tricky to engineer without encouraging a huge growth in personal debt.

In addition, the impact of inequality on our health, wellbeing and crime rates may also have a financial cost. Research recently conducted by the Equality Trust has found that the impact on the UK of some of the social consequences of inequality, including reduced healthy life expectancy, worse mental health, higher levels of imprisonment and murder, could cost the equivalent of over £39bn every single year. If this was broken down to an individual level, it would show that the impact of inequality on every man, woman and child in the UK can be valued at £622.

Yet these figures may be just the tip of the iceberg. Many of the costs associated with inequality remain incalculable. For example, how does one value the higher level of community cohesion, trust, and social mobility associated with less unequal countries? Our estimate is based on a comparison between the level of inequality in the UK and the average level seen in developed countries. In other words, small changes to our level of income inequality would have a huge effect.

In recent weeks, the issue of inequality has gained significant coverage, it is now unquestionably part of the public and political narrative. But we need action to match the rhetoric. A living wage, a fairer tax system, and the creation of jobs with genuine opportunities for advancement should all be part of political party manifestos. But all parties serious about reducing costly social problems must also include an Inequality Test - an explicit goal that the net impact of their policies will be to reduce the gap between the richest and the rest. The benefits of reducing economic inequality are clear - a richer, healthier and quite possibly happier society. But we need politicians to have the courage and conviction to deliver real change.

Duncan Exley is the director of the Equality Trust