Our landmark book revealed the cost of inequality. Fifteen years later, things have only got worse | Richard Wilkinson and Kate Pickett

When we first wrote The Spirit Level: Why Equality is Better For Everyone , we had no idea it would become a bestseller. The book was filled with graphs and statistics, for one thing. But in the 15 years since it was published, we’ve been invited to give well over 1,000 lectures and conferences to tell people about our research. Our findings were clear then and are still true now. When economic inequality gets worse, so does our health and wellbeing. Inequality can affect a society’s death rates, its levels of chronic disease, and the amount of violence (including murders) it experiences. What we weren’t prepared for when we first wrote the book was how much worse things could get.

Today, inequality seeps into almost every facet of life. Children in the most unequal societies are more likely to experience bullying, do less well in school, and have fewer opportunities for social mobility. Research since the book came out has added gambling , domestic violence and the mistreatment of children to the list of problems all correlated to higher levels of inequality. We know that people in more economically unequal societies become more status-obsessed and consumerist , which fuels environmental breakdown. At the same time, social cohesion – the likelihood of people participating in society through things such as neighbourhood associations, political parties, volunteering and clubs – is lower where inequality is worse.

Crucially, since publishing The Spirit Level , we have used established epidemiological criteria to show that these associations are not simply correlations: inequality actually causes poorer outcomes . And our data show that even small differences in inequality matter: marginally reducing inequalitycan have a big impact on people’s health and wellbeing. For instance, if the UK were to reduce its inequality to the average for Organisation for Economic Co-operation and Development countries, our imprisonment rate could be reduced by as much as 25%, saving £1.6bn per year. Although the biggest effects are on the poor, inequality affects a large majority of the population. For example, in more unequal countries, even the better off have worse health than they would if they lived in a more equal society.

The Spirit Level was published during the final months of the last Labour government, and although David Cameron name-checked the book and its findings during his election campaign, his party’s economic policies helped exacerbate inequality. Fourteen years of austerity and cuts to the social safety net, combined with a pandemic, soaring rents and rising energy bills, have all helped to increase poverty and deprivation at the bottom of society – while those at the top have watched their wealth grow. The gap between rich and poor in Britain is now as high as it was in the early 1930s .

In addition, people, and especially young people , are experiencing substantially higher levels of mental distress and lower wellbeing than they were the last time we had a Labour government. Part of the reason is because inequality puts us in a hierarchy, one above the other. It reinforces the idea that some people are worth much more than others, strengthening what psychologists call the “social-evaluative threat”, where people worry more about how they’re seen and judged. This increases status anxiety and exacerbates issues of self-doubt at all levels in society. Income and wealth become the overwhelmingly important measures of status, so that making money and buying things become even more important to people. As a result, some are overcome with a sense of inadequacy and depression, while others respond with narcissism.

This week, we’re presenting the Labour government with a new, updated set of Spirit Level analyses . In an index of health, social and environmental performance, using robust sources of publicly available data, we augment the 10 health and social problems we considered in our book (infant mortality, life expectancy, mental illness, obesity, educational attainment, teenage births, homicides, imprisonment, social mobility and trust) with five environmental issues (the rich’s carbon emissions, recycling, air pollution, progress towards the UN’s Sustainable Development Goals and multilateralism ). There is a clear trend, with more unequal societies having significantly worse scores. The UK is still the most unequal country in western Europe, and ranks 18 out of 22 among the rich market democracies in our index. It does particularly badly on child wellbeing, infant mortality and social mobility. All these are strongly correlated to inequality, which poses a serious obstacle to Labour’s mission of breaking down barriers to opportunity. Our analysis also helps to explain why our prisons have become so overcrowded : the penal system becomes more punitive in more unequal societies, and puts more people behind bars.

So what should we do about this? There are plenty of policies that would help make the UK more equal. The government should implement the socioeconomic duty of the 2010 Equality Act , which requires public bodies to consider how their policies can improve outcomes for people who face social and economic inequalities such as poverty and deprivation. It could end the two-child benefits cap and the bedroom tax , impose wealth taxes , raise taxes on capital gains and unearned income and introduce a proportional property tax . Some people will argue that doing so is unfeasible because it will drive the rich away. But not only does research from the London School of Economics suggests that this is unlikely to happen , but the real truth is that we can’t afford the super-rich. They make us all poorer, and refusing to tax them properly only makes public services more expensive for everyone else.

Indeed, the Equality Trust has estimated that if inequality was reduced to the average levels in other rich nations, Britain could save more than £100bn a year on health and imprisonment alone. Alongside his huge parliamentary majority, Keir Starmer should be emboldened by the popularity of these policies. More than half of voters at the general election cast their votes for progressive parties that championed change, fairness and hope. It is time for an ambitious and overarching mission to focus policymaking squarely on creating a more equal society. It’s what people want , and our evidence suggests that the benefits would be transformative.

• Richard Wilkinson is emeritus professor of public health at the University of Nottingham. Kate Pickett is professor of epidemiology at the University of York

• The Spirit Level by Kate Pickett and Richard Wilkinson (Penguin Books Ltd, £10.99). To support The Guardian and Observer, order your copy at guardianbookshop.com . Delivery charges may apply.