This is what they found in the research they carried out for their book:
'People in more equal societies live longer, have better mental health and have better chances for a good education regardless of their background. Community life is stronger where the income gap is narrower, children do better at school and they are less likely to become teenage parents. When inequaility is reduced people trust each other more, there is less violence and rates of imprisonment are lower.'
Their website has a section called The Young Person's Guide to Inequality and you can find it here