Ever wonder if there was a good argument against widening disparities in income and wealth?  Look no farther:  the UK’s Equality Trust has the goods.They’ve collected evidence from around the globe demonstrating that income inequality is correlated with all sorts of social ills—from mental and physical illness, to poor educational achievement, to low levels of public trust.

Take a look, for example, at the trends in imprisonment.  (Click on the chart to for a more legible version.) The US is certainly an outlier, putting far more people in prison than you’d expect, even given its relatively high levels of inequality.  Greece is an outlier on the low end.  But the general trend is clear:  the more unequal a nation’s distribution of income, the greater the share of its population that nation puts behind bars.

Mind you, these aren’t comparisons based on absolute income, but on income inequality.  Nations with high average incomes, but wide gaps between the rich and everyone else, can still perform poorly on a host of social metrics.  That’s not to say that average income is irrelevant; but apparently, among relatively well-off nations, a sense of equity and fairness is far more important to people’s well-being than having a little extra spending money.

Why is there such a strong link between inequality and social malaise?  Well, I’m not sure I have an answer.  Perhaps it has something to do with the fact that we’re primates—and we’re wired to be on guard against perceived unfairness, and are especially prone to stress-related ailments. But if you’re curious whether there’s a fuller explanation, and you happen to be in Seattle, you should head to Town Hall on Friday night:  Richard Wilkinson and Kate Pickett, two of the founders of the Equality Trust, will be in town promoting their new book—The Spirit Level: Why Greater Equality Makes Societies Stronger.