Professors Wilkinson and Pickett crunch the numbers and show that the same relationship holds true for a range of social problems. Among rich countries, those that are more unequal appear to have more mental illness, infant mortality, obesity, high school dropouts, teenage births, homicides, and so on.
They find the same thing is true among the 50 American states. More unequal states, like Mississippi and Louisiana, do poorly by these social measures. More equal states, like New Hampshire and Minnesota, do far better.
So, it's not the ice hockey that's behind Moynihan's Law of Proximity to the Canadian Border, it's the inequality.
Now, it could be that the Rich Getting Richer is, overall, a bad thing. I don't know. But, the single most obvious quantitative example runs in the opposite direction.
Here's one historical experiment in the effects of the Rich Getting Richer that ought to be familiar to subscribers to the New York Times: homicides in New York City.
New York City always had a lot of income and wealth inequality, but it really took off with Wall Street's boom that started part way through 1982. The rich in New York City became unbelievably rich in the 1990s and 2000s. And what happened? Homicides in New York City hit 2,245 in 1990, then dropped 79 percent by 2009.