Americans are about as likely to move from one income quintile to another as they used to be. That, put as prosaically as possible, was the big economic news of the week, as the epic income-mobility study led by Harvard’s Raj Chetty and Nathaniel Hendren, UC Berkeley’s Patrick Kline and Emmanuel Saez, and the U.S. Treasury Department’s Nicholas Turner generated another data-rich installment.

Yet when it comes to the income distribution in the U.S., quintiles are so 1970s. All the really interesting things over the past few decades have been happening in the top 20%. Consider this chart, which shows the share of aggregate income going to each of the bottom four income quintiles, to those between the 80th and 95th income percentiles, and to the top 5%.

The lines for the bottom four quintiles — 80% of American households — are pretty much parallel, showing almost no change in their relative positions. If you just look at the bottom 80% of the income distribution, then, there’s been no significant increase in income inequality. You do see a steady decline in the share of national income going to the bottom 80%, but in absolute terms, incomes for this group are up modestly over the same period.

Things look a lot different up in the top 20% of the income distribution, the part that’s been getting a rising share of aggregate income. The bottom three-quarters of this quintile (those between the 80th and 95th income percentiles) have grabbed some of that, but the really big gains have gone to the top 5%.

And of course it doesn’t stop there. The Census Bureau survey data used in the above chart doesn’t get more granular than the top 5%. But the aforementioned Emanuel Saez, together with Thomas Piketty of the Paris School of Economics, has for the past decade-plus been using income tax records to compile a rich account of what’s been going on up there in the top 1%. You’re probably familiar with the basic outlines, but it’s worth throwing out a few numbers from their most recent update:

From 1993 to 2013, incomes of the bottom 99% of taxpayers in the U.S. grew 6.6%, adjusted for inflation. The incomes of the top 1% grew 86.1%.

The top 0.1% of U.S. taxpayers claimed 11.33% of overall income in 2012, up from 2.65% in 1978. The top 0.01% got 5.47%, up from 0.86% in 1978.

The average income of the top 0.01% was 859 times that of the bottom 90% in 2012. In 1973 the top-0.01%-to-bottom-90% ratio was just under 80.

Something really dramatic is going on up there in the top 5%, the top 1%, the top 0.01%. But while economists know some things about the impact of increasing overall income inequality, they still don’t know all that much about what this 1% stuff means. In their new paper, Chetty, Hendren, Kline, Saez, and Turner write that their finding of steady intergenerational income mobility “may be surprising in light of the well-known negative correlation between inequality and mobility across countries.” A possible explanation, they continue, is that

[M]uch of the increase in inequality has been driven by the extreme upper tail … [and] there is little or no correlation between mobility and extreme upper tail inequality — as measured e.g. by top 1% income shares — both across countries and across areas within the U.S. Instead, the correlation between inequality and mobility is driven primarily by “middle class” inequality.

That’s the thing about this rise in “extreme upper tail inequality” — most pronounced in the U.S. but by now a clearly global phenomenon. It is one of the most dramatic economic developments of the past quarter century. And it seems like it might be bad thing. But conclusive economic evidence for its badness is hard to find.

Yes, there are theories: All that wealth sloshing around in the top 1% leads to more bubbles and crashes. Extreme wealth corrupts the political process. Income inequality may be slowing overall economic growth. And, as my colleague Walter Frick put it in an email when I brought this up, “given the diminishing marginal utility of income, it’s hugely wasteful for the super rich to have so much income.”

I happen to believe there’s some truth to all four of those. But there are also lots of counterarguments and some counterevidence, and big economic studies like the new one by Chetty & Co. don’t seem to be doing much to resolve the debate.

Which leads me to another theory: I think we’re eventually going to have to figure out what if anything to do about exploding high-end incomes without clear guidance from the economists. This is a discussion where political and moral considerations may end up predominating. And as Harvard’s Greg Mankiw made clear in his maddeningly inconclusive Journal of Economic Perspectives essay on inequality last summer, these are areas in which economists possess no comparative advantage.