Now a mounting body of economic research indicates that the rise in pay for company executives is a critical feature in the widening income gap.

The largest single chunk of the highest-income earners, it turns out, are executives and other managers in firms, according to a landmark analysis of tax returns by economists Jon Bakija, Adam Cole and Bradley T. Heim. These are not just executives from Wall Street, either, but from companies in even relatively mundane fields such as the milk business.

The top 0.1 percent of earners make about $1.7 million or more, including capital gains. Of those, 41 percent were executives, managers and supervisors at non-financial companies, according to the analysis, with nearly half of them deriving most of their income from their ownership in privately-held firms. An additional 18 percent were managers at financial firms or financial professionals at any sort of firm. In all, nearly 60 percent fell into one of those two categories.