Earnings of the average US workers with an undergraduate degree have not kept up with gains in productivity in recent decades, according to research by academics at MIT that challenges traditional explanations of why income inequality is rising.

The findings, which will be presented to the New America Foundation today, come amid widespread unease about the sluggish trend in middle class income growth, both in ab-solute terms and relative to the new superstar class of chief executives, hedge fund managers and otherfinanciers.