The gap between the top and the bottom of the income scale in the US has been growing for decades.

A note from Morgan Stanley's Global Co-Head of Economics and Chief Asia Economist Chetan Ahya included a chart showing how real mean household incomes in the US changed between 1980 and 2015 for each income quintile. While incomes for the top 20% grew by about 60% over that time, the mean income for the bottom 20% has grown only around 10%.

Ahya noted, however, that recent strength in the US labor market has benefitted American workers across the income distribution, pointing out that "the bottom two income quintiles have posted their strongest income gains since the end of the 1990s."

Recent jobs reports from the Bureau of Labor Statistics have suggested that another measure of income growth, average hourly wage growth, has generally been improving over the last year or so, but the figure from last week's January jobs report showing 2.5% year-over-year growth came in lower than economists' expectations.

The note goes on to warn that an increase in protectionist policies in the US could threaten economic growth in the long run.