SAN FRANCISCO, Dec. 20 (Xinhua) — Americans in the bottom half of the country's income distribution have seen their income stalled since 1980, while those on the upper rungs enjoy income growth, according to new research.

The research, in the form of a working paper, was posted Tuesday by the National Bureau of Economic Growth, a private nonprofit organization in the United States committed to undertaking and disseminating economic research among public policymakers, business professionals and the academic community.

Based on tax, survey and national accounts data in the United States, economists Emmanuel Saez and Gabriel Zucman of the University of California, Berkeley, and Thomas Piketty of the Paris School of Economics in France have found that there has been close to zero growth for working-age adults in the bottom 50 percent of the distribution since 1980.

Their other findings include:

— The average pre-tax income of adults in the bottom 50 percent income share in the U.S. has stagnated since 1980;

— The bottom 50 percent's share of income share has collapsed from about 20 percent in 1980 to 12 percent in 2014;

— Between the early 1980s and 2014, the average pre-tax income of the top 1 percent of adults rose from 420,000 U.S. dollars to about 1.3 million dollars;

— In 1980, the top 1 percent adults earned on average 27 times more than bottom 50 percent adults, while they earn 81 times more today;

— The share of women falls sharply as one moves up the income ladder, with only 11 percent in the top 1 percent today.

— In the bottom half of the distribution range, only income for the elderly is rising, while income has fallen for those under the age of 65.

"The top 1 percent income share is now almost twice as large as the bottom 50 percent share, a group that is by definition 50 times more numerous," write Saez, Zucman and Piketty, who are co-directors of the World Wealth and Income Database.