The American middle class, long seen as the economic backbone of the country, is shrinking and no longer constitutes a majority, according to a new study from the Pew Research Center.

After more than four decades of serving as the economic majority, the American middle class, defined as an income between $24,000 and $73,000 for a one-person household, "is now matched in number by those in the economic tiers above and below it," reported The Los Angeles Times, citing Pew. "In early 2015, 120.8 million adults were in middle-income households, compared with 121.3 million in lower- and upper-income households combined, a demographic shift that could signal a tipping point."

Thanks to factory closings and other economic factors such as the housing market crisis and the Great Recession of 2007-2009, the number of American adults living in middle-income households has fallen from 61 percent in 1971 to 50 percent in 2015. "The share living in the upper-income tier rose from 14% to 21% over the same period. Meanwhile, the share in the lower-income tier increased from 25% to 29%," Pew noted.

Not only is the middle class shrinking, but the "nation's aggregate household income has substantially shifted from middle-income to upper-income households, driven by the growing size of the upper-income tier and more rapid gains in income at the top," Pew said.

In 1970, 62 percent of U.S. aggregate income went to the middle class, but that fell to 43 percent in 2014. Over the same time period, the share of income held by upper-income households has increased from 29 percent to 49 percent.

Middle-income Americans have also "fallen further behind financially in the new century. In 2014, the median income of these households was 4% less than in 2000," and because of the financial crisis, "their median wealth (assets minus debts) fell by 28% from 2001 to 2013," Pew said.

Some analysts and policymakers say that the shift signals impending economic and social instability. However, in a counter-intuitive sense, the declining middle class actually represents economic progress, as the upper middle class is expanding and the ranks of the poor contracting, Forbes explained. Overall, the U.S. economy has been growing and we are all getting wealthier, according to Pew.

"Although the middle class has not kept pace with upper-income households, its median income, adjusted for household size, has risen over the long haul, increasing 34% since 1970," Pew said. "That is not as strong as the 47% increase in income for upper-income households, though it is greater than the 28% increase among lower-income households. Moreover, some demographic groups have fared better than others in moving up the income tiers, while some groups have slipped down the ladder. The groups making notable progress include older Americans, married couples and blacks."