Widening wealth gap between Americans slowed the 5-year-old recovery: Standard & Poor

An American in the top 1 percent of earners had an average income of $1.3 million in 2012, the most recent year for which data are available. Average income jumps to $30.8 million for the top 0.01 percent.

Adjusted for inflation, the top 0.01 percent's average earnings have jumped by a factor of seven since 1913. For the bottom 90 percent of Americans, average incomes after inflation have grown by a factor of just three since 1917 and have declined for the past 13 years.

Economists have long argued that a rising wealth gap has complicated the U.S. rebound from the Great Recession.

Now, an analysis by the rating agency Standard & Poor's lends its weight to the argument: The widening gap between the wealthiest Americans and everyone else has made the economy more prone to boom-bust cycles and slowed the 5-year-old recovery from the recession.

Economists have long argued that a rising wealth gap has complicated the U.S. rebound from the Great Recession.

Now, an analysis by the rating agency Standard & Poor's lends its weight to the argument: The widening gap between the wealthiest Americans and everyone else has made the economy more prone to boom-bust cycles and slowed the 5-year-old recovery from the recession.

Read MoreState governments may be expanding wealth gap

Economic disparities appear to be reaching extremes that "need to be watched because they're damaging to growth," said Beth Ann Bovino, chief U.S. economist at S&P.

Tom Williams | CQ Roll Call | Getty Images

Terri Henderson, 6, center, whose mother is El Salvador, attends a rally with members of Congress at Union Station's Columbus Circle to announce the Restore Opportunity, Strengthen, and Improve the Economy (ROSIE) Act, July 29, 2014. The legislation provides incentives for government contractors to pay a living wage and other benefits that would help low-income workers.

The rising concentration of income among the top 1 percent of earners has contributed to S&P's cutting its growth estimates for the economy. In part because of the disparity, it estimates that the economy will grow at a 2.5 percent annual pace in the next decade, down from a forecast five years ago of a 2.8 percent rate.

The S&P report advises against using the tax code to try to narrow the gap. Instead, it suggests that greater access to education would help ease wealth disparities.

The report builds on data from the Congressional Budget Office, theInternational Monetary Fund and academic economists to explain how income disparities can hurt growth. Many consumers tend to become more dependent on debt to continue spending, thereby worsening the boom-bust cycle. Or they curb their spending, and growth improves only modestly, as it has during the current recovery.