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At less than 2 percent, wage growth did not keep up with the 3.5 percent inflation rate in the past year, so typical workers who rely almost exclusively on wages for income would have lost considerable purchasing power without the tax cut.

The Council on Foreign Relations estimates that consumer spending fueled 91 percent of economic growth this year, but less than half of that spending was driven by the usual source - increased incomes. About a third of the spending was paid for by consumers dipping into their savings, and about a fifth was driven by the payroll-tax cut, the group estimated.

“U.S. consumers are maintaining surprisingly brisk personal consumption and retail spending despite low real wage growth,” said iShares Global Chief Investment Strategist Russ Koesterich. “While consumers can maintain spending by reducing saving for a while longer, it is not sustainable over the long term.”

With wages growing at the slowest rate since February 2004, Americans also have become heavily dependent on government transfer programs such as Social Security and unemployment benefits for their income, he said, including the benefits for the long-term unemployed that Congress coupled with the temporary tax-cut extension.

“Such transfer payments have accounted for approximately 60 percent of all income growth over the past 45 months and now constitute 20 percent of disposable income,” Mr. Koesterich said. “Any cutbacks in transfer payments could have a significant and immediate impact on consumers’ ability to sustain their relatively brisk spending.”