Instead, study co-authors Richard Dobbs and Susan Lund found that the biggest impact of quantitative easing by the world's major central banks has been the cost savings delivered to governments. Since 2007, bond-buying programs in the United States, the U.K. and the eurozone have reduced costs for governments by a total of $1.6 trillion.

The finding will come as a surprise to many investors who attribute the rise in stock prices in the United States and elsewhere since the 2007-09 financial crisis at least in part to easy central bank policies.

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All told, major central banks have added $4.7 trillion to their balance sheets over the past five years in an effort to push down long-term borrowing costs while keeping short-term interest rates low.

The findings are sure to resonate among central bankers as they debate when and how fast they may be able to scale down the monetary stimulus they have used to keep deflation at bay and try and pull ravaged economies from the depths of recession.

Higher stock prices are often thought to boost household spending because of the so-called wealth effect — as net worth rises with gains in equity portfolios, people part more freely with their cash.

But quantitative easing does not appear to be driving this kind of wealth effect through rising equity prices, the McKinsey researchers found.

The major boost comes from massive savings to governments that have freed them to borrow and spend more than they otherwise would have, translating to fewer jobs lost than otherwise, the researchers found.

"In practice, the biggest beneficiary has not been Main Street or Wall Street, but it has been the government," Dobbs said in an interview.

That outsized benefit suggests that when the time comes for central banks to reduce quantitative easing, the biggest risk is from rising costs to governments as interest rates increase, and the possibility that governments would then be forced to cut spending.

Already central bankers in the United States and elsewhere have blamed overly austere fiscal policies for hindering what might otherwise be brisker recoveries.

"The most obvious thing that central banks are worried about is, what will be the impact on government finances, and will the tax increases that come from a better economy be enough to offset the rising interest rates — or will we see government being squeezed into more austerity?" Dobbs said.

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