Both the United States and China are pursuing overly accommodative monetary policies, say Harvard historian Niall Ferguson and University of Bonn economist Moritz Schularick.

"Both economies suffer from a severe debt overhang and have become addicted to ultra-loose financial conditions," they write in The Wall Street Journal.

"Both know the medication cannot continue forever. But neither Washington nor Beijing knows how to reduce the dose, much less to quit."

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Each country took steps toward tightening in the summer, but pulled back when interest rates soared.

The two nations have adopted lines from an Amy Winehouse song as their mantra, Ferguson and Schularick say. "They tried to make me go to rehab. I said, 'no, no, no,'" the lyrics go.

"For some economists, the right time to tighten monetary policy is always never," the professors write.

"But there are dangers in procrastination. Having blinked once, both the Fed and the PBOC [People's Bank of China] may soon have a credibility problem. The longer they wait, the frothier asset markets become."

U.S. stocks and big city Chinese housing prices have risen about 20 percent this year, they note.

At this point, there is little evidence the Fed will soon take the professors' advice.

"If Beijing and Washington try to exit at the same time, the global economy could take a big hit," the pair conclude.

Goldman Sachs Chief Economist Jan Hatzius writes in a report obtained by Business Insider that the Fed will likely lower the threshold unemployment rate that would be a pre-condition for the central bank to raise interest rates to 6 percent from 6.5 percent currently.

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