How Low Can Central Banks Go? JPMorgan Reckons Way, Way Lower

On that basis, they estimate if the ECB just focused on reserves equivalent to 2 percent of gross domestic product it could slice the rate it charges on bank deposits to minus 4.5 percent. That compares with minus 0.3 percent today and the minus 0.7 percent JPMorgan says it could reach by the middle of this year.

The Bank of Japan's lower bound on a similar basis may be minus 3.45 percent, while Sweden's is likely minus 3.27 percent, the economists said. Should they also go negative, the Fed could cut to minus 1.3 percent and the Bank of England to minus 2.69 percent in JPMorgan's view, reflecting how the ratio of reserves to assets is higher in their economies than elsewhere.

Concentrating on 25 percent of reserves would allow the ECB to cut to minus 4.64 percent and the Fed to minus 0.78 percent. Making no change to the current regime would allow Draghi to lop to minus 1.36 percent, they said.

Easing the fall is that the JPMorgan economists bet that banks are unlikely to be able to pass on the cost of the policy to borrowers, reducing potential repercussions. They also see limited pressure on bank profits or for a need to stash cash.