Negative interest rates bring additional loosening of monetary policy which can improve the macroeconomic outlook.
Improved prospects for the real economy should reduce the amount of impaired loans, boosting bank's profitability.
The resulting increase should be enough to offset the impact from reduced interest income.
But the flip side of declining interest could also be bank’s reducing lending, particularly if they are worried about maintaining their capital ratios.
When activity in the economy is falling, and unemployment is rising, losses on banks’ existing loans are likely to pick up.
This means their balance sheets - and capital ratios - might be less healthy.
As a result, implementing negative policy rates might be less effective in providing stimulus to the economy.
7. Fight deflation but this creates problems for savers
In economic downturns people typically hold onto their money and wait to see an improvement before they ramp up spending again. As a result, deflation can become entrenched in the economy.
People stop spending, demand declines, prices for goods and services fall, and people wait for even lower prices before spending.
Negative rates fight deflation by making it more costly to hold onto money, incentivising spending.
Theoretically, negative interest rates would make it less appealing to keep cash in the bank.
But the big problem is instead of earning interest on savings, depositors could be charged a holding fee by the bank.
This is a particular issue in the UK where so much money is held in building societies like Nationwide.
The worry is that savers will take their money out of the banks and put it under the bed.
While there are some costs and difficulties with holding cash, such as storage costs and the practicalities of paying bills and receiving wages, the incentive to use it as an alternative to a deposit account is likely to increase if interest rates fall below zero.
Of course not all banks will pass on the negative rate to consumers but this can be difficult for some small banks and building societies.