Nearly 500 million people are living in countries with negative interest rates, according to S&P Global, a factor which could fuel a return to a "cash-only" society.
Negative interest rates are designed to get money flowing in an economy. In theory, the rate, set by a central bank, discourages savers from holding on to their money because of the negative return and encourages banks to lend.
Central banks in Japan, euro zone and several other European countries have introduced negative interest rates, helping push the global stock of sub-zero-yielding sovereign debt to over $11 trillion.
But rather than spend more, negative rates may force consumers to look to hold their cash outside of the official banking sector.