From Brad Delong (full article here):
The problem now is that the natural interest rate – that is, the liquid safe nominal interest rate on short-term US Treasury securities – is less than zero. Thus, the central bank cannot push the market interest rate there. Until something happens to raise the natural interest rate, we are stuck with a depressed economy.
Some blame a global savings glut for this state of affairs, and call for less thrift. But if we were at full employment, we would recognize that the world still has mammoth growth opportunities, and to sacrifice future growth for current well-being is a second-best choice.
Others blame a global investment shortfall driven by a lack of technological opportunities. But, given that this view is expressed in every deep depression, it appears to be an effect of economic stagnation, rather than a cause of it.
Still others say that the problem would resolve itself, at least in the US, if the target for annual inflation were raised from 2% to 5%, because a loss of so much of the real purchasing power that people hoard in cash would induce the needed boost in real investment. I think they are probably right, but former central bankers like Paul Volcker and Alan Greenspan would warn that a 5% inflation target is ultimately unsustainable. People can be happy with a stable 2% target (which is too low to notice), but 5% annual inflation would eventually become 10%, and 10% would eventually become 20%, and then the US would face another deep recession, like in 1982, or even more unpleasant alternatives.
Finally, according to a fourth group of economists, centered around Ricardo Caballero of MIT, the problem is a global shortage of safe assets. This view translates into policies aimed at better mobilizing society’s financial risk-bearing capacity, and that use the public sector to outwit the forces of time and ignorance that curb willingness to engage in risky investments.
So we have four theories, all advocated by smart, thoughtful, and hard-working economists. In a better world, sophisticated debate in a vibrant public sphere would inform economic policy. In the world as it is, we are all Japan in the early 1990’s, looking ahead to two or more decades of lost economic growth.