The first implication, that the central bank has a strong stake in preventing financial instability, has several elements. The central bank needs to be willing to respond to limit financial market bubbles from developing in the first place. This includes not just paying attention to asset price bubbles, but also to related excesses in leverage and in short-term funding markets. As I noted in a speech a few years ago, this is difficult to do in practice.3 After all, bubbles are difficult to identify in real time and the central bank’s policy toolkit to deal with bubbles may be limited. However, this difficulty cannot be an excuse for inaction. Using the bully pulpit, implementing macroprudential measures, or adjusting monetary policy can generate superior results compared to inaction. The central bank also has to ensure that the financial system is sufficiently robust and resilient so that if there are financial market shocks, the banking system can absorb any shocks and continue to perform its credit intermediation function. This has many elements including tough capital and liquidity requirements for banks and strong financial market infrastructures. It also includes taking action to eliminate sources of vulnerabilities whenever they may arise – for example, strengthening the triparty repo system, reducing the vulnerability of money market mutual funds to runs, or forcing derivatives contracts to be standardized and centrally cleared. These actions should be undertaken during “peacetime” rather than when a crisis is already underway.
The result can be disruption to the financial intermediation function with resulting constraints on the availability of credit for households and businesses. This, in turn, can lead to further reductions in aggregate demand that put additional stress on the weakened financial system. Obviously, this is not a favorable dynamic. Financial instability can impact the conduct of monetary policy via three major channels. First, financial instability can generate a sufficiently large shock to aggregate demand that the central bank may encounter the zero lower bound constraint – the constraint that the monetary policy instrument, for example, the federal funds rate – cannot easily be pushed below zero. In such circumstances, it may not be easy to fully offset the shock through the pursuit of a more stimulative monetary policy. At the zero bound, the central bank is not powerless, and may turn to other monetary policy tools such as forward guidance and large scale asset purchases. But these tools may not be as effective as lowering the short-term rate instrument. In particular, the central bank may not be willing to use these nonconventional tools to the full extent necessary to provide the same degree of stimulus as it would provide if it could set interest rates at negative levels. That might be because of uncertainty about how nonconventional tools will work or because of the potential costs associated with the use of such tools in terms of market functioning and the risks of future financial instability.