It is not saying anything new to note that this is a significant source of uncertainty for policy-makers: there is no historical dataset to draw on in estimating how households might respond to a macroeconomic downturn under conditions of  17  As others have pointed out, though, improvements in these various areas do not make for continuing reductions in risk premia and rises in asset valuations – only a one-time adjustment.
It is important that people understand that point, rather than just extrapolating trends. See Paul Tucker (2006), ‘Uncertainty, the Implementation of Monetary Policy and the Management of Risk’ (available at http://www.bankofengland.co.uk/publications/news/2006/056.htm).