The core of the discussion is that house prices and lending sometimes develop in a way that does not appear sustainable – in a way that entails risks that are difficult to quantify or to even capture in analyses and forecasts. These are risks of events that often have a low probability of occurring, but if they do occur could have very serious consequences for the economy. So what is so special about asset prices rising and why do “bubbles” arise? Rising asset prices are not necessarily a bubble… The prices of many assets, such as houses or shares, rise and fall in cycles. And asset prices can rise substantially without this entailing an over-estimation or a financial bubble. A  BIS Review 70/2010  1  bubble is among other things based on speculation and a strongly over-optimistic view of the future. If, for instance, property prices increase substantially this may actually be justified on the basis of fundamental driving forces and realistic views of the future. Factors such as growth, incomes, construction and housing costs may develop favourably. And if one does not, for instance, build new housing at the same rate as the demand for housing increases, then it is not exactly surprising that the price of the existing housing should rise. Even if prices soar during a brief period, grey reality usually catches up before a bubble has arisen. When this happens prices usually adjust without any great drama and do not have any major effects on the rest of the economy.
However, if unrealistic expectations of the future push up prices on a more lasting basis it can lead to serious problems. …but if they are pushed up by unrealistic expectations and lending… Expectations of the future, “mood” and attitude to risk are very important factors with regard to the prices of most assets. During the course of history investors have from time to time behaved as though prices on a particular market would keep on rising for ever. Exaggeratedly optimistic expectations of the future may entice investors into a particular market, prices will then rise, which will attract even more investors in the belief that prices will go on rising. Unless the spiral is broken reasonably early, a bubble will arise. If exaggerated optimism and a high risk propensity over a long period of time have pushed up prices, there is also a considerable risk that the fall from the inflated price level will be very hard once the mood changes. And this will probably have a negative effect on the rest of the economy. It is probably necessary to be most on our guard regarding what developments can be considered sustainable when things appear to be going well. By this I mean times with a high level of growth in the economy and in household incomes, good investment opportunities and low loan losses in the banks. It has happened more than once that a normal economic upturn inspires excessively optimistic expectations of economic developments and the price of risk thus becomes unreasonably low.