Explanations of changes in asset prices as being due to exogenous changes in risk appetite, although arguably controversial, have been popular in the financial community and have also received some attention in attempts to account for recent financial crises. Operational versions of these explanations are based on the assumption that changes in asset prices can be decomposed into a part that can be attributed to changes in riskiness and a part attributable to changes in risk aversion, and that some quantitative measure can capture these effects in isolation.