Evidence of the persistence of misperceptions can be seen in the imbalances in the world, and especially the European, economies. I do not mean to imply that when economic agents make these mistakes they are behaving irrationally. Rather that in a world of intrinsic uncertainty it is far from obvious how to make decisions. The assumption of rational expectations is very helpful for economists when trying to understand the implications of their own models – it is a discipline to prevent the drawing of arbitrary conclusions. In practice, however, households are on their own in a highly uncertain and complex world where they are learning from experience. When it comes to decisions about how much to spend and how much to save, expectations of future incomes are crucial. In the absence of a complete set of markets for future consumption goods – and labour – there is no mechanism to ensure that decisions today, and so the implied plans for tomorrow, will be consistent with the possibilities available in the future. If revisions to expectations of future incomes are uncorrelated across households, then aggregate spending will be relatively stable. The problem comes when many households have similarly over-optimistic views about the future. Aggregate spending and borrowing can then be unsustainably high and lead to an inevitable correction at an unpredictable date when reality dawns. Financial markets both reflect and propagate that common degree of optimism. Sentiment and animal spirits can change very quickly.
Although there is a, by now extensive, literature on financial frictions, including attempts to incorporate them in New Keynesian models, it turns out that such extensions make little difference to the propagation of shocks, to optimal policy, or to the quantitative conclusion that overwhelmingly the most important objective remains inflation stabilisation. 12 There is no doubt that financial frictions such as asymmetric information, credit constraints, and costly monitoring of borrowers, to name but a few, are an important part of the story of how crises happen and why they impact on output. But those models do not provide a convincing account of the gradual build-up of debt, leverage and fragility that characterises the run-up to financial crises. 13  8  This specification of the objective function can be derived as an approximation to the maximisation of the welfare, defined over consumption and leisure, of a representative consumer with an infinite horizon (see Rotemberg and Woodford, 1997). 9  The frontier is named after the US economist John Taylor. 10  To implement such a policy reaction function requires an empirical judgement about the factors that drive the volatility of both inflation and output. In principle, these should include those factors in the banking and financial system, including movements in asset prices, that generate fluctuations in demand and output and affect the way in which shocks are transmitted through the economy. In practice, however, rather little attention was paid to the role of the banking system in determining inflation and output. 11  Brazier et al.