ABSTRACT
I am grateful to Mary Morgan, Frank den Butter and an anonymous referee for the constructive and helpful comments on earlier versions of this chapter. The chapter has also benefited from discussions with the participants at the Tenth Anniversary Conference of the Tinbergen Institute, 14-16 May 1997 and I thank the participants, particularly Ken Wallis and Simon Wren-Lewis, for the good humour and patience with which they endured what must have seemed very naïve questioning. Any naiveté which remains is of course my own fault.
