This chapter has set out in detail the models which are employed below in order to analyse the labour market effects of changes in tax rates and in alterations in the tax structure. The fundamental mechanisms underlying the different approaches have been pointed out. Moreover, vital assumptions have been emphasised. By delineating the models which are used for the subsequent analyses, implicitly statements have also been made about topics or aspects which this study does not cover. For example, all workers and firms are identical ex ante. However, ex-post differences are allowed for, inter alia, if unemploy ment occurs or if some firms have to close down. These restrictions indicate areas of future research insofar as that the findings for homogeneous workers or firms yield an unambiguous proposal for changes in tax rates or the tax structure in order to promote employment. This is because it would be desir able for tax policy to know whether the predicted effects also hold in a world with ex-ante heterogeneity. Furthermore, the product market has not played a role. Therefore, repercussions from labour markets outcomes on product demand - and vice versa - are absent. 55 Moreover, neither the process of capital accumulation, be it physical or human capital, nor substitution pos sibilities between labour and capital in the firms' production function are taken into account. Finally, international competition is not modelled.

In an efficiency wage economy with variable profits, a shift from payroll to employment taxes will reduce unemployment if the tax level is held constant at the initial wage. However, unemployment will rise if firms are constrained to zero profits in the long run and if tax revenues are constant. This reversal of employment effects occurs because the shift in taxes reduces wages. This implies a budget deficit. Hence, taxes will have to be raised if revenues are held constant. If the firm's profits cannot change, the tax increase will cause some firms to close down and unemployment will rise. Thus, the predicted employment consequences of changes in the tax structure depend on assumptions about the time horizon and budget constraint.