the result is that wages are raised above market-clearing, creating involuntary unemployment. This creates a low, or no income alternative which makes job loss costly, and serves as a worker discipline device. Unemployed workers cannot bid for jobs by offering to work at lower wages, since if hired, it would be in the worker's interest to shirk on the job, and he has no credible way of promising not to do so. Shapiro and Stiglitz point out that their assumption that workers are identical (e.g. there is no stigma to having been fired) is a strong one – in practice reputation can work as an additional disciplining device. Conversely, higher wages and unemployment increase the cost of finding a new job after being laid off. So in the shirking model, higher wages are also a monetary incentive. The shirking model does not predict (counterfactually) that the bulk of the unemployed at any one time are those who are fired for shirking, because if the threat associated with being fired is effective, little or no shirking and sacking will occur. Instead the unemployed will consist of a (rotating) pool of individuals who have quit for personal reasons, are new entrants to the labour market, or who have been laid off for other reasons. Pareto optimality, with costly monitoring, will entail some unemployment, since unemployment plays a socially valuable role in creating work incentives. But the equilibrium unemployment rate will not be Pareto optimal, since firms do not take into account the social cost of the unemployment they helped to create. One criticism of this and other flavours of the efficiency wage hypothesis is that more sophisticated employment contracts can under certain conditions reduce or eliminate involuntary unemployment. Lazear (1979, 1981) demonstrates the use of seniority wages to solve the incentive problem, where initially workers are paid less than their marginal productivity, and as they work effectively over time within the firm, earnings increase until they exceed marginal productivity. The upward tilt in the age-earnings profile here provides the incentive to avoid shirking, and the present value of wages can fall to the market-clearing level, eliminating involuntary unemployment. Lazear and Moore (1984) find that the slope of earnings profiles is significantly affected by incentives. However, a significant criticism is that moral hazard would be shifted to employers, since they are responsible for monitoring the worker's effort. Employers do not want employees to