or quality of a worker's effort—and systems of piece rates or commissions are impossible, there may be an incentive for him or her to "shirk" (do less work than agreed). The manager thus may pay an efficiency wage in order to create or increase the cost of job loss, which gives a sting to the threat of firing. This threat can be used to prevent shirking (or "moral hazard"). Minimizing turnover: By paying above-market wages, the worker's motivation to leave the job and look for a job elsewhere will be reduced. This strategy makes sense because it is often expensive to train replacement workers. Selection: If job performance depends on workers' ability and workers differ from each other in those terms, firms with higher wages will attract more able job-seekers, and this may make it profitable to offer wages that exceed the market clearing level. Sociological theories: Efficiency wages may result from traditions. Akerlof's theory (in very simple terms) involves higher wages encouraging high morale, which raises productivity. Nutritional theories: In developing countries, efficiency wages may allow workers to eat well enough to avoid illness and to be able to work harder and even more productively. The model of efficiency wages, largely based on shirking, developed by Carl Shapiro and Joseph E. Stiglitz has been particularly influential. Shirking The shirking model begins with the fact that complete contracts rarely (or never) exist in the real world. This implies that both parties to the contract have some discretion, but frequently, due to monitoring problems, it is the employee's side of the bargain which is subject to the most discretion. (Methods such as piece rates are often impracticable because monitoring is too costly or inaccurate; or they may be based on measures too imperfectly verifiable by workers, creating a moral hazard problem on the employer's side.) Thus the payment of a wage in excess of market-clearing may provide employees with cost-effective incentives to work rather than shirk. In the Shapiro and Stiglitz model, workers either work or shirk, and if they shirk they have a certain probability of being caught, with the penalty of being fired. Equilibrium then entails unemployment, because in order to create an opportunity cost to shirking, firms try to raise their wages above the market average (so that sacked workers face a probabilistic loss). But since all firms do this the market wage itself is pushed up, and