The price of labor. In theory, wages ought to change so that the supply and demand in the labour market are always in equilibrium. In practice, wages are often sticky, especially in a downward direction: when demand for labour falls, wages do not fall. In this situation, the fall in demand results in higher involuntary unemployment. Trade unions may use collective bargaining to keep wages above the market-clearing rate. Furthermore, many governments impose a minimum wage that employers must pay. Firms may choose to pay above the equilibrium wage to increase the productivity of workers. Such so-called efficiency wages may make workers less likely to join another firm, so cutting the employer's hiring and training costs. They may encourage workers to do a better job. They may also attract a higher quality of worker than wages at the market-clearing rate; better workers may have a higher reservation wage (the lowest wage for which they are willing to work) than the market-clearing equilibrium. In recent years, employers have tried to reduce wage stickiness by increasing the proportion of pay that is linked to the performance of their firm. Thus if falling demand reduces the employer's profit the pay of its employees falls automatically, so it does not have to lay off as many workers as it otherwise would. Performance-related wages can also reduce agency costs by giving hired hands a stronger incentive to do a good job.