A possible criticism is that workers do not necessarily view high wages as gifts, but as merely fair (particularly since typically 80% or more of workers consider themselves to be in the top quarter of productivity), in which case they will not reciprocate with high effort. Akerlof and Yellen (1990), responding to these criticisms and building on work from psychology, sociology, and personnel management, introduce "the fair wage-effort hypothesis", which states that workers form a notion of the fair wage, and if the actual wage is lower, withdraw effort in proportion, so that, depending on the wage-effort elasticity and the costs to the firm of shirking, the fair wage may form a key part of the wage bargain. This provides an explanation of persistent evidence of consistent wage differentials across industries (e.g. Slichter 1950; Dickens and Katz 1986; Krueger and Summers 1988): if firms must pay high wages to some groups of workers – perhaps because they are in short supply or for other efficiency-wage reasons such as shirking – then demands for fairness will lead to a compression of the pay scale, and wages for other groups within the firm will be higher than in other industries or firms. The union threat model is one of several explanations for industry wage differentials. This Keynesian economics model looks at the role of unions in wage determination. The degree in which union wages exceed non-union member wages is known as union wage premium and some firms seek to prevent unionization in the first instances. Varying costs of union avoidance across sectors will lead some firms to offer supracompetitive wages as pay premiums to workers in exchange for their avoiding unionization. Under the union threat model (Dickens 1986), the ease with which an industry can defeat a union drive has a negative relationship with its wage differential. In other words, inter-industry wage variability should be low where the threat of unionization is low. Empirical literature Raff and Summers (1987) conduct a case study on Henry Ford’s introduction of the five dollar day in 1914. Their conclusion is that the Ford experience supports efficiency wage interpretations. Ford’s decision to increase wages so dramatically (doubling for most workers) is most plausibly portrayed as the consequence of efficiency wage considerations, with the structure being consistent, evidence of substantial queues for Ford jobs, and significant increases in productivity and profits at Ford. Concerns such as high