One of the earliest postwar institutionalists to attempt to explain inter-industry wage differentials based on real economic conditions was John Dunlop. In 1948, he proposed four reasons for their existence: unequal productivity levels; the proportion of labor to total costs; the relative degree or absence of competitive product market pressures; and an industry's changing skill and occupational composition of the workforce. Similar institutionalist studies soon followed. Though their points of emphasis varied, all those studies significantly departed from the prevailing neoclassical theory of perfect competition tied to marginal productivity.
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