The Puzzle of the Replacement Ratio in the Context of Renewal Theory

The models Feldstein and Rothschild (1974) and Jorgenson (1974) adopted to highlight the nature of the replacement ratio were identical. Yet, even though the theorems they derived from them were complementary and reinforced each other, the authors reached diametrically opposite conclusions. Digging deeper into the controversy that erupted, it emerges that the staying power of the theorem, according to which replacement is a constant propor-tion of the outstanding capital stock, may be attributed to the following reasons. The discernible shift from realism to instrumentalism in the methodology of economics; Its operational advantages; The data that accumulated, thus facilitating research without having to compute capital stock series from scratch; The inertia of the status quo, which is sustained by the absence of a process to decide when a theorem is in conflict with experience and should be set aside, and lastly the lack of a model leading to a more useful theorem than the one un-der consideration. In this light it is concluded that the time has come for research efforts to be directed towards constructing and testing models in which the useful life of capital is deter-mined endogenously in the presence of embodied technological change.