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This paper develops a simple theory governing the relationship between useful energy consumption and national output that explains why the adjusted elasticities found by two economists turned out to be greater than one. The theory is quantified by reference to a wide range of countries covering the whole spectrum of economic development and is tested against the actual experience over 20 to 25 years of individual countries. Conformity to the theory was very close and the efficiency-in-use coefficients computed by economists were found to hold good for periods of 20 to 25 years. Briefly, the hypothesis states that as a country moves through the various stages of its economic development from primitive subsistence agriculture to the ultimate state of fully energy-dependent production, its useful energy elasticity steadily falls from a high value tending asymptotically to one. By itself this proposition is of little use to energy forecasters, because it simply substitutes the requirement to predict energy intensiveness of output for the requirement to predict energy consumption. If we can satisfy ourselves that real GDP per capita is a good measure of energy intensiveness, however, then the relationship derived above can be used as the basis for a mathematical model linking useful energy consumption per capita with GDP per capita.