This paper sketches a theory of the secular decline in morbidity and mortality that takes account of changes in human physiology since 1700.The synergism between technological and physiological improvements has produced a form of human evolution, much more rapid than natural selection, which is still ongoing in both OECD and developing countries.Thermodynamic and physiological aspects of economic growth are defined and their impact on growth rates is assessed.Implications of this theory for population forecasting, measurement of national income, demand for leisure, pension policies, and for the demand for health care are considered.