Résumé
The main theories of population ageing based on recent data on human longevity, life expectancy, morbidity changes, disability trends and fall in mortality show co-existing contradictory tendencies in disability and functioning. These contradictions reflect differences in geographic, cultural, socio-economic, political and medical contexts, for instance: • an increase in the survival rates of sick persons which would explain the expansion of morbidity and/or disability that is now taking place in Taiwan, • control of the progression of chronic diseases which would explain the subtle equilibrium between the fall in mortality and the increase in disability currently observed in the U. K., • an improvement in the health status and health behaviours of the new cohorts of old people which would explain the reduction in morbidity and/or disability now found in France, Switzerland and the U.S. Obviously, all of these co-exist, and future trend scenarios, namely the expansion or reduction in morbidity and/or disability, depend on their respective weighting leading to the need to anticipate the progressive but unavoidable emergence of very old and frail populations (which would explain a new increase in morbidity and/or disability). In this context, the proposed new theory of the ageing population is based on a cyclical movement, where, first, sicker people survive into old age and disability rises, then the number of years lived with disability decreases as new cohorts of healthier people enter old age but finally, the number of years lived with disability rises again, when the average age of death rises so much that many people spent their last years at an advanced age burdened by multiple chronic diseases and frailty. This theory needs to be confirmed by a worldwide harmonization of functional decline measurements and a periodic "international ageing survey" to monitor global ageing by means of a sample of carefully selected countries.