FRANS VAN POPPEL AND JOOP DE BEER
Success can have negative repercussions. Until very recently, demographers emphasized time and again that the popular press and some social science literature overplayed the importance of mortality decline as the cause of the increase in the older population. Their efforts to convince the public that the responsibility for the ageing of the population could not be attributed to the increase in life expectancy have been so succesful that, nowadays, common sense tends to neglect the real effects that changes in mortality can have on population ageing ( Crimmins, 1986: 193; Caselli and Vallin, 1990: 2).
In the more recent past, however, a more balanced situation has arisen. It is now realized that the demographic analyses, on which the statements on the influence of the mortality decline on the ageing of the population were based, were selective in nature and did not relate to levels of mortality that are relevant for current conditions in Western societies. As a consequence, demographers began to pay much more attention to the role of mortality in past and future population changes.
The mortality patterns of the old attracted particular attention. That had, among other things, to do with the fact that the socio-economic consequences of population ageing had become a central area of research. It was realized that the size of the oldest old population depended in large part on the trend of mortality. Demographers also drew attention to the fact that mortality changes in industrialized countries appeared to be less smooth and gradual than had been expected. It had also been proposed that a reduction in the risk of death at high ages could expose the survivors to an increase in the number of years spent in a state of frail health ( Olshansky, 1988). As a consequence of this, the former notion that mortality was the least problematic of the three components of population change--because it tended to change only slowly and relatively evenly, and because the errors in the projected populations caused by incorrect mortality assumptions were small compared with errors caused by wrong fertility or migration assumptions--has been abandoned. Providing reasonable forecasts of mortality has become a central issue in population forecasting. Many studies have been published dealing with mor-