Published online by Cambridge University Press: 13 July 2009
The reproductive capacity of our species is far in excess of its needs. If all women used their reproductive potential to the full they would give birth to about 15 children on average. As averages of 6 or 7 were once sufficient to compensate for the high level of mortality, and higher numbers would have led to unacceptably high levels of population growth, that, presumably, is the level of fertility found in traditional European societies and in developing countries before the onset of fertility decline. In European societies, fertility decline began in the 19th century. It was a reaction to the disequilibrium that resulted from the long-term decline in mortality. However, a clear sequence cannot always be established. The assumption that prevailed in the 1950s and 1960s that this demographic transition, as it is customarily called, would lead to a new equilibrium appears not to have been well founded. Average family size has declined to such low levels that the age structure is seriously affected and the replacement of generations is no longer assured. A new disequilibrium has resulted. If this is a temporary phenomenon it is not an important issue. However, if it has to be assumed that future generations will also limit their reproduction to not more than 10% of their potential, the perspective is different. Even with continued high levels of immigration, population decline will then become inevitable. While some argue this will turn out to be beneficial, it is not without disadvantages.