Summary

The term ‘plasticity’ is not used in demography but the concepts are, although in an analogous fashion and not in any direct physiological sense. Physiological effects upon individuals tend to be rather unimportant as demographic variables. This paper discusses two demographic phenomena relating to immigration which show obvious parallels with Lasker's concepts of ‘plasticity’. Populations can respond, in a plastic or other fashion, to changes in their equilibrium conditions in at least two sorts of ways. One is for individuals to change their behaviour in response to new circumstances. These changes, which in the aggregate may be described as transition or acculturation, may have ‘plastic’ qualities of irreversibility, although the process is psychological and psychosocial, not physiological. The other relates to population dynamic responses to changes in equilibrium.

The first example describes the responses of individual immigrants, measured by their population trends in fertility and interethnic marriage in the ‘host’ countries to which they have moved. In general, immigrants from high-fertility demographic regimes and from traditional closed societies do moderate their birth rates, although to an extent which varies considerably according to the society of origin. Changes between generations are typically faster than those within generations.

At a ‘macrodemographic’ level, populations respond slowly to changes in their underlying vital rates and to immigration. These changes are not truly plastic, being reversible, but they are slowed by the inertia built into population age structures and always tend to return to an equilibrium defined by the underlying vital rates. These population responses to changes in immigration and fertility are important because they define the principles behind the contemporary debate on the ageing of Western society following the reduction of birth rates, and the possible beneficial effects of higher levels of immigration.