Introduction

This chapter is mainly concerned with migration; at the end, however, (§8.10) it also makes reference to certain data which are not strictly demographic but which have an important bearing upon population situations and developments.

Brief mention has been made in §3.26 above of migration as a residual factor in demographic movement. This is a complex subject but one which may not assume much importance in practice unless the net migratory streams have a significant influence on population size and distribution. It is complex because both an origin and a destination are involved, because of the difference between permanent and impermanent movements and because changes of geographic location can not only happen much oftener in life than for the biological elements of fertility and mortality but are also much more subject to social, political and economic influences than those elements are. Migration also has a personal aspect as well as an impersonal one. A consequence of these complexities is that many definitions of migration are possible, and the results of analysis depend very much on the definition adopted. An outcome of the volatility of migration is that it is difficult to record; the definition often has to be chosen according to what one can accurately measure. In all the circumstances, the types of occurrence rates and indexes used for measurement are best kept as simple as possible.

As with births, marriages and deaths, migratory movement varies in intensity from year to year and from season to season, and is associated with such factors as race, sex, age, occupation and geographical region. It may also vary in direction because of these factors.