Several recent studies of migration have emphasized the importance of analyz- ing the flow patterns of return migrants, pointing to the not-surprising empirical fact that the migration rates of people returning to their region of birth are significantly higher than average (Ledent 1980, Lee 1974, Long and Hansen 1975, Miller 1977).
* Appendix A contains the Rogers and Castro data; Appendix C presents the Ledent data.
T A B L E 1 P R P B distribution at stability of national and regional female population of the USA ( r = 0.004361).
Resident in South Resident in North
Born in South Born in North Born in North Born in South
In the next section we follow this advice and introduce higher transition probabilities for return migrants in the multistate projection model. W e shall call the outputs of such models native-dependent projections. In this section, however, we treat first the simpler case of native-independent projections, i.e., projections carried out with models assuming that all of the individuals in a regional population experience identical age-specific probabilities of moving, dying, and bearing offspring.*
3.1 Fertility
In projecting a multistate population forward over time, we shall at times refer t o people by where they live and at other times by where they were born. This poses n o difficulties when we a r e dealing with survivors of a current population; it simply becomes a matter of keeping track of individuals born in each region. It is the births of new individuals that need t o b e examined, because the babies may be born in the region of residence of their parents at the start o r at the end of the unit interval of time, and they themselves may migrate during the same interval into yet another region.
In the conventional multistate projection model, some of the babies born in a given region during a unit time interval ( t , t
+
1 ) may be living in another region at the end of that interval. Consequently, at time t+
1 these babies can be distinguished both by their place of residence, j, and by their place of birth, i. Moreover, they may also b e classified by the region of residence, say k, of their parent at the start of the time interval, because each regional population of parents is a potential contributor of babies t o each PRPB-specific category of babies. For example, in o u r two-region illustration based o n U S A data, we distinguish four categories of babies for parents initially resident in each region. Figure 1 shows the four categories corresponding t o parents initially resident in the South; there are of course four equivalent categories for babies born t o parents initially resident in the North.* Because of the unavailability of the necessary fertility and mortality data, we are unable to introduce native-dependency in birth and death rates.
Dimiter PhiEpoo and Andrei Rogers
Time t
Region of residence of parent
Time (t, t + 1 ) Region of birth of baby
Time ( t + 1 ) Region of residence of baby
FIGURE 1 The four categories of babies born to parents resident in the South at time r.
Let
denote the average number of babies born during the five-year time interval ( t , t
+
1 )in region i and alive in region j at time t
+
1, for every individual between the ages of x and x + 4 living in region k at time t. Summing over all birthplaces i gives the conventional multiregional birth rate (Rogers 1975, p. 121)where
Fh (x) is the annual birth rate of people aged x to x + 4 residing in region h hoL,(O) is the total number of person-years lived between ages 0 and 5 in region j,
per person born in region h
skh(x) is the proportion of people living in region k and aged x to x + 4 that survive to be in region h and aged x
+
5 to x+
9, five years laterlh(0) is the radix of region h (set equal to unity in our calculations) m is the total number of regions
Since, by definition
it is easy to develop computational formulas for bLj(x) by taking the appropriate components from eqn. (2). For our two-region (South-North) example, this gives four equations of the form
for (k,
i)
= (n, n), (s, s), (n, s), (s, n)for parents in two regions who do not migrate between time t and the birth of the infant (i = k), but whose child may or may not migrate before t
+
l ( j = k or j # k ) ; and four equationscorresponding to parents who do migrate between time t and the birth of the infant (i # k), but whose child may or may not migrate before t + l ( j = i or j # i). This implies that a child may migrate without its parents between the ages of 0 and 5.
3.2 Projection
The age-specific birth rates, by region of birth of child, may be incorporated into the standard multiregional projection model (Rogers 1975, Chap. 5) transform- ing that model into a multistate projection model, where the states of interest are places of birth. This transformation makes it possible to generate projections that keep track of the regions of birth, i.e., that produce PRPB projections.
Appendix B describes the matrix model. Note that the Markovian assumption is still retained. All individuals in a region, recent in-migrants as well as established residents, aliens as well as natives, are assumed to experience identical probabilities of transition. This assumption is relaxed in Section 4.
Appendix B sets out the multistate growth matrix for our two-region (South- North), two-state (natives and aliens) example. Appendix B also presents the stable distribution across states that ultimately arises if this projection matrix is applied to any observed population. The stable distribution depends only on the elements of the growth matrix and not on the initial (base-year) population distribution. (Since it is also of some interest to use the matrix to generate projections, a 30-year projection based on the 1968 population is included in Appendix B.)
5 6 Dimiter Philipou and Andrei Rogers
T h e stable growth results in Appendix B may be compared with the results of the conventional projection presented earlier in the same Appendix. Note that the intrinsic rate of growth remains the same (r =0.004361), as does the spatial distribution of the national population (sha, = 34.46% and sha, = 65.54%). T h e national and regional age compositions remain unchanged, with the mean age in the South being 37.94 years and that in the North 36.65. In short, the two projections to stability give identical results, as they indeed must. T h e multistate projection, however, includes additional information: it disaggregates regional populations by place of birth. It reveals, for example, that, at stability, the mean age of the alien population in the South will be about 15.3 years older than that of the native population and some 2.5 years older than the North's alien population. All of these stable growth results, however, could b e obtained without the multistate growth matrix. W e have shown earlier (Table 1 ) that a simple weighting of the stationary multiregional life table population gives identical results. T h e usefulness of the growth matrix, therefore, lies in generating projections such as that presented at the end of Appendix B.
4 NATIVE-DEPENDENT MULTISTATE PRPB POPULATION