3016 migration: internal

mobility of students and migrant workers who
plan to return to their original place of resi
dence within a few months. However, this tem
poral standard is somewhat arbitrary and has
not been universally adopted, nor does it unam
biguously distinguish migrants who have no
intention to return to their place of origin from
those making a longer but essentially imperma
nent sojourn to another city, state, or region for
educational or employment purposes. Second,
and perhaps more problematic, is the fact that
the standard definition of internal migration
does not specify the scale of geographical units
or the distance across which a move must occur
in order to be considered true internal migra
tion. This lack of specificity creates the poten
tial for tremendous variation in the operational
definitions used in research on the topic.
Data on internal migration generally come
from three sources, each of which is character
ized by unique strengths and weaknesses. First,
population registry systems, essentially contin
uous records of citizens vital statistics, typi
cally require individuals to register with the
local administrative office upon moving to a
new area of the country, creating a record of
each internal migration event. Unfortunately,
few countries maintain a population registry
and many of those that do exist contain limited
social and economic characteristics with which
to assess the determinants or consequences of
migration.
In lieu of data from population registries,
migration researchers often rely on data from
periodic population censuses. Based on these
data, the magnitude of net migration (in
migrants minus out migrants) can be assessed
using an indirect method in which the esti
mated natural increase of the population (births
minus deaths) is subtracted from the total
population change in an area, leaving the com
ponent of change attributable to the net addi
tion of migrants. While these estimates are
simple and widely utilized, their reliability
depends greatly on the quality of mortality
and fertility data and they provide little infor
mation about the factors affecting migration or
the influence of migration on the composition
of the population. In most countries, census
records document individuals place of birth,
place of current residence, and perhaps the
place of residence at some intermediate point
of time. Comparing the place of residence at
different points in time provides the basis for
inferring internal migration events and for
estimating population flows between specific
origins and destinations. However, this basic
method fails to capture migration events experi
enced by an individual between the two refer
ence points and the potential for effective
cross national comparisons is undermined by
differences in the mobility intervals and levels
of geography utilized in census items.
Surveys represent a final, somewhat rarer,
source of data on internal migration behavior.
In the US, surveys such as the Current Popula
tion Survey provide regular snapshots of annual
migration behavior and afford the opportunity
to assess the basic association between various
types of migration and a variety of micro level
characteristics. Even more powerful are panel
studies that collect detailed information on the
set of panel members at regular intervals across
an extended period of time. These data make
it possible to trace, prospectively, multiple
occurrences of various types of migration beha
vior by individual panel members and offer the
opportunity to rigorously test theoretical argu
ments about the determinants of migration.
However, these surveys are of limited utility
for assessing the overall magnitude of internal
migration between areas or its impact on aggre
gate population change.
In combination with the definitional ambigu
ity of the phenomenon, the inadequacies asso
ciated with available data sources undermine the
ability to make reliable cross national compari
sons of the magnitude and dynamics of internal
migration or to compare the results of research
that may employ different operational defini
tions. Nevertheless, the combination of various
sources has enabled the development of a rich
and varied literature on the patterns, determi
nants, and consequences of internal migration.

EXPLAINING INTERNAL MIGRATION

Although a wide range of theoretical models
has been employed in migration research, the
pushpull theory remains the most widely used
explanatory framework in the study of internal
migration. While this model has been criticized
for its lack of predictive power, it provides a