DEMOGRAPHYelaboration the population is divided into a num-
ber of states with fixed age-specific migration or
mobility among the states (Land and Rogers 1982;
Schoen 1988). States may be geographic regions,
marital circumstances, educational levels, or whatever.
In part, the value of this theory is in the light it
sheds on how populations work. For example, it
explains how a population can outlive all of its
contemporary members and yet retain its median
age, percent in each race, and its regional distribution.
The fruit of the theory lies in its utility for
estimation and forecasting. Using aspects of the
theory, demographers are able to elaborate rather
modest bits and pieces of information about a
population to a fairly full description of its trajec-
tory (Coale and Demeny 1983). Combined with
this mathematical theory is a body of practical
forecasting techniques, statistical estimation pro-
cedures, and data-collection wisdom that makes
up a core area in demography that is sometimes
called formal demography (United Nations 1983;
Shryock and Siegel 1976; Pollard, Yusuf, and Pollard
1990; Namboodiri 1991; Hinde 1998).
DEMOGRAPHIC DATAGenerating the various rates and probabilities used
in formal demography requires two different kinds
of data. On the one hand are data that count the
number of events occurring in the population in a
given period of time. How many births, deaths,
marriages, divorces, and so on have occurred in
the past year? These kinds of data are usually
collected through a vital registration system (Na-
tional Research Council 1981; United Nations 1985,
1991). On the other hand are data that count the
number of persons in a given circumstance at a
given time. How many never-married women age
twenty to twenty-four were there on July 1? These
kinds of data are usually collected through a popu-
lation census or large-scale demographic survey
(United Nations 1992; Anderson 1988; Anderson
and Fienberg 1999). From a vital registration sys-
tem one gets, for example, the number of births to
black women age twenty. From a census one col-
lects the number of black women at age twenty.
The division of the number of events by the popu-
lation exposed to the risk of having the event occur
to them yields the demographic rate, that is, the
fertility rate for black women age twenty. These
two data collection systems—vital statistics and
census—are remarkably different in their charac-
ter. To be effective, a vital statistics system must be
ever alert to see that an event is recorded promptly
and accurately. A census is more of an emergency.
Most countries conduct a census every ten years,
trying to enumerate all of the population in a
brief time.If a vital registration system had existed for a
long time, were very accurate, and there were no
uncounted migrations, one could use past births
and deaths to tally up the current population by
age. To the degree that such a tallying up does not
match a census, one or more of the data collec-
tions systems is faulty.SOCIAL DEMOGRAPHYOne of the standard definitions of demography is
that given by Hauser and Duncan: ‘‘Demography
is the study of the size, territorial distribution and
composition of populations, changes therein, and
the components of such change, which may be
identified as natality, mortality, territorial move-
ment (migration) and social mobility (change of
status)’’ (1959, p. 2). Each of these parts—size,
territorial distribution, and composition—is a ma-
jor arena in which the relationships between demo-
graphic change and social change are investigated
by social scientist-demographers. Each part has a
somewhat separate literature, tradition, method,
and body of substantive theory.Population Size. The issue of population size
and change in size is dominated by the shadow of
Malthus (Malthus 1959), who held that while food
production can grow only arithmetically because
of diminishing returns to investment in land and
other resources, population can grow geometrical-
ly and will do so, given the chance. Writing in a
time of limited information about birth control
and considerable disapproval of its use, and hold-
ing little hope that many people would abstain
from sexual relations, Malthus believed that popu-
lations would naturally grow to the point at which
starvation and other deprivations would curtail
future expansion. At that point, the average level
of living would be barely above the starvation level.
Transitory improvements in the supply of food
would only lead to increased births and subse-
quent deaths as the population returned to its