Introduction to Demography

Introduction to Demography

Dr. Emily A. Fogarty

SBC115

Fall 2011

Goals for course: This course provides basic background needed for understanding and interpreting quantitative and qualitative aspects of population growth for humans, and the ability to put problems involving population changes into a broader context.

• Demography, the science of human population (or more specifically, the study of population structure and processes), draws together research from a number of disciplines, including economics, sociology, geography, public health, and genetics.

• The name of the field comes from the Greek word demos, meaning “the people” (collectively).

• This first chapter will cover concerns with the characteristics of aggregates of living individuals. Also covered is a cursory overview of the origins of demography and demographic concepts.

• Demography arose from a realization that population statistics could reveal much about the nature of societies and the changes taking place within them.

• Yet the censuses that the Egyptians, Babylonians, Romans and Chinese conducted in ancient times never spurred such reasoning; no one perceive them as more than head counts, mainly of men eligible for taxation, work or military service.

• What finally inspired the invention of demography was not censuses of the living, but lists of the dead, with supplementary information on weddings and christenings.

excerpt from Rowland 2003, pg14

• The 14th century Black Death killed at least a quarter of the population of Europe, and around 50 million in Europe and Asia overall, there followed centuries with further devastating epidemics of bubonic plague….by the

• early 16th century, an ordinance required parish priests in London to compile weekly lists of deaths from plague, called the Bills of Mortality.

• These were intended initially to identify outbreaks and areas for quarantine.

• Later, other causes of death were included, as well as weddings and christenings.


• 17th century London produced the founder of demography, John Graunt (1620 – 74). Graunt became interested in the Bills of Mortality and was moved to begin the studies that led to his book, Natural and Political Observations Made upon the Bills of Mortality, this book quickly brought him to scientific recognition.

• Graunt’s book ranged across many important questions including;

• causes of death• proportions surviving at different ages• health and the environment• the balance of the sexes, family size, age structure,• employment, populations estimates,• population growth and its components and the need for

social statistic in public administration.


• The appearance of Graunt’s book transformed perceptions of population statistics. His ideas stimulated interest in medical statistics on the causes of illness and death, in tables on the statistical likelihood of people’s survival – for the life insurance industry – and in the establishment of national censuses an statistical offices.

• Fruition of some of these ideas, notably the conduct of regular national censuses, took more than a hundred years, but the ideas originated with Graunt.


•Human population trends are centrally important to environmental science because they help to determine the environmental impact of human activities.

•Rising populations put increasing demands on natural resources such as land, water, and energy supplies.

•As human communities use more resources, they generate contaminants, such as air and water pollution and greenhouse gas emissions, along with increasing quantities of waste.

•Population interacts with several other factors to determine a society’s environmental impact. One widely-cited formula is the

"I = PAT" equation, proposed by Paul R. Ehrlich and John P. Holdren in 1974*

Environmental Impact = Population x Affluence (or consumption) x Technology

*J.P. Holdren and P.R. Ehrlich, "Human Population and the Global Environment," American Scientist, vol. 62 (1974), pp. 282–92.

•For generations people have tried to estimate Earth's carrying capacity, or the maximum population that it can support on a continuing basis. This is a slippery undertaking.

•Estimates of human carrying capacity over the past four centuries have varied from less than one billion people to more than one trillion, depending on how the authors defined carrying capacity. Some studies cast the issue solely in terms of food production, others as the availability of a broader set of resources.

•In fact, the question depends on assumptions about human preferences.

•What standard of living is seen as acceptable, and what levels of risk and variability in living conditions will people tolerate?

•Many of these issues are not just matters of what humans want; rather, they intersect with physical limits, such as total arable land or the amount of energy available to do work. In such instances nature sets bounds on human choices*

*Joel E. Cohen, How Many People Can the Earth Support? (New York: Norton, 1995), pp. 212-36, 261–62.

Measuring Earth's carrying capacity at the global level obscures the fact that resources are not allocated equally around the world. In some areas such as the Sahel in West Africa (the transition zone between the Sahara desert and more humid woodlands to the south), population growth is putting heavy stresses on a fragile environment, so food needs are outstripping food production (Fig. 1). Other regions have better balances between populations and resources.

Figure 1. Gully erosion from over-cultivation, Sahel, West Africa Source: Courtesy United States Geological Survey, National Center for Earth Resources Observation Systems International Program.

Population experts can make demographic predictions with more confidence than many other social scientists.

Several basic truths apply to the demographics of all human societies:

•Everyone who is alive one year from now will be one year older at that time than s/he is today.

•Ages 15 to 49 are humans' prime childbearing years, biologically speaking (although resource constraints and social and political factors shape childbearing decisions differently from one country to another).

•Human mortality is relatively high among infants, children, and adults over age 60, and relatively low at other parts of the life cycle.

Putting these observations together, population analysts can develop a reasonably accurate map of how a society's population size, births, deaths, and age structure are likely to evolve in the next several decades.

Birth and death rates are the most important determinants of population growth; in some countries, net migration is also important in this regard.

To calculate population growth rates, demographers take the difference between births and deaths in a given time period, add the net number of migrants (which for the world as a whole is 0), and divide that number by the total population.

For example, there are now about 136 million births and 58 million deaths worldwide annually, adding a net of 78 million new inhabitants to a global population of 6.7 billion, a growth rate of nearly 1.2 percent.

Until the mid-19th century birth rates were only slightly higher than death rates, so the human population grew very slowly.

The industrial era changed many factors that affected birth and death rates, and in doing so, it triggered a dramatic expansion of the world's population (Fig. 1).

Figure 1. Past world population growthSource: Based on data from The World at Six Billion (1999). United Nations Secretariat, Department of Economic and Social Affairs.

•How did industrialization alter population growth rates so sharply? One central factor was the mechanization of agriculture, which enabled societies to produce more food from available inputs.

•As food supplies expanded, average levels of nourishment rose, and vulnerability to chronic and contagious diseases declined over succeeding generations. Improvements in medical care and public health services—which took place more in urban than in rural areas—also helped people to live longer, so death rates fell. After several decades of lower mortality, people realized that they did not have to have so many children to achieve their desired family size, so birth rates began to fall as well.

•In addition, desired family size tended to decrease. As women found many more opportunities to enter the labor force, they were less inclined to devote resources to childrearing rather than paid work, and the jobs they had were not conducive to having children beside them as they worked. The costs of raising children also increased, as slightly wealthier families living in urban areas faced higher expenses for a larger array of physical and social necessities.

•This phased reduction in death and birth rates is a process called the demographic transition, which alters population growth rates in several stages (Fig 2).

1. Change from high birth and death rates to low CBR and CDR (transition means change)

Four stages

Stage 1: High birth rate, high death rate; very slow growth, low RNI Stage

Stage 2: Death rate fall dramatically, birth rate remains high; high RNI Stage

Stage 3: Death rate remains low, birth rate drops; RNI slows

Stage 4: Death rate low, birth rate low; low RNI (as in stage 1)

Highest RNI during stage 2

The Demographic Transition

More Info

The demographic transition is a well-recognized pattern, but it has shown many variations from country to country.

We cannot predict when specific demographic changes will occur in particular countries, and it is hard to specify precisely which factors will shape a given society's path.

Looking forward, a major question for the 21st century is what happens after the demographic transition, and whether some countries in areas such as western Europe, where birth rates are very low, will start striving to raise fertility.*

More important in terms of environment and health, however, is the question of how to help countries that are lagging on the transition path.

*Dudley Kirk, "Demographic Transition Theory," Population Studies, Vol. 50, No. 3 (November 1996), pp. 381–87.

An Overview of Population ScienceObjectives for chapter 1

• To get better acquainted with the field of demography and with the concepts and principles employed in demographic research.

• Basic observations on demographic aggregates.

• Special focus on naturally occurring populations.

• Next consider relationships between aggregates and individuals.

• In general an aggregate is a collection of two or more objects or units: sheets of paper, stones, items of clothing, and so on…

Populations and Other Aggregates

• Demographically speaking there are many kinds of aggregates, depending in part on the extent to which the individuals they contain interact with one another, share the same geographic territory, and maintain prolonged contact over the course of one or more generations.

• For example some aggregates are mere classifications created by the research for analytical purposes, such as the aggregate of all female persons aged 15 to 45 living in the U.S.

Weinstein and Pillai 2001

• Other aggregates are truly interacting groups, such as the specific household that now resides at 105 E. Main St. Anytown, USA

• Many of the principles and techniques of demography can be , and routinely are, applied to all types of aggregates; but demographers do give major consideration to the special type of aggregates known as a population.


• Characteristics of populations and other aggregates with which demography is most directly concerned can be divided into 4 categories.

1) Size: the number of individuals included in and aggregate or population

2) Structure: the ways in which sub- aggregates and subpopulations are formed

3) Geographic Distribution: how an aggregate or population is dispersed over physical space

4) Changes in Size, Structure, and/or Geographic Distribution: the dynamic aspects of these characteristics as they change over time.

Each of these categories can be further divided according to types, subtypes, and other special features.

• Demography is interested in both description and explanation

• The descriptive phase is applied in a wide range of contemporary professions and business, from public administration to wildlife management, from marketing to city planning.

• The explanatory phase assists the understanding of population dynamics…why some aggregates grow faster than others, why population structures vary from time to time and place to place


• The task of explanation begins with the examination of the vital events of demography: birth, death, in and out migration. These are the variables in the “basic balancing equation” of demography

Population Growth = (P1 – P0) = (B – D) + (I – O)

Population Growth = Natural Increase + Net Migration

•P0 is the size of the population at an earlier or initial date•P1 is the size at a later date•B is the ## of births that occurred between the two dates•D is the ## of deaths•I is the ## of in-migrants and •O is the ## of out-migrants

The quantity (B – D) is referred to a “natural increase” and the quantity (I – O) is referred to be“net migration

Elements of Population Study

1. Population growth rates vary greatlyamong the world’s regions

2. Regions have vastly different approaches to family planning (to increase or decrease population).

3. Migration is very important; the greatest migration (in terms of number of migrants) in human history is NOW.

Growth Components as Population Processes

• So far, I have described the causes of population growth as events. Each birth, each death, each move across the border is a population event.

• However, demographers – and indeed the general public – are accustomed to thinking of theses individual events as expressions of underlying collective processes.

• That is, we think of the numbers of births that occur in a population as an expression of the collective fertility of that population; the number of deaths is manifestation of its force of mortality; and the number of moves across the border (in and out) is the process of migration.

• The simplest measure of population process is a crude rate.

• Why construct rates at all? Why not simply count the number of events and let that total be the measure of the process? (discuss)

Yaukey et al. 2007

At bit about rates… and crude rates…

Growth Components as Population ProcessesYaukey et al. 2007

To answer the question… suppose that exactly the same number of deaths occurred in the US during 1900 as occurred in 2000. Can one conclude that the mortality of the US was the same in 1900 as in 2000? most people recognize this statement as misleading…

There were many more people available to die in the year 2000 than in 1900. If the same number of people had actually died in both years then a much smaller percentage of the population would have died in 2000 than did in 1900.

OR, as demographers would express it, the population at risk of dying was greater in 2000, and that should be taken into account when measuring how likely it was that these events occurred.

The basic demographic strategy for doing this is to divide the number of events that occur over a given time by the population that was at risk of experienceing those events.

• This step produces one of the fundamental measurements demographers use, that is, rates.

• The numerator is the number of events; the denominator is the population at risk.

• Generally speaking, rates tell us the probability of a member of the population at risk participating in one of the events.

• SO, a CRUDE approximation of the population at risk for demographic processes is the size of the total population during the specified year. Dividing the number of events by the estimated total population produces CRUDE RATES.

See crude rate pdf on Blackboard for more information

Population Growth and Change

1. Rate of natural increase (RNI) – world average is 1.3 percent/yr: the annual growth rate for a country or region, expressed as a percentage increase or decrease; equals the number of births minus the number of deaths; excluding migration.

2. Total fertility rate (TFR): the average number of children borne by a statistically average woman; world TFR is 2.8, but ranges from 1.4 (Europe) to 5.1 (Africa).

3. Crude birth rate (CBR) the gross number of births divided by the total population, giving a figure per 1,000 of the population; world CBR is 21 per 1,000

Population Growth and Change

4. Crude death rate (CDR): the gross (total)

number of deaths divided by the total population, producing a figure per 1,000 of the population; world CDR is 9 per 1,000

5. Percentage of population under age 15: global average is 30 percent, low is 17 percent (Europe); high of 42 percent (Africa); high number indicates great potential for future growth.

• These four factors – births, death, in and out migration – entirely determine the magnitude and rate of population growth.

• We will cover the fundamental equation in more detail in part three of the book.

Naturally Occurring Populations• Nearly all species of living organisms, including humans

beings, form naturally occurring populations, • A naturally occurring population has certain defining

features that set it off from other kinds of aggregates, among them these primary features:

• Populations are relatively endogamous; their individual members tend strongly to mate and reproduce with one another.

• Populations share a bounded geographic territory. • Human populations are specially complex because, in

addition to having a a common gene pool and territory, their members also share a sociocultural heritage: tools, language, or other knowledge that is transmitted from parents to offspring and from peer to peer through imitation or learning.


• The field of demography is affected by the fact that human populations do have such distinct sociocultural heritages. In general this characteristic divides the field into two branches.

1) formal demography, which focuses on population size, composition, geographic distribution, and the interplay of vital events.

2) social demography, essentially the study of human aggregates. This branch explores the relationships among a population’s size, structure and geographic distribution, its vital events and social and cultural factors.


• The structure of a population is the organization of a population into two or more subpopulations according to sex, age, or any of several sociocultural characteristics.

• Although age and sex are basic dimensions of population structure, many other social and economic characteristics also used to ascribe social roles; for example, how we categorize people and what their social roles many be are influenced by their ethnicity, literacy and education, occupation and income.


• Demography deals with aggregates (or collections) of people, not individual people. The very term “population” refers to the numbers of people resident in some specified geographic area, be it a class room or a nation. Likewise, demography describes characteristics of population, not of individual members in those populations.

• As I have emphasized that demography focuses on the aggregate and not the individual level of human.

• Two important concepts well known to population scientist; 1) ecological fallacy 2) structural effect

• An ecological fallacy is an unwarranted (and possibly false) statement about individual characteristics based on information that relates to characteristics of an aggregate.

• ...an erroneous conclusion that relationships observed a the aggregate level also hold true for the individuals making up the aggregate.

• One of the most interesting discoveries of hidden relationships among individual and aggregate characteristics is credited to a demographer and sociologist of the 1900s era, Emile Durkheim (1858-1917). The discovery is known today as the principle of structural effects.

• The idea that the characteristics and dynamics of populations, cultures, and societies studied in social science are legitimate scientific objects.

• BUT these objects cannot be reduced simply to physical, biological, or psychological phenomena

• Rather, to show that aggregate phenomena do have any independent emergent existence.

• The analysis of structural effects is used to determine whether aggregate-level influences exists.

• It also illustrates that such effects, although related to the characteristics of individuals, operate in their own independent realm.

• This fact has many important implications for understanding the nature and population structure and change.

• Including many ways in which urban residences alters individual beliefs and practices

• a related factor is how migration from one population to another shows individual behavior in the realm of mortality and fertility control

• To review… we defined demography as the study of naturally occurring populations and other living aggregates

• we noted that populations are relatively endogamous, territorial , and are structured along biological dimensions and in the case of human populations, sociocultural dimensions as well

• we stated that populations are part of nature, with a visible concrete existence in space and time

• and that populations can with their own effects transcend the behavior of their individual members.

Aggregate: a collection of items. In demography these items are individual humanBeing, households, etc.

Crude Birth Rate: (CBR) The number of infants born into a population per 1,000Persons (usually measured per year).

Endogamy: the practice of marrying only within one’s own population or social group

Ecological fallacy: An erroneous conclusion that relationships observed at the Aggregate level also hold true for the individual making up the aggregate.

Emergence: characteristics of aggregates that exist independent of the characteristics ofthe individuals making up the aggregate (“the whole is greater than the sum of its parts”)

Population: a kind of aggregate that shares a gene pool, bounded territory, and sociocultural heritage.

Structural effect: the influence that the structure of a population has on its individual members.

Some Key Terminologies

To view all Key Terms for chapter one see page 20 in your textbook

Documents

Introduction to Demography