Demography Health Article

DEMOGRAPHY

Demography is the study of the growth, change, and structure of the human population. Changes in a population's size and structure are caused by changes in the birthrate, the death rate, and the net migration rates. Demographic research focuses on why people have the number of children they do; on factors that affect death rates; and on the reasons for immigration, emigration, and geographic mobility. Understanding a society's demography is an essential tool in determining current and future public health needs.

HISTORY OF THE HUMAN POPULATION

The twentieth century was a very unusual period, demographically. World population grew at a more rapid and sustained pace than at any time in human history, as shown in Figure 1. The global population grew from approximately 1.7 billion people in 1900 to 6 billion in 1999. The annual population growth rate averaged 1.3 percent for the entire twentieth century, and was as high as 2.3 percent between 1965 and 1970. (A sustained 2.3 percent annual growth rate would have meant a doubling of the world's population in thirty years.)

By contrast, throughout most of history the human population grew very slowly. Occasionally, in some regions, there were periods of very rapid population growth—and also very rapid population decline. However, these periods generally averaged out over time, and overall population growth was extremely slow. For example, between the years 1 C.E. and 1750, the average annual population growth rate was only 0.06 percent. (At this rate, the population would double, on average, only once every 1,250 years.) A period of rapid population growth began around 1750 in Europe and North America. Rapid population growth in most other parts of the world began between 1920 and 1960.

Why did the population begin to grow rapidly first in Europe and North America, next in Japan,

Figure 1

Australia, and New Zealand, then in most of Asia and Latin America, and finally in Africa and the rest of the world? The answer lies in how a population grows or declines. A change in the size of a population occurs in only a few ways: Either births and immigrants add new members to the population, or deaths and emigrants remove members from the population. Throughout most of human history both birthtates and death rates were high, though birthrates were slightly higher than death rates on average. Slightly higher birthrates than death rates meant that the population was growing, although at a very slow rate. Migration added to some populations and subtracted from others at different periods in history.

Beginning in the eighteenth century, however, death rates began to decline, slowly at first and then more rapidly. For example, death rates declined from about 35 to 45 deaths per 1,000 population in the period from 1750 to 1850 to around 8 to 12 deaths per 1,000 in low-mortality countries (Europe, North America, Japan, and Australasia) in the late twentieth century. This decline began in different parts of the world at different times. In North America and Europe, the timing of the mortality decline was closely tied to the beginning of the Industrial Revolution. In Asia, Latin America, and Africa, declines in death rates took place mostly during the twentieth century. Declining death rates in combination with continuing high birthrates triggered the rapid growth of the population. Simply put, many more people were born into the population each year than left it through death.

Historical research shows that much of the mortality decline in Europe and North America occurred before most modern changes in medical technology and treatment, and therefore was caused by other factors. These factors include improvements in public health (including sanitation, waste disposal, clean water supply, and quarantine); changes in personal hygiene (including bathing, handwashing, and household cleanliness); improved standards of living (including better nutrition and housing); and improved political, economic, and transportation systems, which led to better responses to food shortages and drought.

These factors also played an important role in reducing death rates in Asia, Latin America, and Africa during the twentieth century. However, improvements in medical and public health technology were also important in these regions. For example, immunization programs, pesticide spraying against mosquitos that spread malaria and yellow fever; oral rehydration therapy for diarrhea; antibiotics; and improved and more widely available health care have all contributed to mortality reduction.

Despite continuing gains in health and survival, the pace of population growth began to slow in industrialized countries in the mid–twentieth century and in other regions of the world in the last three decades of the twentieth century. The reason is that birthrates began to decline. In some European countries, birthrates fell so low by the end of the twentieth century that their population growth rates became slightly negative, meaning that the number of people in these countries is declining slightly. For example, between 1995 and 2000, Italy had a birthrate of 9 per 1,000 population, or an average of about 1.2 births per woman. During this period, Italy's death rate was 10.4 per 1,000 population, so the Italian population became slightly smaller each year. Birthrates have also fallen to historically low levels in many countries in Asia and Latin America. There is also substantial evidence that birthrates are declining in many African countries as well. However, there is still great variability in birth and death rates among regions of the world, as the figures in Table 1 show.

The decline in birthrates is due to dramatic changes in economic and social conditions, ideas about the family and the role of children and women, the availability of family planning programs, and the acceptance and use of contraception. Although much of the fertility decline in

Table 1

Europe and North America occurred before many modern contraceptive methods were available, the development and widespread use of contraceptive methods has played a major role in reducing fertility throughout the world. Contraceptive methods include the hormonal pill, the intrauterine device (IUD), sterilization (vasectomy for men and tubal ligation for women), hormonal injections and implants, and barrier methods such as condoms, spermicidal foam and jelly, diaphragms, and cervical caps. In some countries, such as the former Soviet Union and Japan, induced abortion has also played an important role in reducing the birthrate.

Even though birthrates have fallen substantially in many countries, their populations continue to grow because of the effects of their age structure, or "population momentum." For example, the U.S. population continued to grow at almost 1 percent per year during the 1980s and 1990s despite a very low birthrate. The reason is that a substantial proportion of the population was in their childbearing years because of the "baby boom" in the 1950s and early 1960s. The effects of population momentum is temporary: In the absence of immigration, if birthrates remain low for the next fifty years, the size of the U.S. population will begin to decline. However, immigration is likely to continue during this period, keeping the U.S. population growing at a relatively slow pace.