POPULATION COMPOSITION: AGE, SEX, RACE/ETHNICITY

The age structure of a society has a profound impact on its demographic and social character.

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Statistical Techniques

Demographers have developed statistical techniques to overcome some of the shortcomings of the basic data with which they work. They apply these techniques to the best data available to compute estimates of the actual population counts and measures. Although estimates based on good data can be quite accurate, users of these estimates should not forget that estimates are only approximations of the true number. Likewise, users of demographic data always should question the source and quality of the data that underlie the rates, ratios, and proportions they cite. Judging the quality of data is one of the most important skills demographers must learn. Indeed, everyone would benefit from taking a hard look at the myriad statistics we encounter daily. References

Where Do Demographic Data Come From?

Demographers use a variety of rates, ratios, and other measures to study population. But these measures are only as accurate as the data from which they are calculated. Where do these demographic data come from? How accurate are they? Most demographic measures are based on counts of people or demographic events (for example, births) in a specific area during a specific time period. There were 281,422,509 residents in the United States in April 2000, for example, and 1,730,000 births recorded in Iran during 1990. Counts come from population censuses, vital registration systems, national registers, and surveys. Their accuracy varies greatly by country and even by region within countries.

americans forming smaller households/families

In the United States, the mix of household types has changed enormously over the last three decades.2 One of the most notable changes is the decline in the proportion of family households and the rise in single-person households. In 1970, 81 percent of all households were family households, but this was down to 68 percent by 2005. The retreat from marriage and the general aging of the population are increasing the number of single-person households. Americans are waiting longer to get married, if they choose to marry at all. Married couples are more likely to get divorced than they were in the 1970s. More of America's elderly live alone after the death of a spouse. In 2005, 26 percent of all U.S. households consisted of just one person, compared with 17 percent in 1970 (see figure). Many European countries have seen a similar rise in single-person households for similar reasons. The U.S. baby-boom generation hastened many of the changes in the makeup of U.S. households. When the first of the baby-boom generation entered their late teens and early 20s in the 1960s, they moved out of their parents' homes and set up their own households, often alone or with housemates. They waited longer to marry than the previous generation, and they were more likely to divorce. Americans born after the baby boom are delaying marriage even longer. In 1970, 89 percent of women ages 25 to 29 had been married at least once. In 2005, only 58 percent of women ages 25 to 29 had been married.3 The choices these women make about marriage and childbearing help determine the present and future makeup of U.S. families and households.

Households and Families

Individuals relate to society through their families and households. When these units add or lose members—or when the household members grow older, divorce, or marry—there can be profound social and economic consequences. Divorce can bring financial hardship. Marriage can add additional income, as well as stepchildren or mothers-in-law. The birth of a child can bring new financial expenses, but it also can encourage stability. Households and families are basic units of analysis in demography. They are not the same thing. A household is composed of one or more people who occupy a housing unit

Life Stage Determines Living Arrangements

Life Stage Determines Living Arrangements An individual's living arrangements usually change at different stages of life. In the traditional scenario, a person starts out in a family household, leaves to create a new household alone or with friends, then forms a family household with a spouse and eventually children. In old age, an individual may live in a single-person household again because of divorce or death of the spouse. Of course, not everyone follows this pattern; many people skip or repeat stages. The average size and composition of households are highly sensitive to the age structure of the population. But they also reflect social and economic changes. An economic squeeze may prolong the time adult children live at home; a rise in the divorce rate may increase the number of single-person households. Relaxed social rules about marriage may boost the number of unmarried couples setting up house.

Defining Families

Not all households contain families. Under the U.S. Census Bureau definition, family households consist of two or more individuals who are related by birth, marriage, or adoption, although they also may include other unrelated people. Nonfamily households consist of people who live alone or who share their residence with unrelated individuals. These official definitions do not necessarily reflect changing attitudes about marriage, childbearing, and the roles of men and women. Households that consist of unmarried couples living together and gay and lesbian couples, for example, would be counted as nonfamily households even though they might share many characteristics of a family. If these couples live with children from their current or a previous relationship, the household moves into the family category. Life

Sample Surveys

Surveys often provide estimates of demographic events where registration systems are inadequate. They also aid in developing estimates of population size during the long interval between censuses. Surveys usually collect data for a sample group within a specific geographic area. In the United States, a monthly national survey is used to track the unemployment rate as well as many demographic indicators. But surveys suffer from many of the same accuracy problems as censuses and registration systems, and their data are subject to varying degrees of error.

Censuses: The Most Basic Source

The population census forms the cornerstone of demographic analysis. In many countries the census—an enumeration of all households—is the main source of national population data. More than 90 percent of the world's population was covered in a national census conducted during the 1990s. But censuses usually miss a small percentage of the population, especially in hard-to-enumerate areas such as the mountains of Turkey or low-income neighborhoods within some U.S. cities. The population characteristics that censuses record also are subject to error. Residents may lie about their income or forget the exact ages of some household members.

________________is the only one that really looks like a pyramid because each age cohort is larger than the one born before it

a rapid growth population

Vital Registration Systems

Vital events—births, deaths, marriages, and divorces—are usually recorded in national vital registration systems. These are the source of the counts used to calculate fertility, mortality, marriage, and divorce rates. But in countries in which mothers give birth at home, or where many residents are illiterate, a large proportion of vital events are never recorded. Less than half of the world's population lives in countries that have "complete" vital registration systems. Even "complete" systems may miss up to 10 percent of a country's vital events.

A population that is not growing, or is decreasing, produces a very different shape. The base of Italy's population "pyramid" is narrowing because its birth rate has been falling. The 1995-1999 birth cohort was barely one-half the size of the 1965-1969 cohort. If fertility remains below replacement, the pyramid's base will continue to constrict, and Italy will undergo natural decrease. If Italy's TFR rises to the replacement level of 2.1, its age and sex structure would eventually assume a rectangular shape because similar numbers of births would occur each year. Because mortality is low, this shape would be maintained until the older ages, when mortality would eat away at the top bars. At the very top, the female bar is almost always longer because women live longer than men. A slow-growth population is generally in the process of changing from a rapid-growth to a near-zero growth shape in response to changes in fertility and mortality. The United States is typical of these "middle-age," slow growth societies. Population pyramids also can be shaped by migration. Because migration is age selective, it alters the shape of age-sex pyramids in both the place of origin and destination. Migrants tend to be young adults; a steady migration stream is likely to make the place-of-origin population older, and the place-of-destination population younger. There are plenty of exceptions to this pattern.

low fertility narrows base

Population pyramids depict the general shape of a population's age structure, but they do not provide rates or measures that can be compared over time or with other populations. One such measure of age composition is the median age—the age at which exactly half the population is younger and half is older. In general, less developed countries have rapidly growing populations with low median ages, while more developed countries have slowgrowing or declining populations with higher median ages. The UN calculated the median age of Ethiopia at 17 in 2005, compared with 42 for Germany and 43 for Japan. More than four-fifths of the world's population lives in less developed countries, and the world's overall median age is young—about 28 years. U

median age

The majority of people in rapid-growth societies are young. This creates tremendous momentum for future growth because that large pool of young people makes up the parents of the future. Even if they have only four children apiece (the average for some less developed countries), their children's generation would be twice the size of their own. Ethiopia's population age structure is typical of a young, rapid-growth society. Low

momentum for future growth

This ______________primarily from sustained high fertility. If couples in one generation average eight children, for example, their children's generation will be about four times larger than their own. The pyramid's base would be about four times as wide as its middle. The distinctive pyramid shape also results from declines in mortality. Because of high mortality in the past, older age groups have relatively few surviving members and occupy a small section of the pyramid. The base is broadened by the fact that mortality, particularly infant mortality, is declining. This increases the proportion of the younger birth cohort that will survive to enter the next age group.

pyramid shape results

____ reflect historical events—wars, famines, baby booms or busts, and changes in immigration policies— that have affected one of the three demographic variables. Consider the tumultuous events portrayed in the pyramid for Germany in Figure 7. Births plummeted during the two world wars and a severe economic crisis, for example. Migration streams that are predominately male—as is labor migration to Middle Eastern countries—create an unbalanced pyramid, illustrated by Figure 8.

pyramids

Some of these impacts have been noted already—for example, the effect of age structure on population growth and on the average age of a society.

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The age structure of the United States looks more like a bowling pin than a pyramid (see Figure 6, page 19). This shape was created by drastic swings in the number of births—from the historic low of the 1930s, to the babyboom peak of 1957, down to the baby-bust low of the mid-1970s, and back to the baby boomlet of the 1980s and early 1990s. The pyramid's middle-age bulge is composed of the baby-boom cohort, which has been likened to a swallowed pig moving through a python.42 The narrower base is made up of the baby-bust cohorts born since the late 1960s. Each year the U.S. population gets older, primarily because of the aging of the baby boomers and low fertility. Increases in average life expectancy also have contributed to the aging of the population. The U.S. median age was 36 in 2005, up from 28 in 1970. The U.S. median age could reach 39 years by 2050.43

u.s age structure