Regional Demographic Characteristics
The key characteristics of regional demography of concern for health services provision include the size, age structure, and sex structure of the population and its rate of growth and comparative measures of fertility and mortality.
Sources of Population Data and Methodology
The population and mortality estimates for various regions summarized here are based on different data sources and methods, and thus are not strictly comparable. This primarily concerns the impact of different estimates of deaths by age and sex on population size and structure. Because the effect of mortality on population size and structure is generally modest, such differences have little impact on the findings reported in this chapter. The population estimates are based on data the United Nations (UN) Population Division compiled and analyzed for its biennial assessment of global population trends and regional demographic patterns (United Nations 2003). The UN Population Division estimates population size and vital rates (births and deaths) from censuses, vital registration, and demographic and health surveys and evaluates the data for completeness, accuracy, and consistency. Where necessary, it adjusts the data to achieve internal consistency and cross-country comparability. The baseline from which the UN projections are made is mid-2003. Because the 2002 revision was produced without complete data for 2001 for all countries, the baseline estimates are also projections, and the population figures in this chapter are therefore a mixture of both observed and projected data.1
The UN Population Division assesses a number of demographic parameters to produce country projections. In addition to total population, the baseline assessment includes a breakdown of population by sex and age (in five-year aggregates). Fertility is specified as age-specific fertility rates for females and mortality rates are based on survival probabilities from life tables. Age-specific patterns of migration are also incorporated for countries in which migration flows are observed or are thought to occur. When these inputs are not available from any of the sources listed earlier, the UN uses demographic models, such as model life tables or indirect mortality estimation techniques, to generate the information. Additional modeling is applied to estimate mortality patterns in countries with significant HIV/AIDS prevalence levels.
The UN Population Division provides a limited amount of information about the data in its reports, including the dates of censuses, the adjustment factors applied to total census populations, and the type and year of the latest surveys that contained mortality and fertility estimates. It does not provide information about the adjustments made to reported fertility rates, age and sex structures, or mortality rates. Basic information on population size and composition is available for most countries for 1990, and with the exception of Sub-Saharan Africa, for 2000 (or thereabouts) as well (table 2.1). Around both dates, censuses covered more than 90 percent of populations in all the regions except Sub-Saharan Africa. Thus, the basic population estimates developed by the UN Population Division and summarized in this chapter have a reasonable evidence base.
The UN projections of population size and vital rates are based on assumptions about levels and trends in vital rates. Fertility is assumed to follow a path modeled on the experience of countries with declining fertility, except when a country's recent fertility trend deviates considerably from the model pattern, in which case the country-specific pattern is followed (United Nations 2003).
Our 2001 estimates and future projections are generated on the basis of the cohort component methodology. This approach applies estimated trends in birth and death rates and migration by age and sex to a baseline age and sex structure. Population growth rates are determined by the levels of age-specific fertility and mortality rates and migration and the size of the initial age groups (base year population) against which these levels are applied. We constructed demographic estimates for the aggregate regional and income groupings used for the second edition of Disease Control Priorities in Developing Countries (Jamison and others 2006) from the UN Population Division country-level estimates by aggregating populations in specific age and sex groups and age-specific fertility rates. The aggregates are thus weighted by the different population sizes of individual countries.
The mortality estimates presented in this chapter are developed from other sources using methods different than those the UN employed, as described later. As a result, the age and sex structures reported here, as well as any indicators derived from them (such as crude birth and death rates) are not strictly internally consistent. In particular, the mortality rates estimated for this chapter would, in some cases, have produced different age and sex population structures than those estimated by the UN, as well as different numbers of births and deaths. These differences are unlikely to be large, however, as the estimated age-specific mortality rates reported later in this chapter agree quite closely with those of the UN, except for Sub-Saharan Africa.
Population Size and Growth
Between 1990 and 2001, global population increased from about 5.3 billion to 6.1 billion people, an average rate of increase of 1.4 percent per year, equivalent to about 220,000 people per day (table 2.2). During the decade, the growth rate in developing regions ranged from 0.2 percent in Europe and Central Asia to 2.6 percent in Sub-Saharan Africa.
Estimates at the global level conceal large differences in population growth among regions, which in turn consist of countries that may have quite different demographic trends. For example, Europe and Central Asia added just 1 million people per year between 1990 and 2001, whereas South Asia added 25 million people each year.
The World Bank regions (see map 1 inside the front cover of this volume) vary substantially in terms of population size, with East Asia and the Pacific accounting for about 30 percent of the global population and South Asia for roughly another 20 percent.Thus,about half the world's population live in the low- and middle-income countries of these two regions. The smallest region in terms of population size is the Middle East and North Africa, with just 5 percent of the world's population. Just over 10 percent of the world's population live in Sub-Saharan Africa. Another 15 percent live in high-income countries, a proportion that is declining.
Distribution by Age, Sex, and Location
How populations are distributed by age matters a great deal for public health, because many aspects of risk behavior, as well as disease and injury outcomes, are strongly associated with age. While many other factors contribute to mortality and fertility levels, the age distribution of a population is an important factor in explaining differences in demographic and epidemiological indicators. Regions differ significantly in how their populations are distributed across age groups, with almost 45 percent of the population of Sub-Saharan Africa being younger than 15, compared with 20 percent of the population in high-income countries, where fertility has been low for decades. Nevertheless, the trends during 1990-2001 show a great deal of similarity: in all regions the proportion of the population in the youngest age groups was lower in 2001 than in 1990, with most of the increase occurring in the 15 through 69 age group. As a result, the median age of the population has increased in all regions. At the same time, the population aged 70 and older has been increasing in most regions as mortality has declined, and this age group now represents more than 10 percent of the population in the high-income countries.
These changes in the relative age distribution of populations since 1990 reflect changes in the growth rates of different age groups (figure 2.1). In three of the six regions (East Asia and Pacific, Europe and Central Asia, and the Middle East and North Africa), as well as the world as a whole, the number of children under five was smaller in absolute terms in 2001 than in 1990. The highest growth rates during this period were in the 40- through 55-year-old age group and among those over 70. The irregularities in growth rates of different age groups reflect past trends in the initial size of each cohort and its subsequent mortality and migration experiences. This is particularly evident for Europe and Central Asia, where the impact of the regional conflicts in the early 1990s on demographic structure is particularly evident.
Along with the progressive aging of the population, the relentless trend toward increasing urbanization has continued, with consequences for health in terms of both health service provision, which, in principle, is better with urbanization, and risk of exposure to chronic disease, which is, on balance, worse (Ezzati and others 2005). Almost half the world's population lived in urban areas in 2001, up 4 percentage points from 1990. The increase in urbanization was particularly marked in East Asia and the Pacific (increase from 29 to 37 percent of the population) and in Sub-Saharan Africa (from 28 to 34 percent). Overall, 42 percent of the population in low- and middle-income countries now live in urban areas.
In general, more boys than girls are born, with sex ratios at birth of between 1.03 and 1.06 in most countries, though in some Asian countries, sex-selective abortions have skewed this ratio to more than 1.10. Differential mortality and, to a limited extent, migration, shape the sex ratio at other ages (figure 2.2). In South Asia, higher mortality for girls and for women during their childbearing years leads at first to an increasing and then to a constant sex ratio to about age 45, after which male mortality is higher. Excess mortality of adult males in Europe and Central Asia explains the particularly low sex ratio observed there (Lopez and others 2002). In all regions, the higher mortality of males relative to females accounts for the sharp decline in the population sex ratio after age 50 or thereabouts.
The overall effects of the age-specific mortality differences between the sexes are relatively minor in terms of total population sex ratios. All regions have roughly equal numbers of males and females in the population, with the proportion of males being slightly higher in Europe and Central Asia and in the high-income regions (51 to 52 percent) than in East Asia and the Pacific and South Asia (49 percent).
Table 2.2 shows recent trends in fertility, as indicated by the total fertility rate for the period, that is, the average number of children a woman could expect to have if she were subject indefinitely to current age-specific fertility rates.Even though fertility levels vary a good deal among regions, all low- and middle-income regions witnessed large declines in fertility levels during the 1990s. Overall fertility levels in low- and middle-income countries fell by almost 20 percent over the decade, a remarkable decline, with levels falling by as much as 33 percent in the Middle East and North Africa, and even by 10 percent in Sub-Saharan Africa. However, fertility rates in Sub-Saharan Africa remain high, with the total fertility rate of 5.6 being about twice as high as that for any other region.
Few low- and middle-income countries experienced increasing fertility during 1990-2001,2 though a few high-income countries have seen small upturns from previously low levels. Fertility is below replacement levels (about two children) in all but five high-income countries (Brunei Darussalam, Israel, Kuwait, Qatar, and the United Arab Emirates), as well as in most countries in Europe and Central Asia. When fertility drops to below replacement levels, population growth often continues for several decades,as the number of births exceeds the number of deaths because of the high proportion of women of childbearing age.