Perhaps the most fundamental of these characteristics is the age distribution of a population. Demographers commonly use population pyramids to describe both age and sex distributions of populations. A population pyramid is a bar chart or graph in which the length of each horizontal bar represents the number (or percentage) of persons in an age group; for example, the base of such a chart consists of a bar representing the youngest segment of the population, those persons less than, say, five years old. Each bar is divided into segments corresponding to the numbers (or proportions) of males and females. In most populations the proportion of older persons is much smaller than that of the younger, so the chart narrows toward the top and is more or less triangular, like the cross section of a pyramid; hence the name. Youthful populations are represented by pyramids with a broad base of young children and a narrow apex of older people, while older populations are characterized by more uniform numbers of people in the age categories. Population pyramids reveal markedly different characteristics for three nations: high fertility and rapid population growth (Mexico), low fertility and slow growth (United States), and very low fertility and negative growth (West Germany).

Contrary to a common belief, the principal factor tending to change the age distribution of a population—and, hence, the general shape of the corresponding pyramid—is not the death or mortality rates, but rather the rate of fertility. A rise or decline in mortality generally affects all age groups in some measure, and hence has only limited effects on the proportion in each age group. A change in fertility, however, affects the number of people in only a single age group—the group of age zero, the newly born. Hence a decline or increase in fertility has a highly concentrated effect at one end of the age distribution and thereby can have a major influence on the overall age structure. This means that youthful age structures correspond to highly fertile populations, typical of developing countries. The older age structures are those of low-fertility populations, such as are common in the industrialized world.

Sex ratio

A second important structural aspect of populations is the relative numbers of males and females who compose it. Generally, slightly more males are born than females (a typical ratio would be 105 or 106 males for every 100 females). On the other hand, it is quite common for males to experience higher mortality at virtually all ages after birth. This difference is apparently of biological origin. Exceptions occur in countries such as India, where the mortality of females may be higher than that of males in childhood and at the ages of childbearing because of unequal allocation of resources within the family and the poor quality of maternal health care.

The general rules that more males are born but that females experience lower mortality mean that during childhood males outnumber females of the same age, the difference decreases as the age increases, at some point in the adult life span the numbers of males and females become equal, and as higher ages are reached the number of females becomes disproportionately large. For example, in Europe and North America, among persons more than 70 years of age in 1985, the number of males for every 100 females was only about 61 to 63. (According to the Population Division of the United Nations, the figure for the Soviet Union was only 40, which may be attributable to high male mortality during World War II as well as to possible increases in male mortality during the 1980s.)

The sex ratio within a population has significant implications for marriage patterns. A scarcity of males of a given age depresses the marriage rates of females in the same age group or usually those somewhat younger, and this in turn is likely to reduce their fertility. In many countries, social convention dictates a pattern in which males at marriage are slightly older than their spouses. Thus if there is a dramatic rise in fertility, such as that called the “baby boom” in the period following World War II, a “marriage squeeze” can eventually result; that is, the number of males of the socially correct age for marriage is insufficient for the number of somewhat younger females. This may lead to deferment of marriage of these women, a contraction of the age differential of marrying couples, or both. Similarly, a dramatic fertility decline in such a society is likely to lead eventually to an insufficiency of eligible females for marriage, which may lead to earlier marriage of these women, an expansion of the age gap at marriage, or both. All of these effects are slow to develop; it takes at least 20 to 25 years for even a dramatic fall or rise in fertility to affect marriage patterns in this way.

Ethnic or racial composition

The populations of all nations of the world are more or less diverse with respect to ethnicity or race. (Ethnicity here includes national, cultural, religious, linguistic, or other attributes that are perceived as characteristic of distinct groups.) Such divisions in populations often are regarded as socially important, and statistics by race and ethnic group are therefore commonly available. The categories used for such groups differ from nation to nation, however; for example, a person of Pakistani origin is considered “black” or “coloured” in the United Kingdom but would probably be classified as “white” or “Asian” in the United States. For this reason, international comparisons of ethnic and racial groups are imprecise, and this component of population structure is far less objective as a measure than are the categories of age and sex discussed above.