Population pyramid

A population pyramid (age structure diagram) or "age-sex pyramid" is a graphical illustration of the distribution of a population (typically that of a country or region of the world) by age groups and sex; it typically takes the shape of a pyramid when the population is growing. Males are usually shown on the left and females on the right, and they may be measured in absolute numbers or as a percentage of the total population. The pyramid can be used to visualize the age of a particular population. It is also used in ecology to determine the overall age distribution of a population; an indication of the reproductive capabilities and likelihood of the continuation of a species. Number of people per unit area of land is called population density.

A population pyramid often contains continuous stacked-histogram bars, making it a horizontal bar diagram. The population size is shown on the x-axis (horizontal) while the age-groups are represented on the y-axis (vertical). The size of each bar can be displayed either as a percentage of the total population or as a raw number. Males are conventionally shown on the left and females on the right. Population pyramids are often viewed as the most effective way to graphically depict the age and distribution of a population, partly because of the very clear image these pyramids provide. A great deal of information about the population broken down by age and sex can be read from a population pyramid, and this can shed light on the extent of development and other aspects of the population.

The measures of central tendency (mean, median, and mode) should be considered when assessing a population pyramid. For example, the average age could be used to determine the type of population in a particular region. A population with an average age of 15 would be very young compared to one with an average age of 55. Population statistics are often mid-year numbers.

A series of population pyramids could give a clear picture of how a country transitions from high to low fertility rates. If the pyramid has a broad base, this indicates that a relatively high proportion of the population lies in the youngest age band, such as ages 0–14, which suggests that the fertility rate of the country is high and above replacement fertility level. If a population is below replacement fertility level, the older population is declining with age, due to a combination of mortality and an increase in the number of births over time. There are usually more females than males in the older age ranges since, for a variety of reasons, women have a greater life expectancy.

The shape of the pyramid can also reveal the age-dependency ratio of a population. Populations with a high proportion of children and/or of elderly people have a higher dependency ratio. This ratio refers to how many old and young people are dependent on the working-age groups (often defined as ages 15–64). According to Weeks' Population: an Introduction to Concepts and Issues, population pyramids can be used to predict the future, known as a population forecast. Population momentum, when a population's birth rates continue to increase even after fertility rate has declined to replacement level, can even be predicted if a population has a low mortality rate since the population will continue to grow. This then brings up the term doubling time, which is used to predict when the population will double in size. Lastly, a population pyramid can even give insight into the economic status of a country from the age stratification since the distribution of supplies is not evenly distributed through a population.

In the demographic transition model, the size and shape of population pyramids vary. In stage one of the demographic transition model, the pyramids have the most defined shape. They have the ideal big base and a skinny top. In stage two, the pyramid looks similar but starts to widen in the middle age groups. In stage three, the pyramids start to round out and look similar in shape to a tombstone. In stage four, there is a decrease in the younger age groups. This causes the base of the widened pyramid to narrow. Lastly, in stage five, the pyramid starts to take on the shape of a kite as the base continues to decrease. The shape of the population is dependent upon what the economy is like in the country. More developed countries can be found in stages three, four, and five, while the least developed countries have a population represented by the pyramids in stages one and two.

Each country will have a different population pyramid. However, population pyramids can be categorised into three types: stationary, expansive, or constrictive. These types have been identified by the fertility and mortality rates of a country.

Gary Fuller (1995) described a youth bulge as a type of expansive pyramid. Gunnar Heinsohn (2003) argues that an excess in especially young adult male population predictably leads to social unrest, war, and terrorism, as the "third and fourth sons" that find no prestigious positions in their existing societies rationalize their impetus to compete by religion or political ideology.