Population pyramid

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A population pyramid, also known as an age-sex pyramid, is a graphical illustration that shows the distribution of various age groups in a population (typically that of a country or region of the world), which normally forms the shape of a pyramid. It typically consists of two back-to-back bar graphs, with population plotted on the X-axis and age on the Y-axis, one showing the number of males and one showing females in a particular population in five-year age groups (also called cohorts). Males are conventionally shown on the left and females on the right, and they may be measured by raw number or as a percentage of the total population.

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 wide base of the pyramid indicates a large number of children, and the steady upwards narrowing shows that more people die at each higher age band. The pyramid indicates a population in which there is a high birth rate, a high death rate and a short life expectancy. This is the typical pattern for less economically developed countries, due to little access to and incentive to use birth control, negative environmental factors (for example, lack of clean water) and poor access to health care.

Note that there tend to be more females than males in the older age groups, due to females' longer life expectancy.

Types of population pyramid
While all countries population pyramids differ, three types have been identified by the fertility and mortality rates of a country.

Stationary pyramid - A population pyramid showing an unchanging pattern of fertility and mortality.

Expansive pyramid - A population pyramid showing a broad base, indicating a high proportion of children, a rapid rate of population growth, and a low proportion of older people.

Constrictive pyramid - A population pyramid showing lower numbers or percentages of younger people.

Youth bulge

 * See also Baby boom



The expansive case was described as youth bulge by Gary Fuller (1995). 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. Heinsohn claims that most historical periods of social unrest lacking external triggers (such as rapid climatic changes or other catastrophic changes of the environment) and most genocides can be readily explained as a result of a built up youth bulge, including European colonialism, 20th century Fascism, and ongoing conflicts such as that in Darfur and terrorism. One problem with this line of reasoning is that under conditions prevailing before the introduction of modern medicine, death rates were much higher than they are now and almost all societies had youth bulges, even when their population growth rate was negligible. It's not just that most periods of unrest occurred in societies with youth bulges, but that most pre-modern periods of any sort existed in societies with such bulges.

Another problem is that it ignores the social consequences of poverty, corruption and mass unemployment among young males in developing countries, where most of the world's current population growth is occurring. The "youth bulge" is not an accurate predictor of social unrest, war and terrorism, because they are the product of far more complicated and interlated set of factors of which demographics only plays a part.

Youth bulge theory represents one of the most recently developed theories of war and social unrest and has become highly influential on U.S. foreign policy as two major U.S. proponents of the theory, U.S. political scientist Jack Goldstone and U.S. political scientist Gary Fuller, have acted as consultants to the U.S. government. Samuel Huntington has adapted youth bulge theory as the foundation of his clash of civilizations model: "I don’t think Islam is any more violent than any other religions, and I suspect if you added it all up, more people have been slaughtered by Christians over the centuries than by Muslims. But the key factor is the demographic factor. Generally speaking, the people who go out and kill other people are males between the ages of 16 and 30".

Uses of population pyramids
Population pyramids can be used to find the number of economic dependents being supported in a particular population. Economic dependents are defined as those under 15 (children who are in full time education and therefore unable to work) and those over 65 (those who have the option of being retired). Of course, in some less economically developed countries children start work well before the age of 15, and in some more economically developed countries it is not usual to start work until 18 or 21, and people may work beyond the official retirement age of 65, but the definition provides an approximation. The government must plan the economy in such a way that the working population can support these dependents.