Population pyramid
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
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. However, they certainly did not experience such youth bulge as prevails today in some parts of the world or as prevailed in twentieth century Germany or in Africa and the Middle East nowadays. It's not just that most periods of unrest occurred in societies with youth bulges, but that some of the pre-modern periods of any sort existed in societies with such bulges as well. Nevertheless since the improvement of medicine and its introduction the element of youth bulge has become far more salient than before. Therefore perhaps it cannot explain massacres throughout human history, but it can serve as rather plausible theory to explain the terror, social unrest, and uprisings of today.
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 interrelated set of factors of which demographics only plays a part. Yet even when there are other factors and circumstance to enable mass unrest, a youth bulge is likely to be one of them.
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 dont think Islam is any more violent than any other religions, 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".
- Afghanistan shows a classical youth bulge.
Afghanistan shows a classical youth bulge.
- Angola shows the same, even more pronounced.
Angola shows the same, even more pronounced.
- China had an extreme youth bulge until the 1960s, since sharply curbed as an effect of the one-child policy.
China had an extreme youth bulge until the 1960s, since sharply curbed as an effect of the one-child policy.
- Compare the population pyramid of the USA which was bulging until the 1960s and has steadily slimmed since.
Compare the population pyramid of the USA which was bulging until the 1960s and has steadily slimmed since.

# 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 developed countries children start work well before the age of 15, and in some developed countries it is not unusual to start work until 18 or 21, and people may work beyond the age of 65. Therefore, the definition provides an approximation. In many countries, the government plans the economy in such a way that the working population can support these dependents. Age pyramid has similar uses to population pyramid.

# Notes
-  Goldstone, Jack A.: "Revolution and Rebellion in the Early Modern World", Berkeley 1991
-  Fuller, Gary: "The Demographic Backdrop to Ethnic Conflict: A Geographic Overwiew", in: CIA (Ed.): "The Challenge o Ethnic Conflict to National and International Order in the 1990s", Washington 1995, 151-154
-  'So, are civilizations at war?', Interview with Samuel P. Huntington by Michael Steinberger, The Observer, Sunday October 21, 2001.