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Population Pyramids Get the lesson plan here
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A population pyramid is a graph that allows us to see the gender and age structure of a population. There are different shapes to the pyramids which tell us different things about the population of the country. It is often useful to divide population pyramids into 3 distinct age groups or COHORTS- 0-15 or the Young, 16-65 or the working age and 65+ as the retired sectors of the population.  Knowing the percentage of people in these sectors can allow us to calculate a Dependency Ratio. This is the ratio between those of working age and those of non-working age. The no-working age are known as DEPENDENTS because they depend upon the working age people to provide for their needs.  This is calculated as:

The ratio for an MEDC usually lies between 0.5 and 0.75. The ratio for an LEDC is typically higher. Mexico, with a youthful population structure, has a dependency ratio of 1.04. The higher ratio, the greater the number of dependents that have to be provided for from the taxes on the workforce. A ratio of 1 means that for every working age person there is one dependent person.  The UK had a ratio of 0.49 in 2010 (CIA FACTBOOK) meaning that for every young or old dependent person there were approximately 2 working age people.

Stage 1 LDCs (Least Developed Countries) that are High Fluctuating

These countries have a concave profile because they have high birth rates and high death rates, plus low life expectancies.  Very few people survive to old age (the average life expectancy in Sudan was 52 years in 2010) and birth rates are very high.  This cause the population at the base to be prominent and grow through momentum every year, but these groups are compensated by 

Stage 2 LEDCs that are RAPIDLY GROWING

LEDCs have a triangular shaped pyramid. They have lots of children and people do not tend to live for a long time (low Life expectancy). These countries populations grow rapidly as many more children are added to the population than people die. The Philippines pyramid shows this perfectly and has a population growing at 1.93% per year.  This does not sound like a big percentage. However, when you do the math, 1.93% of the 99.99million people that live in the Philippines means that the population should grow by 1.9 million people!  These are typical of countries at STAGE 2 of the Demographic Transition Model and have HIGH DEPENDENCY

Stage 4 MEDCs with STABLE POPULATIONS

Most MEDCs have a space rocket shape, with old people living for a long time (high life expectancy), lots of workers and reasonable numbers of children. These populations are stable and are growing slowly as the number of young is just above the number of people dying.

Some MEDCs actually have declining populations where there are not enough children being born each year to replace those dying. Germany is experiencing a period of negative growth (-0.1%). As negative growth in a country continues, the population is reduced. A population can shrink due to a low birth rate and a stable death rate. Increased emigration may also be a contributor to a declining population.

Below you can see how these pyramids relate to the Demographic Transition Model

Population pyramid typical of stage 1

A population pyramid typical of stage 1 - note the wide base, low life expectancy and concave profile
Population pyramid typical of stage 2

A population pyramid typical of stage 2 - note the wide base, lengthening life expectancy and pyramidal profile
Population pyramid typical of stage 3

A population pyramid typical of stage 3 - note the narrowing base, increasing life expectancy and rocket shaped profile
Population pyramid typical of stage 4

A population pyramid typical of stage 4 - note the narrowing base, long life expectancy and vase shaped profile
Population pyramid typical of stage 5

A population pyramid typical of stage 5 - note the ever diminishing base and very long life expectancies
Population pyramids and how they link to the Demographic transition model