- Stage 1: Low Growth-Stage one areas have existed for most of Earth's history. High changes of crude and death rates between years were common and varied over most areas. Over long time periods, the CBR and CDR proved to be very high with an NIR near zero and an unchanging population. Around 8000 B.C. Earth's population exploded from 5 to 800 million people due to the agricultural revolution. The revolution occured after humans began using domesticated animals and raising plants and animals as resources. Today, there are no countries that remain in stage one.
- Stage 2: High Growth-A sudden burst of population occured in the late 18th century as several counties moved to stage 2 of the model. During Stage 2, CDR drops dramatically while CBR remains the same. The NIR becomes drastically higher and leads to much higher population growth. Industrial revolutions and developments in healthcare and hygiene can lead to stage 2 growth patterns. LDCs move to stage 2 as medical technology filters from developed countries and increases their life expectancies and progress. Medical revolutions are important in this process. Many African countries remain in Stage 2. Cape Verde is one country off the coast of West Africa that continues to have a high birthrate and dropping CDR.
- Stage 3: Moderate Growth- A drop in the CBR signals the change from stage 2 to stage 3. The CDR drops again, but not drastically while the CBR becomes much lower. Population growth is still steady as CBR remains above CDR, but the gap narrows. Most developed countries moved to this stage during the early 20th century. Changes in social aspects of society lead to drops in CBR. People choose to have and need less children. As more infants survive, less are born and less are used to complete chores or support parents on farms. Urbanization decreases the ability to have many children because of limits on space and jobs. Many Latin American countries, such as Chile, are now in stage 3. Chile moved to stage 3 in the 1960s with support from government through use of family planning and economic development.
- Stage 4: Low Growth- When the CBR equals the CDR a country enteres stage 4. A country in stage 4 has Zero Population Growth as the NIR gets close to zero. Most European countries, for example Denmark, have entered stage 4, as has the United States. Social customs such as increasing women working, birth control, and new groups of people that do not desire children contribute to the low CBR.
Population Structures
Population Pyramids are often used to display the general shape of a population. They show the different percentages of the population that exist in 5 year age groups. The youngest group (0-4 yrs) becomes the base with older groups at the top. Males are usually on the left and females on the right.
Age Structures become evident through population pyramids. The different ways that the age groups are structured can clue the reader in to how a population is developing. Many stage 2 countries would have a broad base and small top, while stage 4 countries would look more like a square shape. Dependency Ratio shows the number of people that are too young or old to work compared to groups that make up the productive members of society. Large amounts of dependents can strain social programs such as healthcare, schooling, and aid to the poor. Often these people do not have a way to get help and cannot become productive parts of society.
Sex Ratios also become clear through population studies. Sex Ratios are the number of males per hundred females in a group. In general more males are born, but only slightly. Sex ratios in Europe and North America are different from the rest of the world with around 95 males for every 100 females.
Worldwide Impact
Worldwide, two big breaks are needed to increase the social progress of areas across the world.
Drop in CDR due to technology
Drop in birth rate due to social customs
