Human geographers have determined that all nations go through a four-stage process called the demographic transition model (DTM). Developed in 1929 by American demographer Warren Thompson, the DTM’s function is to demonstrate the natural sequence of population change over time, depending on development and modernization. This can help geographers, and other scientists examine the causes and consequences of fertility, mortality, and natural increase rates. Though controversial, the DTM is used as the benchmark for forecasting human population growth regionally and globally.

Stage 1: Low Growth Rate 

We have lived in the first stage of the Demographic Transition Model for most of human existence. In this first stage, CBRs and CDRs fluctuated significantly over time because of living conditions, food output, environmental conditions, war, and disease. However, the natural increase of the world was pretty stable because the CBRs and CDRs were about equal. However, around 8,000 BC, the world’s population began to grow dramatically due to the first agricultural revolution. During this time, humans learn to domesticate plants and animals for personal use and became less reliant on hunting and gathering for sustenance. While this transition allowed for more stable food production and village populations to grow, War and disease prevented population growth from occurring on a global scale.

Stage 2: High Growth Rate

Around the mid-1700s, global populations began to grow ten times faster than in the past for two reasons: The industrial revolution and increased wealth. The Industrial Revolution brought with it a variety of technological improvements in agricultural production and food supply. Increased wealth in Europe, and later North America, because of the Industrial Revolution, meant that more money and resources could be devoted to medicine, medical technology, water sanitation, and personal hygiene. Investments in sewer systems installed in cities led to greater public health improvements. All of this caused CDRs to drop around the world dramatically. At first, CBRs stayed high as CDRs decreased; this caused populations to increase in Europe and North America. Over time, this would change.

Africa, Asia, and Latin America moved into Stage 2 of the demographic transition model 200 years later for different reasons than their European and North American counterparts. The medicine created in Europe and North America was brought into these emerging nations, creating what is now called the medical revolution. This diffusion of medicine in this region caused death rates to drop quickly. While the medical revolution reduced death rates, it did not bring with it the wealth and improved living conditions, and development that the Industrial Revolution created. Global population growth is highest in the regions that are still in Stage 2.

Stage 3: Moderate Growth Rate

Advances in technology and medicine cause a decrease in IMR and overall CDR during Stage 2. Social and economic changes bring about a reduction in CBR during Stage 3. A state moves from Stage 2 to Stage 3 when CBRs begin to drop while CDRs simultaneously remain low or even continue to fall. It should be noted that the natural rate of increase in states within Stage 3 is moderate because CBRs are somewhat higher than CDRs. States that begin to acquire wealth tend to have fewer children as they move away from rural-based development structures toward urban-based structures because more children survive, and the need for large families for agricultural work decreases. Additionally, women gain more legal rights and chose to enter the workforce, own property, and have fewer children as states move through Stage 3.

Europe and North America have moved through Stage 3 of the demographic transition model. The United States, Canada, and countries in Europe entered this stage in the early 20th Century. Latin American states entered this stage later in the century.

Stage 4: Low Growth Rate

A state enters Stage 4 of the demographic transition model when CBRs equal to or become less than CDRs. When CBRs are equal to CDRs, a state will experience zero population growth (ZPG). It should be noted that sometimes a state could have a slightly higher CBR, but still experience ZPG. This occurs in many countries where girls do not live as long before they reach their childbearing years due to gender inequality.

When a country enters Stage 4, the population ages and fewer children are born. This creates an enormous strain on the social safety net programs of a country as is tries to support older citizens who are no longer working and contributing to the economy. Most of Europe, the United States and Canada can be found in this stage.

A state in the first two stages of the transition model will have a broad base of young people and a smaller proportion of older people. A country in Stage 4 will have a much smaller base of young people (fewer children), but a much larger population of elderly (decreased CDR).  A state with a large youth population is more likely to be rural with high birthrates and possibly high death rates. This can tell geographers a lot about the health care system of that country. Moreover, a country in Stage 4 with a large elderly population will have much fewer young people supporting the economy. These two examples represent the dependency ratio, mentioned earlier in this chapter. This ratio is the number of people, young and old, who are dependent on the working force.

Stage 5: Negative Growth Rate

In Stage 5 of the DTM a country experiences loss to the overall population as the death rate becomes higher than the birth rate. The negative population growth rate is not an immediate effect however. Based on demographic momentum, in which total population growth increases even while birth rates decline, it will take a generation or two before a negative population growth rate is observed.

In this scenario it is the economy that is the driving force behind further limits on family size and the use of contraception. Whether persuaded by the high costs of raising a family in cities or the enticing opportunities of employment that delay child bearing, birth rates decline well below replacement level (2.1 children per woman). What occurs is an aging citizenry that will eventually lead to a decrease in total population. Other explanations for declining birth rate can be linked to biological or political causes, and consequently fertility rates differ from country to country.

In recent years a few countries, primarily in Eastern and Southern Europe, have reached a negative rate of natural increase as their death rates are higher than their birth rates. Possible examples of Stage 5 countries are Croatia, Estonia, Germany, Greece, Japan, Portugal and Ukraine. According to the DTM each of these countries should have negative population growth but this has not necessarily been the case. Complicating the Demographic Transition Model’s framework is the impact of migration across national borders. Even with smaller birth rates countries are still growing because of positive net migration rates.