Demographic Transition Model
Definition: The Demographic Transition Model (apprev. DTM) has five stages that can be used to explain population increases or decreases. The DTM is a key tool for understanding global and regional population dynamics. You need to be able to recognize the 5 stages of the DTM when looking at a population pyramid.
Stage one of the DTM has a high birth rate and a high death rate. Because of this the natural increase rate is close to zero. Zero population growth is when the crude birth rate and crude death rate are equal and the population remains the same. The birth rate and the death rate are both high and equal each other out.
No official country in the world is currently in stage 1. There may be small communities of humans that are in stage 1 of the DTM. Some Indigenous groups in the Amazon or Sub-Saharan Africa that are in stage one, but not all pre-contact Indigenous peoples have high birth rates and high death rates.
For most of human history, the entire world was in stage one. For example, 30,000 years ago, the life expectancy of humans was around 30 years
. Remember that it took the world about 100,000 years to reach one billion people!
Stage two is the early expanding stage where population begins to rise. It has a high birth rate, but the death rate drops. Because of this the natural increase in population rate goes way up! Infant death rates are often high in stage 2 communities but people who do survive birth live longer. A key feature of stage 2 of the Demographic Transition Model is the emergence of grandparents. Longer life expectancies allow for 3 generations to share a part of their live spans. Grandparents are part of every stage of the DTM, but will be more rare in societies with shorter life expectancies.
Identify stage 2 of the DTM on a population pyramid. A number of countries in Sub-Saharan Africa (Niger, Democratic Republic of the Congo, Somalia) are currently in stage two. This is the population pyramid for Niger from 2019. The population of of Stage 2 countries is rising and their doubling time is short. However, the population will not continue to go up at the same rate.
The population pyramids
of these countries have a very wide base that gets thinner and thinner the higher you go.
Stage three is the late expanding stage. The birth rate goes down, while the death rate remains low. The population continues to grow, but not nearly as quickly as stage two because low births and low deaths are at equilibrium. The natural increase rate is still positive, but not as high as in stage two.
More adults often means more workers. Stage three countries start to become more industrialized, which means they are more urbanized and the total fertility rate goes down. Countries that are currently in stage three are Mexico, India, Colombia, and South Africa.
The population pyramids
of these countries are wider in the middle ages and have more of a pear shape.
Stage four is the low stationary phase. With countries in stage 4 the birth rates get lower, while death rates start to rise as people are getting older. The natural increase rates (NIR) in these countries is close to zero.
For example, in the United States the NIR would be zero if you took away the net-in migration that it has every year. Other countries currently in stage four are China, Brazil, and Argentina.
The population pyramids
of these countries are even throughout the age groups and somewhat resembles a skyscraper.
Stage five countries the birth rates remain low, and the death rates go up. Because of this, countries have a negative NIR, which leads to the population decreasing. These countries have graying populations, with less men and women in their childbearing years.
Countries currently in stage five are Japan and a number in Eastern Europe (Germany, Estonia, Ukraine). Fewer young adults are having children. Some stage 5 governments promote pro-natalist policies to try and stunt the population decrease by incentivizing having children.
in these countries are wider at the top and start to look like an upside down pyramid.
Epidemiological Transition Model
The ETM describes causes of death in each stage of the DTM. The Epidemiological Transition Model focuses on why death rates are high or low.
Most people die because of pandemics, like infectious and parasitic diseases (the Black Plague and Malaria).
They will also die because of environmental factors like drought, earthquakes, floods, and also things like starvation and malnutrition. More infants die overall.
People begin to live longer because of change in conditions. Changes that can move a society from stage 1 to stage 2 are improved nutrition, breakthroughs in medicine, end to warfare, and/or improved sanitation. In this stage, not as many people die of infectious diseases because of epidemiology, which is the branch of science that studies disease, the causes and cures.
Because of better infrastructure (hospitals, sewers, better plumbing) less people die of parasitic diseases.
People are living much longer. However, chronic diseases associated with age become a challenge. Things like cancer and heart disease are the leading causes of death.
People are still living longer, but because of better healthcare, treatments, and medical technology people are able to survive cancer and heart disease.
Stage five has the highest death rates because the population is older. This leads to a negative NIR.
Experts cite three different reasons for this.
The first is disease evolution. Infectious diseases have evolved and established resistance to drugs and other treatments.
The second is poverty. Even the most modern societies have homelessness and poverty. Infectious diseases spread more easily in these pockets of society.
The last is increased connections. Through air travel, trains, highways the world is more connected than ever. Because of this, diseases like AIDS, which spread from person to person via blood or sexual fluids, can be unknowingly spread around the world.