Demographic transition theory

The "Demographic Transition" is a model that describes population change over time. It is based on an interpretation begun in 1929 by the American demographer Warren Thompson, of the observed changes, or transitions, in birth and death rates in industrialized societies over the past two hundred years or so.

The demographic transition model seeks to explain the transformation of countries from having high birth and death rates to low birth and death rates.

The model is based on the change in crude birth rate (CBR) and crude death rate (CDR) over time. Each is expressed per thousand populations. The CBR is determined by taking the number of births in one year in a country, dividing it by the country's population, and multiplying the number by 1000.

The crude death rate is similarly determined. The number of deaths in one year is divided by the population and that figure is multiplied by 1000.

Stage I

Prior to the Industrial Revolution, countries in Western Europe had a high CBR and CDR.

Birth Rate is high as a result of:

• Lack of family planning 
• High Infant Mortality Rate: putting babies in the 'bank' 
• Need for workers in agriculture 
• Religious beliefs
• Children as economic assets

Death Rate is high because of:

• High levels of disease 
• Famine 
• Lack of clean water and sanitation 
• Lack of health care 
• War 
• Competition for food from predators such as rats
• Lack of education

The high CBR and CDR were somewhat stable and meant slow growth of a population.

Stage II

In the mid-18th century, the death rate in Western European countries dropped due to improvement in sanitation and medicine. Out of tradition and practice, the birth rate remained high. This dropping death rate but stable birth rate in the beginning of Stage II contributed to skyrocketing population growth rates. Over time, children became an added expense and were less able to contribute to the wealth of a family. 

Death Rate decreases as a result of:

• Improved health care (e.g. Smallpox Vaccine) 
• Improved Hygiene (Water for drinking boiled) 
• Improved sanitation 
• Improved food production and storage 
• Improved transport for food 
• Decreased Infant Mortality Rates

Stage III

Along with advances in birth control, the CBR was reduced through the 20th century in developed countries. Populations still grew rapidly but this growth began to slow down. 

There are several factors contributing to this eventual decline:

• Infant Mortality Rate declined.
• Increasing urbanization changes the traditional values placed upon fertility and the value of children in rural society. Urban living also raises the cost of dependent children to a nuclear family (education acts and child labor acts increased dependency through the late 1800s). People begin to assess more rationally just how many children they desire or need. Once traditional patterns of thinking are broken the decline is likely to accelerate.
• Increasing female literacy and employment lower the uncritical acceptance of childbearing and motherhood as measures of the status of women. 
• Improvements in contraceptive technology.

Stage IV

In the late 20th century, the CBR and CDR in developed countries both leveled off at a low rate. It is characterized by stability. In this stage the population age structure has become older. In some cases the fertility rate falls well below replacement and population decline rapidly.

Drawbacks of Model

The model does not provide "guidelines" as to how long it takes a country to get from Stage I to III. Western European countries took centuries through some rapidly developing countries are transforming in mere decades. The model also does not predict that all countries will reach Stage III and have stable low birth and death rates. There are factors such as religion that keep some countries' birth rate from dropping.