The Demographic Transition Model can be divided into pre-industrial, transitional, and industrial segments. A pre-industrial country (like Somalia, for example) will have high birth rates and high death rates. This is the starting place for all of humanity—many people being born and many people quickly dying. The next phase is transitional; in this phase, first the death rate starts dropping, and then shortly thereafter, the birth rate begins to drop too. The final phase is called "industrial." When a country is well-developed and women are relatively liberated, then birth rates and death rates remain fairly stable and much, much lower than in a pre-industrial society.

In highly developed countries like the United States the natural increase rate is generally very low, it might even be negative. But, the natural increase rate only reflects the changes in population caused by births and deaths; it does not consider immigration and emigration. Much of the population growth in the United States is driven by immigration and the fact that newly arrived immigrants are much more likely to have large families.

The stationary population level (SPL) is the level at which a population cannot grow any more.

The crude death rate or CDR is the ratio of the number of deaths yearly per 1,000 people in a given population.

The stationary population level (SPL) is the level at which a population cannot grow any more.

Demography is the study of human population. Specifically, demography generally involves the study of statistics concerning things like birth and death rates, fertility and public health, income, ethnicity, and outbreaks of disease.

The crude death rate or CDR is the ratio of the number of deaths yearly per 1,000 people in a given population.

In highly developed countries like the United States the natural increase rate is generally very low, it might even be negative. But, the natural increase rate only reflects the changes in population caused by births and deaths; it does not consider immigration and emigration. Much of the population growth in the United States is driven by immigration and the fact that newly arrived immigrants are much more likely to have large families.

All of these countries are in either stage two or stage three (the transitional stages) of the demographic transition model, except Japan, which is in stage four. Stage four is generally considered the final stage of the demographic transition model. In stage four, the birth rates and the death rates are both low and generally steady. In Japan, the death rate is very low and the birth rate is very low, leading to a steady population and perhaps even a slight population decline.

Distance decay is the name of the theory that states that as the distance between two places increases, the interaction between those two places decreases. This is not necessarily determined by the actual geographic distance between two places, but might instead be determined by how easy it is to access one place from another place. For example, London and New York are much further away than London and the Shetland Islands (off the Northern coast of Scotland), but it takes less time to get from London to New York in the modern era.