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.

A population pyramid is used to show the percentage of the population who fit into various age groups. If a population pyramid is rectangular, that means the bottom section (where babies are represented), the middle section (where working adults are represented), and the top section (where older people who are often retired are represented) are roughly similar sizes. This generally means the population of a country is close to steady, but is possibly growing slightly. A country like Norway or the United Kingdom would have a rectangular population pyramid.

Most geographers and demographers agree that over the course of the twenty-first century, the population growth will slow considerably. This is based on the belief that the current rapid population growth is an anomaly in Earth’s history and that the planet cannot sustain such rapid growth for another century.

The Natural Increase Rate and the Demographic Accounting Equation are both calculations used to determine the current or projected population growth of a region. The Demographic Accounting Equation is more comprehensive because it takes into account immigration and emigration, whereas the Natural Increase Rate only takes into account the birth and death rates.

An Essay on the Principle of Population, published at the end of the eighteenth century, was written by Thomas Malthus. In this essay, Malthus argues that the rate of food production grows arithmetically, whereas population grows exponentially. Malthus argued that this would lead to a situation where population outstrips food production, leading to widespread famine and starvation. His writings are extremely influential even to this day; however, many geographers dispute his conclusions and argue that technology augments food production, allowing it to keep up with population growth. It remains to be seen whether Malthus’ predictions will come true in the twenty-first century or beyond.

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.

Malthus’ “principle of population” states that eventually, population growth will exceed the capacity of agricultural production, which will lead to widespread starvation and suffering. Over the years, many detractors have proposed various arguments in opposition to Malthus’ theories. Two of the most prominent arguments are that a) population growth will slow due to the availability of contraception and the desire of governments to limit growth and b) technological innovation can lead to dramatic improvements in agricultural production, which allow it to keep up with the growth in population.

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.

The natural increase rate does not consider immigration or emigration. It is determined by subtracting the number of deaths from the number of births in a given region. So, if there are ten thousand deaths and thirteen thousand births in a given region, in a given time period, the natural increase rate is three thousand.

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.