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.

The demographic transition model demonstrates the changing nature of birth and death rates as related to the economic development of various countries. In the first phase, birth and death rates are both high; in the second phase, death rates fall, but birth rates remain high (leading to rapid growth); in the third phase, birth rates fall to a level similar to death rate and population growth begins to slow; in the fourth phase, birth rates and death rates are fairly stable and very low, which usually leads to slow population growth. Many demographers have in recent years argued for a fifth phase that reflects when death rates begin to outstrip birth rates again, leading to a negative “natural increase rate.” It could be argued that Italy and Japan (to take two examples) are currently in this speculated “fifth phase.”

In stage two of the demographic transition model, the death rate is falling due to improved access to education, health care, shelter, sustenance, etc., but the birth rate continues to be high. This causes massive population growth. This is currently happening in Bangladesh and many of the more developed countries in Africa and South Asia.

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 demographic transition model primarily equates the rate of population growth with the various levels of economic development around the world. The first phase is “preindustrial” and the fourth phase is “industrial.” This model has been criticized by many demographers and geographers as being too simplistic.

The crude birth rate (CBR) is an important statistic that describes the number of births per thousand people in a given period of time (usually a year). The average number of children born to a woman during the course of her lifetime is called the total fertility rate (TFR).

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.

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.

The “demographic accounting equation” is used to predict future population growth for a region or country. It is determined first by calculating the “natural increase rate,” which is the number of births minus the number of deaths. Then by adding the “natural increase rate” to the “net migration number.” The “net migration number” is the number of immigrants minus the number of emigrants in a given time period.