Populations in Stage I of demographic transition have these features. Stage II features a high birth rate, decreasing death rate, and slowly increasing population growth. Stage III features a decreasing birth rate, low death rate, and rapidly increasing population growth. Stage IV features a low birth rate, low death rate, and a stagnating population. Stage V is hypothetical and features the birth rate dropping below the death rate, causing a very slow population decline.

The demographic accounting equation is used to predict population growth and future population of a country or region. It takes into account birth rates, death rates, immigration, and emigration and so is seemingly quite thorough. The problem with the demographic accounting equation is that all of these statistics (birth rates, death rates, immigration rates, emigration rates) are subject to dramatic change over time. As a result, using contemporary statistics to project into the future becomes a dubious proposition.

The physiological population density of a country refers to the number of people in that country divided by the size of arable land in that country. It essentially refers to how much land is being used to provide sustenance for the population of a country. It is different from arithmetic population density, which simply refers to the number of people in a country divided by the size of the total land area of that country.

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).