The dependency ratio of a country reflects the number of people in a country whose labor supports the rest of the country that is incapable of working. It is usually determined by adding the number of retired people and children on one side, and the number of working adults on the other. In a country where the population is aging, like Italy and much of Northern Europe, the dependency ratio is growing as fewer and fewer people are supporting more and more retirees. Some geographers would argue that this is “unsustainable,” but there is as of yet insufficient evidence to conclusively say this is true.

In recent years, many politicians, scientists, and activists have raised the issue of the environmental changes to the Earth being caused by human consumption of resources. Almost all informed individuals agree that such extensive consumption and population growth cannot continue without dire consequences. While people in the developed world tend to decry overpopulation and rapid population growth as the main cause of environmental problems in the world, most politicians and activists in the developing world blame the disproportionate consumption of resources by the developed world.

The “dependency ratio” refers to the percentage of people within a population who are either too young or too old to work and must therefore be supported by the labor of working adults within that population. As the “baby boomers” continue to age, they exacerbate the burden on the Social Security system and the average American worker. In the United States, this is somewhat tempered by immigration and the high natural increase rate of the immigrant population, but in some European countries, the problem could soon become disastrous if the aging of the population continues.

A population pyramid is used to show the percentage of the population who fit into various age groups. If a population pyramid is triangle-shaped that means the bottom section (where babies and children are represented) is bigger than the middle section (where working adults are represented), which is in turn bigger than the top section (where retired older people are represented). This means that much of the population is young and that the population is increasing rapidly. A country like Nigeria or Bangladesh would have a triangle-shaped population pyramid.

Crude birth rate indicates the number of live births occurring during a year per every 1,000 people in a given population in a given year.

The “Green Revolution” is the name given to a series of technological innovations in the 1940s, 50s, and early 60s that led to a massive growth in agricultural production around the world in the late 1960s. This led to massive population growth, particularly in the developing world.

The “natural increase rate” is a statistic used to measure the growth of population in a region, exclusive of immigration and emigration. It only considers the crude birth rate and the crude death rate. If the death rate is higher than the birth rate, then the region has a “negative natural increase rate.”

The total fertility rate is the average number of children born to each woman in a given region during the course of her lifetime. Anything higher than four is a very high total fertility rate and anything lower than two is a very low total fertility rate. Most of the developed world is fairly close to two and much of the developing world is close to, or in excess of, four.

The maternal mortality rate reflects the number of women, per thousand, who die during childbirth. So, if the maternal mortality rate is high (like in Sierra Leone or Bangladesh) then the number of women who die during childbirth is high. To provide possible clarification, “maternal” means motherly and “mortality” means death.

The "baby boom" generation began in 1946, the year after the end of the Second World War, and lasted until 1964. This generation is primarily unique to the United States, although other Western countries experienced a similar spike in birth rate and lowering of the death rate. The baby boom generation began when the millions of soldiers returned home from war and were offered unprecedented job opportunities and access to higher education. Marriage rates and fertility rates both rose as the population skyrocketed.