Population growth is not as simple as watching the total number of humans rise each year. This lesson explores how the experts actually calculate population growth rates, and how different theories explain why various regions of the world experience population growth changes in such different ways.
Understanding Population Growth Rate
As of 2014, the world population has reached over seven billion human inhabitants. This is quite a jump from just 500 years ago, when the world population was likely under one billion. Although the population growth rate is actually down as of 2013, at around 1%, compared to a recent peak of 2.2% in 1963, the overall population of the world continues to increase in sheer numbers.
To understand population growth, we must separate the rate of growth from the total numbers themselves. Even though the world population has continued to rise in number, the rate at which the population is growing has slowed. Most specifically, different populations, or groups of people living in specific areas, around the world have been experiencing shifts in their population growth in very different ways. What accounts for these differences?
Although there are several models for observing and predicting the world's population growth (or shrinkage), one of the most commonly accepted is the demographic transition model (DTM). To calculate population growth, most scholars use the following equation, which captures both natural (birth and death) and mechanical (immigration and emigration) changes in population:
Population Growth Rate (%) = (Births - Deaths) + (Immigrants - Emigrants)
Malthusian theory is the term used to describe the position of the 19th-century political economist, Reverend Thomas Robert Malthus, in his arguments about how and why population changes. His works, such as An Essay on the Principle of Population, seek to explain the natural pushes and pulls of population fluctuation.
At their core, his arguments remind us that humans are ultimately tied to their environment. Even the most highly developed society can be obliterated by a severe enough drought or plague. Even with the most modern technological advancements in agriculture, Earth will eventually reach its carrying capacity, rendering further population growth impossible.
Malthus' theory consists of principles, the first of which states that human population grows at a geometric rate, or exponentially with each generation. His second principle points to the difference between this geometric rate of growth for human populations and the arithmetic rate of food production, which means that with each generation, the food supply will only increase by the same set number.
For example, although each woman is projected to produce four children, and each of those children will produce four children, the food supply will only increase by two units each generation. According to these first two principles, the population will outgrow the food production capabilities at a certain point in the future.
Demographic Transition Model (DTM): Four-Stage Theory
A commonly used phrase in the discussion of population growth is demographic transition, which describes a progressive movement from high birth and death rates to low birth and death rates. The demographic transition theory argues that population growth is inextricably tied to a society's level of technology. As the society advances in its usage of technology, in every industry from healthcare to crop production, its birth and death rates shift, directly impacting the population numbers and growth rates.
Stage one of the demographic transition model, or the DTM, is associated with pre-industrial society. In the first stage of human society, birthrate is high and death rate is high, producing a population with a relatively stable size and a slow growth rate. Closely tied to the nature and the limitations of geographical resources, this stage was observable for most of human history.
If the land was fertile that year, then the people were fertile that year. If drought or disease became severe and prolonged, the population shrank. Stage one follows Malthusian theory closely, as it reveals the inherent weakness of a population to thrive beyond the carrying capacity of its natural environment.
Stage two, the next stage, takes place in a developing nation. In the second stage, although the birth rate remains high, the death rate drops, particularly as it applies to infant mortality. Children under five are no longer dying at such high rates, leading to a larger youth population in a given community. This resulting population pyramid, with the large group of youth at the base, reveals a rapidly growing population. This imbalance can be temporary; it occurs as the result of a sudden improvement in the quality of life.
For example, the Industrial Revolution of Western Europe in the mid-1800s led to advances in sanitation, hygiene, and public health initiatives. At the same time, advances in agricultural technology led to higher and more reliable crop yields. As women and children gained access to education and opportunity off the farm, the new urban lifestyle led to a rapid change in population growth. Population growth rates exploded!
Stage three can be seen in a newly developed nation. In the third stage, birthrates drop while death rates remain constant. Attributed to the continued shift in values and lifestyle in the post-Industrial Revolution urban society, women choose to have fewer children. Technology allows for contraception, although it is usually a slow introduction into mainstream usage. More access to health education and more emphasis placed on children remaining in school both contribute to a drop in birthrate.
Suddenly, it was more expensive to raise children, with school clothes and books becoming a necessity even in poor communities. Pressure mounted to raise educated, productive members of society, replacing the previous practice of producing as many children as possible to aid in farm work. During this stage, the population pyramid rounds out, with relatively even numbers of youth and aging members.
The last known stage, stage four, can be observed for an established, developed nation. In the fourth stage, both birth rates and death rates drop, meaning that population growth is actually stable. The high levels of food production and other technology of established, developed nations reveal that, contrary to Malthus' ideas, society can become noticeably detached from the cycles of nature, such as drought and famine. Additionally, if birthrates drop below replacement rates (two children per couple), the population actually shrinks over time.
This phenomenon is particularly dangerous to society's stability in the first couple of generations of shrinkage, as the still-large aging population retires and relies on a smaller pool of working youth for support. Other complications in this stage include the possibility of the aging population experiencing a slight increase in the death rate, due to the increased rates of obesity and related diseases, which were historically only applicable to the wealthy upper class.
Potential for Additional Stages
Is there an agreed-upon next step? Studies in the science of futuring, the field where experts predict several potential scenarios for how the future could look, have led to two distinct options for a stage five. Either the population shrinkage of stage four's developed nations continues until the society experiences a devastating loss of population, or the developed nation turns away from this shrinkage in what some sociologists recognize as the J curve. In the latter scenario, highly developed nations might eventually experience a rebound in fertility rates, leading to a more rapidly growing population than is commonly seen in developed nations.
Missing Factors in the DTM
What about mass migration? What about an AIDS epidemic? What about a country where the authoritarian government style or the strict religious community does not allow the population to take advantage of the available technology, from birth control pills to antibiotics, that other comparable nations are enjoying?
The DTM cannot cover every nation and every 'what-if' situation. It, like most theories, has its limitations and weaknesses. Although these four stages have been observable in many nations throughout history, they're not so applicable for some. Historically, the United States flipped the order of progression by first experiencing a birthrate drop and later experiencing the death rate drop - with approximately 100 years in between the two!
What will happen in the future is only a guess. Yes, we can make educated guesses, and futuring scientists can certainly apply a variety of methods and techniques to predict how things could play out. While the demographic transition model (DTM) seems to explain the shifts in population nicely and neatly, exceptions abound, particularly in very poor nations. Which regions of the world will have stable population growth in the next century? In the end, we'll just have to wait and see.
Once you've finished with this lesson, you should have the ability to:
- Describe the Malthusian theory of population growth
- Summarize the four stages of the demographic transition model (DTM)
- Define the science of futuring and identify a potential fifth stage of the DTM
- Explain the limitations of the DTM