Download Factors Affecting Population Growth: Exponential vs. Linear - Prof. John M. Pratte and more Study notes Physics in PDF only on Docsity!
Population
Introduction
On October 12, 1999, the Earth welcomed a newborn child that brought our planet's population up to the 6 billion mark, according to the U.N. Population Fund. While this is an astoundingly large number of people to have on Earth, what makes this figure even more remarkable is that as recent as 1960, there were only a little over 3 billion people on the planet. For the last century, we have been averaging a rate of increase in the world's population that would double it about every 50 years. If this continues, it means that we will have close to 12 billion people on the planet by the middle of this century. By some people's estimates, this would amount to complete environmental devastation if it were to happen.
While the world's population has been growing at a staggering pace, it has not been a homogenous, global growth. Some areas of the world are not experiencing any growth at all. In fact, some areas of Europe are actually experiencing a decline in their population. Other areas of the world offset these declines by growing at a rapid clip. Both of these situations bring about their own unique set of problems. In this chapter, we will look at the factors that affect population and discuss how global population might change over the coming century. Before we do that, we need to define a few terms and formulae that are important to this discussion.
Demographic Basics
Demography is the study of populations, whether they be human, plant, or animal. Human demography is that subdiscipline that looks specifically at humans, and it relies very heavily on statistics, geography, psychology, and sociology. Like any area of study, it has its own unique terms that, while they exist in everyday language, describe something very specific that might be different from their everyday definitions. For example, demographers have several terms that they use to describe how a population can change. The terms that they use are birth rate, death rate, natural growth rate, growth rate, replacement-level fertility, and total fertility rate. The first two of these are quite simple and are what you would expect.
Birth rate : Commonly used as the crude birth rate , this is the number of births per 1,000 people within a certain population.
Death rate : Commonly used as the crude death rate , this is the number of deaths per 1,000 people within a certain population.
The formulae for determining crude birth rate and crude death rate for a given time interval, such as a year, are as follows:
total number of births in a population x 1,000 = crude birth rate total population
total number of deaths in a population x 1,000 = crude death rate total population
The next two terms are not quite as obvious, but are easily explained. The natural growth rate is just the crude birth rate minus the crude death rate, i.e. the number of people who are born minus the number that died per 1,000 people. Many people who read this think “Well, that is obvious. The growth is just how many new babies are born minus the number of people who die.” However, this neglects the effect of immigration and emigration, and it is why this term is called the natural growth rate. The term growth rate includes the effects of immigration (people entering a country) and emigration (people leaving a country) and is defined as the natural growth rate plus the migration rate (immigration minus emigration).
To explain this another way, let us look at an example. Suppose that a country of 100,000 people has 15,000 births in a year in which 10,000 people die. At the same time, 540 people move into the country to settle and 780 leave the country live in the Arctic tundra of Canada. With these values, the terms we have defined so far are:
birthrate = x =
deathrate =
x 1,000 = 100
natura lg rowthrate = 150 − 100 = 50
migrationrate =
x 1,000 = −2.
growthrate = natrua lg rowthrate + migrationrate = 50 + (−2.4) = 47.
Thus, this country has a total growth rate of 47.6 people for every 1,000 citizens that it has. At this rate of growth, it would take 21 years for the population to double, as 47.6 people per year times 21 years is equal to 1,000 people (actually, 999.6 people, but there is no such thing as .6 of a person).
We must recognize that this way of expressing the growth is a little different than some other ways of expressing it. In some instances, we express the growth as a percentage of the population. It is easy to convert between the two, as all one needs to do is to divide the crude growth rate by 1,000 people and then multiply by 100%. In our example, this is
percentagegrowth =
47.6 people
1,000 people
x 100% = 4.76%
We need to note, though, that if the population continues to grow at this percentage , then the amount of time to double will not be 21 years; it will be less. The reason for this is quite simple, as growth at a fixed percentage means that each year sees greater overall growth than the year before. In our example, growth at 4.76% means that 4,760 people are added to the country the first year. The next year, the number of people who are added will be 4.76% of 104,760, or 4,762. While this is only 2 people more, this will continue to grow in succeeding years, which means that doubling will be reached sooner, as we will show below.
Replacement-level fertility : This is the average number of births that couples in a population must have in order to keep the population stable. At first glance, one might think that this number is 2.0, as it is a simple matter of each member of a couple replacing himself or herself. However, not all children will survive until they reach puberty when they could legitimately take part in producing the next generation of children. For this reason, replacement-level fertility (RLF) is higher than 2.0 and depends greatly upon the infant and childhood mortality rates of a country. In more developed countries, this number is approximately 2.1, while in developing countries with poor medical care, the number can be as high as about 2.3.
Total fertility rate : This is the average number of children a woman in a population has in her lifetime. Excluding immigration and emigration from the equation, if the total fertility rate exceeds the replacement- level fertility in a population, then the population is growing. If the total fertility rate is below the replacement-level fertility, then the population is reducing. Germany in 2007, for example, had a total fertility rate of 1.4, which is well below the RLF. With this rate, the population of Germany should be shrinking; yet, it is actually stable. The reason for this is in two parts. First of all, Germany is
Some of these textbooks forget, though, to illuminate into what type of society John Eli Miller and his progeny were born when discussing this growth. Miller and his descendents were farmers that were raised in an agrarian community in a developing country (the U.S. was not a fully developed country in the 1800’s). His example is not like one would find from an industrialized community like one would find in a fully developed country like America, Europe, and Japan today or even a semi-industrialized community like that found in a developing country.
As a contrary example, let us look at the Jules Francis Pratte family. Jules was born in the late 1800's in a small town near St. Louis. Like John Eli Miller, he had a fairly large brood of children (Miller had 7 in total, Pratte had 6). However, Pratte's six children only produced 14 grandchildren, much less than the 63 of Miller. These 14 grandchildren only produced 21 great-grandchildren, which is a far cry from the 341 great-grandchildren of Miller. As Figure 1 shows, while Miller’s family growth is exponential (increase by same percentage), Pratte’s is an example of linear growth (increase by same amount).
The difference is that all of Pratte's family have had occupations in the industrial or professional sector of the American economy. This has a profound difference in the rate of growth of a population. In an agrarian society, especially one without modern fossil-fuel driven machinery, every child you have is another free field hand. Children usually start working on the farm at an early age, and will be responsible for producing more food than they eat. This results in a net income for the family. In an industrial society with strict child labor laws, every child you have is another mouth that you have to feed that does not bring in any additional income. In a very real sense, every child you have will be a serious drain on your financial resources. This societal difference accounts for a large portion of the difference that you will find in the population growth rates in the two different kinds of cultures even though most industrialized societies have a longer life span.
Fig. 1: Miller (circles) versus Pratte (diamonds)
Other Growth Factors
While the exact reason a person has a child will vary, it usually falls into one of several categories. The most obvious reason is a biological imperative to procreate, i.e. an innate desire to have children. This is a very strong reason, and should not be neglected. However, there are additional reasons that people have children that go beyond this base desire, and can cause growth even when other factors to stop having children become strong. For example, some people have babies for societal reasons such as to carry on the family name. Anybody who has ever had their mother or father ask them when they are going to get married, settle down, and have kids knows this reason very well. This drive to carry on the family name is very strong and can often overcome drastic measures not to have children. China’s “One Baby Policy” in which couples pay fines and lose health benefits if they have more than one child in urban environments has resulted in a disparity in the number of males and females. Some families have resorted to abortion or giving their child up for adoption if they find out it is a female in order for them to have a male child to carry on the family name. This has led to a situation where there are almost 10% more males under the age of 30 than there are females. Even with the shorter lifespans of males, this ratio might never reverse, and could be a tremendous societal problem for China in the future.
Some people have children for religious reasons. For some faiths, it is a matter of not doing anything to prevent having children. For instance, the Roman Catholic faith believes that it is immoral to use artificial means to prevent conception. For other faiths, there are actual dictates to have more children, as the more children there are, the more souls there will be available to go to a good place in the afterlife. These reasons tend to spawn larger families than are needed to exceed RLF.
These two reasons, along with the basic instinct to procreate, are probably the two strongest reasons for growth in the U.S. and some other developed countries. In developing countries where growth rates are the largest, there are often many other very important reasons. For instance, a lack of education and economic opportunity for women has been very strongly correlated to increased birth rates. Women in these situations generally do not have information about birth control. The lack of economic opportunities further compounds the problem as their only “job” is to be married and have children. This is a double- edged sword. If they are able to find employment, then there is pressure not to have more children, as each child means that they will have to be away from work, and thus, decrease the earnings for the family. If there are no economic opportunities available, then the only way for women to provide for themselves later in life is to have children who will take care of them as they age. Hence, if there are economic opportunities, there is pressure to decrease family size while a lack of them causes pressure to increase it.
Another very important factor in determining growth is the age at which women get married. The average woman has about 30 years in which she can have children. If she starts having children at an early age, then she is much more likely to have a lot of children. This factor is also tied to economic opportunities and education, as these two things often delay the age at women get married.
Developed Countries and Immigration
When George Washington started the first Presidency of the United States of America, he was governing less than 4 million people who occupied an area of 2,300,000 square kilometers 1. It was an agrarian society, with 95% of the population living on farms and only 5% in cities or towns of more than 2,500 2. There was plenty of land, and a vast array of natural resources, just waiting to be tapped.
Today, we live in a country of over 300 million people 3. While we are one of the major food producers in the world, it is no longer due to us being an agrarian society. Today, most people live in towns and cities, with less than 25% of the population living in rural areas. And even though we have grown to a physical size of over 9.8 million square kilometers, our population density has increased from the 2 people per square kilometer of Washington’s day to almost 30 people per square kilometer today.
From where did all of these people come? The increase in territory that we experienced over that time did increase the number of people in the U.S. from the simple fact that there were already people living there. We have also expanded by immigration, with wave after wave of people entering this country to find new life and new opportunity. Neither of these, though, accounts for the largest segment of growth. Both put together still account for less than 80 million people 4. The largest sector of people is here due to birth.
Fig. 2: Historical U.S. Population ( Data: U.S. Census )
This same thing is true of most countries. Immigration is often a small factor to the overall growth of the country. However, it can have a powerful effect on the growth of a country, as the birthrate can be greatly affected by it. In the early days of the U.S., the growth from births came from all sectors of society. However, as the following story shows, the growth of a developed country can come mostly from the descendents of immigrants.
States, thereby promoting environmental sustainability in these countries through foreign aid
initiatives.
Immigration advocates say that the United States should always be a "safe harbor" for victims of
human rights abuses or armed conflict around the world. They also claim that immigrants infuse
skills and labor into the workforce and improve the economy of the host country. Karen’s family
was a classic example. Her grandparents emigrated from Sicily to the vibrant Italian-American
community in New Haven, Connecticut, in the early 1900s and prospered in subsequent
generations through a dedication to education and hard work.
Karen sat back in the chair and sighed heavily. The epiphany, the tiebreaker, or the revelation she
sought had once again eluded her. This wasn't going to be easy. She sat forward, propped her
elbows on the table, stared at her ballot, and began running her fingers through her hair…
If you were Karen, what would you do?
Describe how your family’s immigration history in the United States. Did an understanding of
this history affect your views on the subject?
Do you think the current U.S. immigration regulations are appropriate? Explain why or why not.
If you think current immigration policies are inappropriate, provide specific suggestions on how
they should be modified.
Predicting the Future
For all of its futuristic stylings, the television show Star Trek was, like most science fiction, a commentary on the state of society at the time it was written (late 1960’s). One of the more famous episodes of that series was the one entitled The Mark of Gideon, which dealt with a planet that was so overpopulated that people did not have anywhere to sit down. At the time of the show, the Earth’s population was at about 3.5 billion, and was increasing at an incredible rate. The effects of such a large population on the environment were beginning to become abundantly clear, and people had begun to wonder just how many more people the Earth could hold. This Star Trek episode was just one of a number of television shows, movies, and books at that time (ex. Soylent Green ) that foretold of an ominous future if we did not begin to do something.
All of these shows were predicated on predictions made by demographers at the time of what the world’s population would be at some point in the future. Each generation has done this, sometimes with startling accuracy, and sometimes missing the mark completely. For example, in 1798, Thomas Malthus, an English demographer and economist, predicted that the world’s population would one day outstrip mankind’s ability to feed itself, and that this would be reached when the population was 1 billion people. Of course, Malthus could not foresee the introduction of the internal combustion engine and the production of artificial fertilizers that have increase our capacity to feed.
We now have more than 6.5 billion occupants of Earth, and we are continuing to grow. As previously stated, at current rates of growth, we will reach 12 billion by the year 2050. However, this prediction is most assuredly wrong, as it relies on everything going along just as it is. As Malthus shows us, one cannot assume this. In order to make a more sophisticated prediction, we need to look deeper at the
data to see what it is telling us. One tool that aids in that regard is the age structure diagram, also known as the population pyramid.
Demographic Profiles
While it is sometimes called a population pyramid, an age structure diagram does not have to have a pyramid shape. What this diagram actually shows is the distribution by ages of females and males within a certain population in graphic form. Figure 3 shows a diagram in which the ages and sexes for the United States population are arranged so that ages are grouped together such as 0 – 4 years, 5 – 9 years, and so on. The population of each is group is represented as a bar extending from a central vertical line, with the length of each bar dependent upon the pop total for that particular group. The centerline separates the females from the males. The female and male populations for each group are represented by the distance from t centerline, with females on the right and male
Fig. 3: Age Structure diagram for the U.S.
ulation
he
s on the left.
Age and Sex Distribution
By looking closely at the age structure diagram, one will notice slightly more boys in the younger age groups than girls; however, the ratio tends to reverse in the upper age groups, as females tend to outnumber males. Many countries have a female majority as a result of the longer life expectancy for females. In the United States, this ratio change is clearly shown in the table below showing age and sex distribution in the census year 2000. Notice that at about age 35, the majority changes.
Number Age (^) Both sexes Male Female
Total population 281,421,906 138,053,563 143,368,
Under 18 years 72,293,812 37,059,196 35,234, 18 to 64 years 174,136,341 86,584,742 87,551, 18 to 24 years 27,143,454 13,873,829 13,269, 25 to 44 years 85,040,251 42,568,327 42,471, 45 to 64 years 61,952,636 30,142,586 31,810, 65 years and over 34,991,753 14,409,625 20,582,
Median age (years) 35.3 34.0 36.
more children in the 0-4 age bracket 20 years from now than there are today. Thus, the total population of the country will grow, as all age brackets will have more people in them then than today.
Japan’s diagram shows the opposite of this. In it, you should note how pre-reproductive age groups (0 – 14 years) have smaller populations than the reproductive age groups (15 - 44 years). Thus, unless the total fertility rate increases, there will be even fewer kids in the 0-14 age group 20 years from now than there are today, which means that the population will decrease. Of course, this will present different problems for Japan, as a shrinking younger age group means that there will be fewer and fewer younger workers to support the elderly. One way to rectify this is to increase immigration at the lower age groups, which will bring more workers into the country. While this will provide the necessary workers to take care of the elderly, it can be problematic to implement in a homogenous population like that found in Japan or Italy, another country that has this same problem.
Iceland, on the other hand, shows a more stable population. Except for the post-reproductive groups (45+ years), the populations for the age groups extend generally the same lengths. There is a slight bulge outward, but if you account for infant and childhood mortality, the number of children in the 0-4 age range that make it to the 20-24 age range should be just about the same as the number of adults in that age range today. This means that there should be little to no growth, with the possible exception of people living longer and increasing the raw numbers by essentially extending the diagram upwards.
Additional Reading
U.S. Census Bureau
Topic : Population Projections for the U.S. Summary : Information on projected population increases in the U.S. Link : http://www.census.gov/population/www/projections/ppl47.html
References
1 http://www.census.gov/population/censusdata/table-2.pdf, October 16, 2004. 2 http://www.census.gov/population/censusdata/table-4.pdf, October 16, 2004. 3 http://www.census.gov/main/www/popclock.html, October 16, 2004. 4 http://eh.net/encyclopedia/?article=cohn.immigration.us, October 16, 2004.
