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Ask Dr. Weld 3 — The Demographic Rap: Terms and Definitions

Image Credit: Dr. Madeline Weld.

Madeline Weld, B.Sc., M.S., Ph.D., is the President of the Population Institute Canada. She worked for and has retired from Health Canada. She is a Director of Canadian Humanist Publications and an editor ofHumanist Perspectives.

Scott Douglas Jacobsen: What are some of the more common terms, and their definitions, used with demography?

Dr. Madeline Weld: A glossary of all the common terms would take many pages and probably put most of your readers to sleep. But I’ve listed many of the most relevant terms and invite readers who want to know more to check online glossaries like the ones at the links below.

Abortion rate: The number of abortions per 1,000 women ages 15–44 or 15–49 in a given year.

Abortion ratio: The number of abortions per 1,000 live births in a given year.

Birth control: Practices that permit sexual intercourse with reduced likelihood of conception and birth. Abortion is included in the definition of birth control.

Carrying capacity: This is an ecological term that you won’t find in a glossary of demography although it is relevant to humans. Carrying capacity refers to the number of organisms of a given species that can be supported indefinitely in a given environment. (See also Overshoot.)

Cohort: A group of people sharing a common temporal demographic experience who are observed through time.

Contraception: Practices that permit sexual intercourse with reduced likelihood of conception. Modern methods include the pill, injectable hormones (such as Depo-Provera), implants (small hormone-releasing rods implanted in the upper arm), intra-uterine devices or IUDs, condoms, and sterilization.

Contraceptive prevalence: Percentage of couples currently using a contraceptive method.

Crude birth rate: Births per 1000 population.

Crude death rate: Deaths per 1000 population.

Demographic transition: The historical shift of birth and death rates from high to low levels in a population. The mortality decline usually precedes the fertility decline, resulting in rapid population growth during the transition period.

Demography: The scientific study of human populations, including their sizes, compositions, distributions, densities, growth, and other characteristics, as well as the causes and consequences of changes in these factors.

Doubling time: The number of years required for the population of an area to double its present size, given the current rate of population growth.

Emigration rate: The number of emigrants departing an area of origin per 1,000 population in that area of origin in a given year.

Family planning: The conscious effort of couples to regulate the number and spacing of births through artificial and natural methods of contraception. Family planning connotes conception control to avoid pregnancy and abortion, but it also includes efforts of couples to induce pregnancy.

Fecundity: The physiological capacity of a woman to produce a child.

Fertility: The actual reproductive performance of an individual, a couple, a group, or a population. See general fertility rate.

General fertility rate: The number of live births per 1,000 women ages 15–44 or 15–49 years in a given year.

Growth rate (or population growth rate): The annual rate of change in the size of a population. This change includes the increase (or decrease) from births over deaths and the net migration (immigration minus emigration), expressed as a percentage of the population at the beginning of the time period.

Immigration rate: The number of immigrants arriving at a destination per 1,000 population at that destination in a given year.

Infant mortality ratio: The number of deaths of infants under age 1 per 1,000 live births in a given year.

Life expectancy: The average number of additional years a person could expect to live if current mortality trends were to continue for the rest of that person’s life. Most commonly cited as life expectancy at birth.

Maternal mortality ratio: The number of women who die as a result of pregnancy and childbirth complications per 100,000 live births in a given year.

Migration: The movement of people across a specified boundary for the purpose of establishing a new or semi-permanent residence. Migration can be international (between countries) or internal (within a country).

Net migration: The estimated rate of net migration (immigration minus emigration) per 1,000 population. For some countries, data are derived as a residual from estimated birth, death, and population growth rates.

Net migration rate: The net effect of immigration and emigration on an area’s population, expressed as an increase or decrease per 1,000 population of the area in a given year.

Overshoot: This is not a term that you are likely to find in a glossary of demography, although it should be there. In population ecology, overshoot occurs when a population temporarily exceeds the long-term carrying capacity of its environment. This situation arises when a species or population encounters a rich and previously unexploited stock of resources that promotes its increase. When the stock is exhausted, the species faces a precipitous population decline or crash. Many ecologists think that the age of oil has sent the human population into overshoot.

Population: The total number of persons inhabiting a country, city, or any district or area.

Population control: A broad concept that addresses the relationship between fertility, mortality, and migration, but is most commonly used to refer to efforts to slow population growth through action to lower fertility

Population density: Population per unit of land area; for example, people per square mile or people per square kilometer of arable land.

Population increase (or population growth): The total population increase resulting from the interaction of births, deaths, and migration in a population in a given period of time.

Population momentum: The tendency for population growth to continue beyond the time that replacement-level fertility has been achieved because of the relatively high concentration of people in the childbearing years.

Population projections: Computation of future changes in population numbers, given certain assumptions about future trends in the rates of fertility, mortality, and migration. Demographers often issue low, medium, and high projections of the same population, based on different assumptions of how these rates will change in the future.

Replacement level fertility: The level of fertility at which a couple has only enough children to replace themselves, or about two children per couple.

Rule of 70: You aren’t likely to find this term in a demography glossary but it’s very useful to determine the approximate doubling time of a population based on the annual growth rate. To get the doubling time, divide 70 by the annual growth rate. For example, populations growing at 1, 2, and 3% annually have respective doubling times of 70, 35, and 23 years.

Total fertility rate (TFR): The average number of children that would be born alive to a woman (or group of women) during her lifetime if she were to pass through her childbearing years conforming to the age-specific fertility rates of a given year. This rate is sometimes stated as the number of children women are having today.

Unmet need: Women with unmet need for spacing births are those who are able to become pregnant and sexually active but are not using any method of contraception (modern or traditional), and report wanting to delay the next child or limit their number of births.

Zero population growth: A population in equilibrium, with a growth rate of zero, achieved when births plus immigration equal deaths plus emigration.

You can get more information about terminology at these and many other sites:

Jacobsen: How are these terms properly and improperly used within the mainstream and colloquial discourse?

Weld: I would say that there is an overall lack of understanding of how enormously consequential human population growth is to everything that is happening in the world today. How many people know that Syria’s population quadrupled from 5 million to 20 million between 1950 and 2010? Once self-sufficient in wheat, Syria has become increasingly dependent on more expensive imported wheat. The 2007–2010 drought was the worst in modern history its water resources dropped by 50% between 2002 and 2008. Crop failures led to hundreds of thousands of people from predominantly Sunni rural areas into coastal cities traditionally dominated by the Alawite minority. I’m not saying that population growth is the only cause of the political violence in Syria, but I am saying that things would probably have gone a lot better if Syria’s population had remained stable at 5 million. The same applies to many other countries that are currently conflict zones and sources of massive outmigration. The pivotal role of human population growth in the global environmental crisis is also often underplayed.

There is an inherent bias in reporting about growth in general. If a city, region or country has a decreasing population, it is often reported in terms such as “suffering a population loss” while growth will be reported in positive terms. More fuss has been made in the media about Japan’s shrinking population than about the out-of-control growth in many sub-Saharan countries, Syria, Gaza and some others. Yet Japan is coping much better with its decreasing population than the others are with their growing populations.

As for the use of demographic terms, many people probably couldn’t give dictionary-perfect definitions of a lot of them and many may confuse such terms as fecundity and fertility (both defined above). Nevertheless, the gist of some terms can be intuitively grasped. For example, the definition of total fertility rate (TFR) given above may sound a bit convoluted, but in a nutshell it is the average number of children that women of a given country or region have in their lifetime. Most people would probably get some sense of that from the term itself.

I am going to discuss six areas where I think there is often a lack of understanding of issues even when demographic terms are technically used correctly. And I am also going to discuss the concept of overshoot, a term used by ecologists but which many think is applicable to the trajectory of the human population.

(1) Population density

This refers to the number of people per unit land area. Canada is said to have a low population density based on dividing its total surface area by the number of inhabitants. But that crude statistic does not reflect the reality of Canada. Most of us live within 150 km of the US border and much of the southern part of Canada is as densely populated as any other developed country. It would be more realistic to determine Canada’s population density by dividing about one-tenth of its surface area by its population. Our immigration policy is based on this myth of endless space, but most newcomers not surprisingly congregate in our cities instead of trying to make a living in all that wide open tundra. Because of this rapid population growth in our southern belt, our cities are experiencing significant stresses on infrastructure and social services and we are losing farmland, wildlife habitat, and biodiversity.

(2) Population growth and population growth rate

Population growth or population increase refers to the increase (or decrease) of the absolute numbers of a population. It is the sum of births over deaths and immigration minus emigration. A country like Canada, with a below replacement fertility rate of 1.6, can grow rapidly because of high immigration levels.

The population growth rate refers to the annual rate of change in the size of a population as a percentage of that population at the beginning of the time period. A population can be growing in absolute numbers even if its rate of growth is slowing down. The growth rate of the global population has in fact slowed down a lot in the past several decades. This has led to a perception among many (including in the media) that the problem is solving itself. But the absolute number of people being added annually has gone up, because the size of the population is bigger. This is illustrated in the table below.

Image Credit: Dr. Madeline Weld.

It is the absolute number of people that puts pressure on the environment. Yet many people seem to think that a decreasing growth rate solves the problem.

(3) Total fertility rate (TFR)

This refers to the average number of children women in a given country or region have during their lifetime. For 2018, the global TFR was 2.5, according to the UNFPA’s State of World Population for that year. The plummeting TFR of recent decades is a good thing, and again this is often considered to indicate that the population problem is solving itself. But to consider only the global TFR, as is sometimes done when the topic is global population growth, is a big mistake, because the global figure includes huge disparities among individual regions and countries. For example, the countries defined by the UNFPA as “more developed regions” have a TFR of 1.7, while those in the “less developed regions” have a TFR of 2.6, and the “least developed countries” have an average TFR of 4.0. The TFR of Somalia is 6.1 and of Niger 7.1. Many, probably most, or the countries with very high TFRs are failed or failing states, with emigration pressures that are already huge and that will likely only worsen with time. Africa’s population is projected to explode from 1.2 billion today to 2 billion by 2050 and over 4 billion by 2100.

(4) Contraceptive prevalence versus unmet need

Contraceptive prevalence refers to the percentage of couples using a contraceptive method. Unmet need refers to women who are able to become pregnant, sexually active, do not want to become pregnant but are not using contraceptives. There is sometimes considered to be a direct correlation between contraceptive prevalence and unmet need (i.e., all reproductive-age women who are not trying to become pregnant but not using contraceptives represent an unmet need). However, in some cases a low contraceptive prevalence does not reflect a lack of access to contraceptives (unmet need) but a desire for a large family. That is why the completely hands-off approach taken at the 1994 International Conference on Population and Development (Cairo conference) with regard to promoting small families represents a huge failure, in my opinion. People were to “freely and responsibly” decide on the number and spacing of their children, but this idealistic thinking did not take into consideration the strong influence of cultural norms, religion, and tradition on desired family size. The UN developed no programs to educate people about the impact of population growth and to promote smaller families. (Unlike, for example, programs to promote and implement child immunization that were developed right after the World Health Organization was created.)

Lack of access is only one of many reasons for a low contraceptive prevalence. For example, according to the 2008–09 Demographic and Health Survey (USAID), 96% of married women in Kenya and 98% of their husbands knew about modern contraceptives. Of the married women who were non-users, 40% did not intend to ever use contraception. Among all non-using married women, 8% gave as their reason the desire for more children. Among the reasons given for not using contraception by women who were not pregnant and did not want to become pregnant, only 0.8% cited lack of availability of contraceptives, and 0.4% cited cost. The top four reasons among those who are still fecund: (1) concern with the medical side effects of contraceptives (31%); 2) religious prohibition (9%); (3) personal opposition (8%); and (4) opposition from the husbands (6%). (The information on the DHS survey is from a December 2012 paper by William Ryerson of the Population Media Center.) In a 2015 presentation by Dr. Ryerson summarizing the major reasons given for non-use of contraceptives in over 30 rapidly growing countries, lack of access was the main reason in only 1% (a single country) or below of the people surveyed in every country. The major reason was fear of side effects, followed to varying degrees depending on country by health concerns, religious prohibition, opposition by spouse, and lack of knowledge.

Changing high fertility rates is much more than a matter of making contraceptives available at an affordable cost (although that is extremely important). People must understand the benefits in terms of health, wealth, and the environment, and overcome their fears that contraceptives are dangerous or that family planning is unacceptable for religious or other reasons.

(5) Population projections

Population projections (or estimates of what a given population will be at some future time) are based on assumptions about fertility, mortality, and migration and are therefore only as good as those assumptions. It is because of these uncertainties that demographers often have low, medium, and high population projections based on differing assumptions about the parameters affecting population growth. At one point there was optimism that the world population would peak at 9 billion before 2100 and then decline, but current projections are for a still-growing population of over 11 billion in 2100. Almost all of the increase in the projected global population is because fertility rates did not fall as quickly in sub-Saharan Africa as had been assumed. In 2004, the United Nations projected a population for Africa in 2100 of 2 billion, but by 2015 had upped its projection for 2100 to 4 billion. The increase in the projected population of sub-Saharan Africa accounted for almost all of the increase in the projected global population.

Assumptions are sometimes based on what had happened previously in other countries. It has often been assumed that the “demographic transition” that occurred in developed countries would automatically occur all over the world. Unfortunately, this has not proven to be the case in sub-Saharan Africa and some other countries.

One thing that population projections do not take into account is the depletion of resources. The human population may not undergo the gradual decline that demographers foresee based on their assumptions of fertility decline, but a rather more abrupt crash based on resource shortages, starvation, war, the outbreak of diseases resistant to antibiotics, and other dystopian factors (see Overshoot below).

(6) Demographic transition and demographic transition theory (DTT) or model (DTM)

Many people have heard of the demographic transition theory. There is probably no theory that has done more to make people complacent about human population growth than the DTT. The videos on the subject by the charming and entertaining Hans Rosling spring to mind.

The demographic transition theory posits that societies will transition from having high fertility and high mortality to low fertility and low mortality as a natural consequence of socioeconomic modernization. The transition is usually divided into four stages. In the first stage, the population of a society is fairly stable because the high birth rate is balanced by a high death rate. In the second stage, as the society develops and health and hygiene improve, the death rate falls but the birth rate remains high, leading to rapid population growth. In stage 3, population growth starts to decrease as the birth rate falls due to better economic conditions, more education and an improvement in the status of women, and more access to contraception. In stage 4, both the birth rate and the death rate are low, and population growth is negligible or even declining.

The demographic transition theory describes reasonably accurately what happened when Europe transitioned from the middle ages to a modern industrial society. The mistake that many people made and continue to make is to assume that what happened in Europe would automatically be repeated everywhere else. The DTT was embraced by the 1987 Brundtland Commission on sustainable development. Sustainable development would be achieved through economic growth in developing countries, social equity, and environmental protection. But how would these be achieved without controlling population growth? The demographic transition would take care of it because people would have fewer children as they became richer. The same thinking guided the International Conference on Population and Development in Cairo in 1994.

Since 1987, when the Brundtland Commission published its report Our Common Future, the global population has grown from 5 billion to over 7.6 billion. Much of the hoped for development has stalled because of population growth, and the natural world and its biodiversity have been massively affected. As discussed above, large families are still considered desirable in many countries experiencing rapidly population growth, and little is being done to change this mindset.

Much emphasis has been placed on things like education for girls and economic development to indirectly address population growth. There is indeed a negative correlation between the level of female education and the TFR, and education and equal opportunities for girls and women are desirable in their own right. But, as Dr. Jane O’Sullivan has shown, expecting an increase in wealth to lead to a reduction in fertility is putting the cart before the horse. O’Sullivan showed that fertility decline typically preceded marked increases in wealth, and per capita wealth growth was accelerated when fertility fell to between two and three births per woman. Girls’ education was neither a pre-requisite nor a sufficient measure to drive fertility decline, while fertility decline was a necessary if not sufficient condition for sustained economic growth. O’Sullivan asserts that the data do not support the mantra that “development is the best contraception,” but that they make a case for “contraception is the best development stimulus.” (Jane O’Sullivan, “Revisiting the demographic transition: correlation and causation in the rate of development and fertility decline,” 2013)

There is an urgent need to make population growth an issue in its own right. Some countries, such as Bangladesh and Thailand, have done so. But most have not, nor has the United Nations made population growth a central part of its Millennium Development Goals (launched in 2000) or its Sustainable Development Goals (launched in 2016). The American population activist Rob Harding has proposed a UN Framework Convention on Population Growth, in which every country would take responsibility to bring its own population to a sustainable level. Other countries could help rapidly growing countries to achieve a sustainable population, but would not be expected to take in their surplus population (which appears to be the objective of the UN’s Global Compact on Safe, Orderly and Regular Migration).

In short, the demographic transition theory has been accorded far too much respect as a predictive too for population growth in much of the world and should be knocked off its pedestal. We can no longer rely on population growth being slowed indirectly through development and education. All countries must find ethical, culturally appropriate ways to slow to bring and keep their population at a sustainable level. We are nowhere near that point yet.

(7) Overshoot

William Catton’s book Overshoot was published in 1980. It is as relevant today as it was then and every decision-maker who thinks that there can be infinite growth on a finite planet (i.e., pretty much all decision-makers) should be forced to read it. The gist of Catton’s book is that oil provided the energy for humans to draw down the world’s resources, which has allowed the human population to greatly exceed the long-term carrying capacity of the Earth (i.e., to go into overshoot). When resources become scarce or run out, there won’t be enough to support the human population, which is likely to undergo a steep decline or a crash. The world is an ecosystem with limits to growth and nature will have the last word.

It took until 1804 for the human population to reach one billion. It increased to 2 billion by 1927, and 3 billion by 1960. The next three billion were in 1974, 1987, and 1999. In 2011, the human population reached 7 billion, and is now over 7.6 billion. Our population increases by 1,000,000,000 every dozen or so years. There is an eerie parallel of this spectacular increase in the growth of the reindeer population on St. Matthew Island, a remote outcrop in the Bering Sea, 300 km from Alaska. In 1944, the US Coast Guard established a LORAN (long-range radio navigation system) on St. Matthew Island, and released 29 reindeer as a backup food source for personnel. The station was decommissioned within months but the reindeer stayed and found themselves in a paradise rich in their favourite food, lichen. There were also no natural predators on the island. In the summer of 1957, researchers went to the island and found that the 29 reindeer had increased to 1350. They were in good health from their nutritious diet. Researchers visited again in the summer of 1963, and counted 6000 reindeer. But the lichen had all been consumed and the reindeer were feeding on sedge grass. Their health had deteriorated. In 1966, researchers again visited the island. They found it littered with reindeer skeletons, with only 42 reindeer remaining: one infertile male, 41 cows and no calves. The reindeer population on the island would die off completely. A graph of the phase of population increase of reindeer on St. Matthew Island bears a disconcerting resemblance to the graph of the increase in the human population since the beginning of the twentieth century. Let’s hope the human population graph doesn’t follow the trajectory of the reindeer population!

It’s true that the Earth is bigger than an island, and humans are smarter than reindeer, but we are exploiting resources globally on a colossal scale and the negative impacts of this drawdown of resources are becoming ever more evident. We would be wise to take heed.

Jacobsen: Thank you for the opportunity and your time, Madeline.

Image Credit: Madeline Weld.

Scott Douglas Jacobsen is the Founder of In-Sight: Independent Interview-Based Journal and In-Sight Publishing.




A journal with educational content via subject matter expert interviews.

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Scott Douglas Jacobsen

Scott Douglas Jacobsen

Scott Douglas Jacobsen is the Founder of In-Sight: Independent Interview-Based Journal and In-Sight Publishing. Jacobsen supports science and human rights.

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