Population momentum is a consequence of the
demographic transition
In demography, demographic transition is a phenomenon and theory which refers to the historical shift from high birth rates and high death rates in societies with minimal technology, education (especially of women) and economic development, to lo ...
. Population momentum explains why a
population
Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...
will continue to grow even if the
fertility rate
The total fertility rate (TFR) of a population is the average number of children that would be born to a woman over her lifetime if:
# she were to experience the exact current age-specific fertility rates (ASFRs) through her lifetime
# she were t ...
declines. Population momentum occurs because it is not only the number of children per woman that determine population growth, but also the number of women in reproductive age. Eventually, when the fertility rate reaches the replacement rate and the population size of women in the reproductive age bracket stabilizes, the population achieves equilibrium and population momentum comes to an end. Population momentum is defined as the ratio of the size of the population at that new equilibrium level to the size of the initial population.
Population momentum usually occurs in populations that are growing.
Example
Assume that a population has three generations: First (oldest), Second (child bearing), and Third (children). Further assume that this population has a fertility rate equal to four (4). That is, each generation is twice the size of the previous. If the population of the first generation is arbitrarily set at 100, the second is then 200, and the third is 400. The spreadsheet below shows the initial population in the first row.
First note that the second and third generation of the initial population are each twice the size of the previous. The total of the initial population is 700 = 100 + 200 + 400.
Then assume that at the end of the third generation, fertility falls to replacement (for simplicity assume that to be two). Now take the population forward in time to the next generation, line two of the spreadsheet. The first generation dies, and the new generation, the fourth, is equal to the third (because now fertility is replacement). Repeat the process again to reach the fifth generation (line 3 in the spreadsheet). The fifth generation is again equal to the fourth and now the population’s three generations are equal, and the population has reached equilibrium.
The initial population has grown from 700 to 1,200 even though fertility dropped from four to replacement (two) at the end of the third generation. Population momentum carried the population to higher levels over the next two generations.
Further steps to zero population growth
Population momentum impacts the immediate
birth
Birth is the act or process of bearing or bringing forth offspring, also referred to in technical contexts as parturition. In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the f ...
and
death rates
Mortality rate, or death rate, is a measure of the number of deaths (in general, or due to a specific cause) in a particular population, scaled to the size of that population, per unit of time. Mortality rate is typically expressed in units of de ...
in the population that determine the
natural rate of growth. However, for a population to have an absolute zero amount of natural growth, the US National Library of Medicine National Institutes of Health suggests that three things must occur.
1. Fertility rates must level off to the
replacement rate
The total fertility rate (TFR) of a population is the average number of children that would be born to a woman over her lifetime if:
# she were to experience the exact current age-specific fertility rates (ASFRs) through her lifetime
# she were t ...
(the
net reproduction rate
In population ecology and demography, the net reproduction rate, ''R''0, is the average number of offspring (often specifically daughters) that would be born to a female if she passed through her lifetime conforming to the age-specific fertility ...
should be 1). If the fertility rate remains higher than the replacement rate, the population would continue to grow.
2.
Mortality rate
Mortality rate, or death rate, is a measure of the number of deaths (in general, or due to a specific cause) in a particular population, scaled to the size of that population, per unit of time. Mortality rate is typically expressed in units of de ...
must stop declining, that is, it must remain constant.
3. Lastly, the age structure must adjust to the new rates of fertility and mortality. This last step takes the longest to complete.
Implications
Population momentum has implications for population policy for a number of reasons.
1. With respect to high-fertility countries (for example in the
developing world
A developing country is a sovereign state with a lesser developed industrial base and a lower Human Development Index (HDI) relative to other countries. However, this definition is not universally agreed upon. There is also no clear agreem ...
), a positive population momentum, meaning that the population is increasing, states that these countries will continue to grow despite large and rapid declines in fertility.
2. With respect to
lowest-low fertility countries (for example in
Europe
Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
), a negative population momentum implies that these countries may experience population decline even if they try to increase their rate of fertility to the replacement rate of 2.1. For example, some
Eastern European
Eastern Europe is a subregion of the European continent. As a largely ambiguous term, it has a wide range of geopolitical, geographical, ethnic, cultural, and socio-economic connotations. The vast majority of the region is covered by Russia, whi ...
countries show a population shrinkage even if their birth rates recovered to replacement level. Population momentum can become negative if the fertility rate is under replacement level for a long period of time.
3. Population momentum shows that replacement level fertility is a long-term concept rather than an indication of current population growth rates. Depending on the extant age structure, a fertility rate of two children per woman may correspond to short-term growth or decline.
Calculation
![Population pyramid of India 2015](https://upload.wikimedia.org/wikipedia/commons/1/1b/Population_pyramid_of_India_2015.png)
To calculate population momentum for population A, a theoretical population is constructed in which the
birth rate
The birth rate for a given period is the total number of live human births per 1,000 population divided by the length of the period in years. The number of live births is normally taken from a universal registration system for births; populati ...
for population A immediately becomes replacement level. Under such conditions, the population will eventually stabilize into a stationary population, with no year-to-year changes in age-specific rates or in total population. The population momentum is calculated by dividing this final total population number by the starting population. Momentum, Ω, can be expressed as:
In this equation, ''b'' is the crude birth rate while ''e
o'' is the life expectancy at birth. ''Q'' is the total number of births per initial birth.
This equation is used to derive Q (total births per initial birth), ''r'' is the growth rate and ''µ'' is the unchanging population mean age at childbearing. R
o is the Net Reproduction Rate of the non-changing population.
Causes
Population momentum is typically caused by a shift in the country's
demographic transition
In demography, demographic transition is a phenomenon and theory which refers to the historical shift from high birth rates and high death rates in societies with minimal technology, education (especially of women) and economic development, to lo ...
.
When
mortality rate
Mortality rate, or death rate, is a measure of the number of deaths (in general, or due to a specific cause) in a particular population, scaled to the size of that population, per unit of time. Mortality rate is typically expressed in units of de ...
s drop, the young survive childhood and the aging population live longer.
Fertility rates
The total fertility rate (TFR) of a population is the average number of children that would be born to a woman over her lifetime if:
# she were to experience the exact current age-specific fertility rates (ASFRs) through her lifetime
# she were t ...
remain high, causing the overall population size to grow.
According to population momentum, even if high fertility rates were immediately replaced with replacement level fertility rates, the population would continue to grow due to the pre-childbearing population entering childbearing years.
See also
*
Human overpopulation
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
*
Demography
Demography () is the statistics, statistical study of populations, especially human beings.
Demographic analysis examines and measures the dimensions and Population dynamics, dynamics of populations; it can cover whole societies or groups ...
*
List of population concern organizations
This is a list of organisations who promote a moderation of the size of the human population.
Intergovernmental organizations
Worldwide
Regional
Asia-Oceania
* All countries – Asian Forum of Parliamentarians on Population and Development
A ...
*
Population stabilization
Zero population growth, sometimes abbreviated ZPG, is a condition of demographic balance where the number of people in a specified population neither grows nor declines; that is, the number of births plus in-migrants equals the number of death ...
References
External links
* Mason, A., Lee, S. and Russo, G. (2000). Population Momentum and Population Aging in Asia and Near-East Countries.
book
A book is a medium for recording information in the form of writing or images, typically composed of many pages (made of papyrus, parchment, vellum, or paper) bound together and protected by a cover. The technical term for this physical arr ...
Strategic and Economic Analysis Office. Available at
ccessed 28 Mar. 2018
* PAPP103 - S08: Population dynamics and momentum, United Nations Population Fund - UNFPA
{{Population country lists
Human overpopulation
Population ecology
Environmental controversies