This paper estimates the relative contributions of four demographic components—fertility, mortality, migration, and the current age structure of population— on future population growths of selected countries. The analysis is based on the data compiled from the 2022 Revision of World Population Prospects ¹⁰ , which is available online under the file type “standard projections estimates and projection scenarios”. The study has been carried out for the six most populous countries and the global population as of 1 January of projected years.

The UN projection used cohort-component method for projecting population (CCMP), which relies on information about fertility by age of mother to determine the number of births taking place each year; mortality by sex and age to determine the number of deaths; and net international migration by sex and age to determine the levels and patterns of population shifts across international borders ² . Methodologically, several updates have been incorporated in the 2022 Revision of WPP and some of the important modifications are: ten deterministic projection scenarios were constructed to illustrate the impact of differing assumptions from the medium scenario, five-year age groups by five-year periods (5X5 matrix) into single-year age groups by one-year periods (1X1 matrix), population reference dates changed from 1 July to 1 January to help align the calendar year. For more details about the methodology adopted in the 2022 Revision of WPP, please refer to the comprehensive note on methodology published by the UN in 2022 ² .

To calculate the relative effects of fertility, mortality, migration, and population momentum, on population changes from base year 2022, we use the standard decomposition technique as explained hereunder. The input data to estimate effects of four demographic factors are four variants of 2022 Revision of WPP projection results, viz., i) medium variant (base values), ii) zero-migration variant (natural change in fertility and mortality), iii) instant replacement fertility (fertility set to instant replacement as of 2022 and other parameters are equivalent to medium variant), and iv) momentum variant (fertility set to instant replacement as of 2022, mortality set to 2022 level and zero migration from 2022). Projected results as of 1 January are used.

The population change from base year takes the summative form of population changes caused by fertility, mortality, migration, and age structure which can be written as,

1. Population changes due to population momentum

Population momentum ( P _Mom ) refers to an inherent driving force for population growth resulted from the existing age structure ¹ . Effect due to population momentum is calculated by subtracting the population projected figure under Momentum variant from the base year (here 2022).

2. Population changes due to fertility

Fertility component contributes to population growth positively or negatively depending on whether fertility rates are above or below replacement level fertility. Replacement level of fertility is an important concept in demographic transition and considered to be it’s total fertility rate (TFR) value around 2.1 while considering survival chances of females through their reproductive lifespan. Difference of population projected figures based on instant replacement scenario from figures under zero-migration scenario attributed to population changes due to fertility. This is calculated on a year-on-year basis.

3. Population changes due to mortality

Mortality pattern changes the contours of future population growth in either way. Usually gains in mortality situation cause spurts in population growth whereas the recent Covid-19 pandemic marginally impacted population decline in many countries. The difference in total population between the instant replacement and Momentum variants indicates the effect of anticipated change in mortality on future population size.

4. Population changes due to migration

Future trends in net-migration influences positively or negatively the population size of a country. Since migration is closed for the global population, its impact is not calculated however, computed for countries by subtracting projected figures under the zero-migration scenario from the figures of median variant.

Finally, calculated figures have been checked for the consistency as per Equation 1 by summing these above-mentioned four figures with the population differences computed from base year under median variant for projected years. The above methodology is largely based on a seminal technical paper by Andreev K et al. published in 2013 by the Population Division of UN DESA which looked at the demographic components of future population growth ⁶ .

5. Selection of six countries

The six most populous countries based on 2022 population data have been selected for the analysis and it is interesting to note that they represent different continents (none belong to Europe and Latin America though), passing through different stages of demographic transition, financially different economic categories, and follow different geo-political systems. In this analysis, population projection figures refer to 1 January unless otherwise specified.

The analysis started by looking at the positional changes of 10 most populous countries in different decades based on projected population figures. According to the medium variant 2022 Revision WPP, the 10 most populous countries are ordered and plotted in the Figure 1 below for period 2022 – 2100. Six of the top ten countries in 2022 appear in the list until 2100 with minor positional changes. India is likely to overtake China by May 2023 and continues to lead the list with China in the second spot until 2100 (projection period ends). The projected population shows that India will be the only country with a population of over one billion after 2080 as China’s population continues to decline and falls below 1 billion by 2080.

In 2030, Mexico is edged out by Ethiopia in the top 10 list while other countries have stuck to their positions except that Russia moved to tenth from ninth in the list. Two changes have been noticed in the 2040 list – i) DR Congo entered the list by edging Russia out of top 10 and ii) Pakistan and Nigeria moved up to take fourth and fifth positions respectively while Indonesia pushed to sixth from fourth position. The 2050 list is marked by a few more reshufflings – i) Nigeria edges past Pakistan to occupy fourth position, ii) Bangladesh is pushed to tenth from eighth position and iii) DR Congo and Ethiopia moved up in the ladder placed in eighth and ninth positions respectively.

The next two decades, i.e., 2060–70 did not observe any dramatic positional changes except that Brazil moved to ninth position from seventh. Tanzania entered the list of ten populous countries in 2080 by edging out Bangladesh while DR Congo replaced Indonesia to occupy sixth position. The last decade of this century will see Egypt entering the list and Brazil out of the list of ten most populous countries. DR Congo sneak past the USA to occupy the fifth position; finally bringing the tally of four countries from Asia, five countries from Africa and one from the Americas.

Figure 2 presents the trend of projected population for the total, China and India under medium variant scenario provides an overview of the future population size. The world population is likely to stabilise in 2086 at 10.43 billion, which is twenty-two years after India’s population expected to stabilise at 1.70 billion in 2064. China’s population is already showing declining trend in 2022 suggesting that its population has achieved population stabilisation. Interestingly, the projected population of China shows that by 2079 it will likely leave India alone in the one billion plus club as its projected population dropped to 989.8 million from 1.01 billion in this year – after its population crossed the 1 billion mark nearly 100 years ago in 1982.

To provide clear picture of population age-sex composition and future trends, population pyramids are created for the six countries and for the World population and displayed below ( Figure 3a to Figure 3g).

2022 age-sex population pyramid of China suggests likelihood of declining population due to rapid decline in fertility below replacement which is expected to stay well below the replacement level for nearly three decades. However, the USA’s population pyramid for the year 2022 portrays an interesting shape signifying a nearly stable population – meaning considerably slower growth in the future.

Population pyramids of India and Indonesia represent an “ expanding slowly” scenario in which age structure, improved mortality situations and net-migration are likely to drive future population growth with fertility transition being completed.

Population pyramids of both Nigeria and Pakistan represent scenario of “ expanding rapidly” implying both fertility transition is yet to occur and help population growth more than other demographic factors.

The World’s 2022 age-sex population pyramid represents early signs of “ expanding slowly” as fertility is getting closer to replacement level in the near future.

Figure 4 provides the details of trends in TFR forecasted by the 2022 WPP for six countries considered for the analysis to show how fertility transition is envisaged in these countries. Fertility rates in Nigeria and Pakistan are well above 3 in 2022 and beginning to converge with other countries towards the end of this century. Fertility transition in other four countries has already been completed in the base year 2022. China’s fertility is forecast to increase from just above 1.19 in 2022 to reach 1.48 by the end of this century.

1) Factors affecting World Population Growth

Since the migration factor is closed for the world population, the contributions of the other three factors – fertility, mortality, and age structure - are calculated and presented in the form of horizontal bars on primary axis in Figure 5 below. The figure also presented the changes in population from 2022 in the form of two lines representing additions and subtractions to the population due to changes in fertility, mortality, and age structure on the secondary axis.

During 2022–30, the world’s population is likely to increase by 570 million of which age structure or population momentum of past growth contributes 78.1 percent while 12.5 percent is driven by the gain in mortality conditions and the marginally above replacement level of fertility attributed the rest 9.4 percent. Globally, 1.75 billion people will be added to the 2022 population level during the 2022–2050 period and two-thirds (66 percent) of this growth would be due to the population momentum, gains in mortality accounts for one-fourth (25 percent) increase and the contribution of above replacement level fertility is estimated at 8.5 percent.

The global population continues to grow at a slower pace in the second half of the century with the contributions from mortality decline touching 50 percent; now the leading cause of population growth by overtaking the contributions from age structure (46.6 percent) during 2022–80 period. In this period the population is likely to grow by 2.47 billion as fertility accounts for only 3.3 percent of the population increase. Finally, the last decade of this century shows decline in population growth at the global level with the gains in mortality contributing nearly two-thirds (65.4 percent) of the 2.41 billion population projected to be added between 2022-2100. Since fertility stays below replacement level ensuring 259 million fewer in the total population.

2) Factors affecting China’s population change

Contributions of factors affecting changes in China’s projected population are provided in Figure 6 below and tell different story. Impact of population momentum on growth in the population disappear in the next three decades – from 95 percent in 2022–30 to no impact in the 2022–60 period and negative impact thereafter – signifying fast changing population age structure resultant of fertility remaining below replacement level for a longer period in the past. The projected population of China shows negative population growth starting between 2022–30 with 9 million less population in this decade. Net-migration (4 to 5.8 percent) and fertility below replacement level (94 to 95 percent) are likely to contribute to the negative population growth until mid-century. During this period, both age structure and gains in mortality enhance the moderate population growth. Impacts of age structure turned out to be influencing negative population growth starting 2060 onwards for the country by leaving gains in mortality the lone influencer of population growth. By the end of the century, China expected to cutdown 0.65 billion in population size compared to its 2022 population count under the medium scenario assumptions.

3) Factors affecting India’s future population growth

India’s population growth story is presented in Figure 7 below. The projected population of India shows an increase of 115 million people due to population momentum and gains in mortality while a decrease of 18 million due to fertility below replacement level and net outmigration resulting in net addition of 97 million during 2022–30 period. Age structure contributes more than 80 percent of 115 million increase and the remaining 20 percent contributed by the gains in mortality situations. More than three-fourth (76.3 percent) of the 18 million less population impacted by the below replacement of fertility levels and 23.7 percent due to outmigration during the same period.

The same trend continues until the end of this century as population momentum and longer expectation of life driving population gains while fertility below replacement levels and net outmigration are likely to reduce the population with the impact in mortality gains overtaking population momentum in the period 2022–2080 as the major contributor in share of population increase.

4) Factors affecting Nigeria’s future population growth

Nigeria starts as the sixth populous country before moving up to third populous country by 2060 signifying a typical population growth story. The population of Nigeria is expected to double from 216 million in 2022 to 436 million in 40 years making it one of the fastest growing countries in the world. Figure 8 presents the factors affecting Nigeria’s population growth over the next few decades. Fertility above replacement level of fertility (ranging from 55 to 65 percent) contributes significantly to the population growth followed by the population momentum (ranging from 14 to 40 percent) and marginally by mortality gains (ranges between less than 1 percent initially and 23 percent by the end of the century) as shown in the figure below.

5) Factors affecting USA’s future population growth

Population of the USA is projected to grow at a very slow pace from 337.5 million in 2022 to 394 million in 2100; an addition of 56.5 million in 72 years under the medium scenario. The analysis shows that growth of population in the US is contributed by age structure, mortality gains, and net in-migration whereas, fertility levels below replacement contribute to decrease in population in the coming years and decades. Figure 9 below provides the details. Between 2022–30, the population is projected to increase by nearly 30 million people due to age structure (31.6 percent), mortality gains (39.3 percent) and net in-migration (29 percent) while lower fertility rates contributed to decrease of 16 million in the same period resulting in a net addition of 13.8 million people to the total population of the country. Factors propelling population increase between 2022–50 are net in-migration (36.3 percent), mortality (54.5 percent) and age structure (9.2 percent) of the 99.7 million population while lower fertility rates below replacement contributes to 62.1 million less population resulting in 37.6 million net increases in the population in the same period.

6) Factors affecting Indonesia’s future population growth

Under medium projection scenario, presently the fourth populous country, Indonesia, is projected to grow moderately from 274.6 million in 2022 to 319.4 million in 2060 before population decline starts. At the end of this century in 2100, Indonesia’s population is projected to fall below the 300 million mark and likely to be the eighth populous country. Figure 10 shows the contributions of factors affecting the population change on primary axis and changes in population sizes during different periods on the secondary axis.

Both mortality and age structure propelling population increase while net out-migration and lower fertility levels below replacement fertility contribute to the population decrease throughout this century for the country. Population momentum contributes more than 78 percent and improvements in mortality conditions cause 21 percent of the 17 million population increase for the 2022–30 period. Analysis shows that net outmigration (72 percent) and low birth rates (28 percent) bring down population by 0.61 million in the same period. During the 2022–60 period, Indonesia is likely to add 44.8 million people to its total before stabilising the population with 62 percent of population increase (59.6 million) contributed by population momentum and 38 percent of increase due to improved mortality conditions in the same period. Below replacement of fertility levels (83 percent) and net outmigration (17 percent) bring down the 14.8 million population between 2022 and 2060 in Indonesia. The trends in contributions of factors affecting population changes continue until the end of the projection period as fertility contributes to 92 percent and net outmigration covers 8 percent of population decrease while shares of mortality gains 63 percent and population momentum 37 percent of population increase estimated for the country.

7) Factors affecting Pakistan’s future population growth

Like Nigeria, Pakistan too follows a rapid population growth trajectory as its population is projected to double in 40 years from 215.9 million in 2022 to 431.4 million in 2061 ( Figure 11 below). The decomposition analysis shows that mortality (8.6 percent) and net out-migration (91.4 percent) account for nominal population decline of 3.9 million during 2022–40 while fertility higher than replacement level fertility (38 percent) and age structure (62 percent) drive population increase of 90.6 million resulting net addition of 86.7 million from the 2022 base population in Pakistan.

Since then, three factors – fertility, mortality, and age structure – drive population growth with net out-migration lone contributor of population decrease for the rest of the projection period in the country. The shares of age structure declined from 62 percent to 39 percent, gains in mortality increased from 0.9 percent to 14.6 percent and higher fertility above replacement level from 37.1 percent to 46.7 percent in population size increase from 2022 to the end of different decades.

Population momentum, a force that drives future population growth resulting from the existing age structure with constant levels of mortality and net zero migration continues to grow even when fertility still is constant at the replacement level, which turns out to be a large young population accumulated due to high fertility in the past. Population momentum can also trigger negative population growth if the existing age structure is old thus, could cause positive or negative population growth depending on young or old age structure, respectively.

The 2022 UNDESA medium scenario population projection results suggest that the global population continues to grow from 7.94 billion in 2022 to 10.43 billion until 2086 before it starts declining. Age structure is found to be the major driver (66 percent) of the 1.75 billion population increase at least for the next three decades from 2022. This result corroborates results from a previously conducted study using 2019 UNDESA data ¹¹ . Population momentum is the only common factor among four demographic factors driving population increase in six most populous countries considered for the analysis at least until 2050.

Population growth caused by population momentum is inevitable unless significant changes in mortality and migration (at country level) parameters impacted on population. WPP 2022 rightly summarised that further actions by Governments aimed at reducing fertility would not have a major impact on the pace of growth between now and mid-century, beyond the gradual slowdown anticipated by the projections ¹² . Population momentum is a natural demographic transition and has already been in motion fuelling population growth. The effect of population momentum on China’s population is found to be declining and likely to have a negative impact starting from 2060 onwards. China’s population growth pattern provides a clue to an argument that cumulative effect of lower fertility, if continued over several decades, could result in a more substantial reduction of population growth in later decades. When we compare the results from 2010 WPP data by Andreev et al. ⁶ , the impact of age structure on population growth in the year 2022 significantly reduced for both China and India. In absolute numbers, 447 million people would have projected to add between 2010 and 2100 due to young age structure in India significantly reduced to 174 million people between 2022 and 2100 whereas, China would have added 146 million between 2010 and 2100 now 142 million fewer people than 2022 population as per the 2022 WPP analysis.

Population growth patterns of other five countries are different than China’s as the population momentum continues to influence population growth with varying degrees in these five countries during the projection period.

Fertility rates above replacement level of fertility influences the population growth. During demographic transition, fertility rates play important roles in prospects of population growth of any country. In the present global population scenario, fertility is still a deciding factor influencing positive population growth at least until 2050. The population of Nigeria and Pakistan too are positively influenced by their higher levels of fertility rates however, in the other four countries where fertility rates have reached below replacement level of fertility so it is no more a deciding factor shaping population growth in future. Decades of sustained much lower fertility than replacement level of fertility in China greatly changed the country’s population growth trajectory as more than 94 percent of the decline in population was attributed to the fertility factor alone during 2022–2050. The one-child policy adopted by the country during the 1980–2015 period (which was reversed to a two-child policy in January 2016 and further to a three-child policy in May 2021 ¹³ substantially lowered the fertility rates and curbs the population growth. Decline in population due to consistently lower fertility rates than replacement level in China was calculated at 425 million between 2010 and 2100 ⁶ against the 621 million between 2022 and 2100 in this study.

On the other hand, prolonged higher fertility rates along with population momentum will have significant impact on the population growths of Nigeria and Pakistan. Both the countries will see a surge in population in the coming several decades with a doubling time of 40 years and are likely to be the third and fourth most populous countries by the second half of this century.

Impact of fertility on curtailing population growths of India, Indonesia and USA is evident due to the sustained lower than replacement fertility rates observed in these countries. Once fertility was synonymous with the population growth in many developing countries since 1950s but now with the wider use of family planning methods it is no longer influencing population growth rather restricting population.

Globally, better healthcare services, improved living conditions and hygiene practices in recent decades help people live longer, which is found to be one of the driving forces of future population growth. Gains in mortality conditions in five of the six countries (except Pakistan) analysed here show a positive impact on population growth until 2050 and going forward will contribute to the population increase by accounting for a major share (more than 50 percent). Pakistan’s case was found to be exceptional due to the upward adjustment of mortality estimates accounting for excess Covid-19 deaths pushing negative growth of population in 2022 WPP projections until 2040. This upward adjustment of mortality estimates was carried out for all the countries however, its negative impact was visible only for one or two years in the projection period for other five countries analysed here. Gains in mortality and population momentum started clearly offsetting the decline in fertility rates while positively impacting population growth after 2040 in five countries apart from China where the share of age structure shifted to groups of factors (fertility and migration) that influences negative population growth since 2040.

Net migration irrespective of whether in-migration or out-migration is likely to change the population growth of many developed countries (USA in this analysis) where natural population growth is nearly zero as noted by a previous report ⁶ . The results suggest that population growth in the US is characterized by cumulative sum of age structure, gains in mortality and net in-migration of which the last two factors contribute nearly 91 percent of population growth during the 2022–50 period. The situation is different in developing countries like India and Indonesia where more than one-fifth (21.1 percent) of the negative population growth for India (21.1 percent) and Indonesia (21.4 percent) attributed to net out-migration in the same period whereas migration did not show much impact on population growth of China, Nigeria, and Pakistan (at least volume-wise). International migration is considered to be an important factor in propelling population growth (negative or positive) in the coming years, it has been largely regulated by geo-political and policy environment. Thus, policies related to the international relations are likely to change the assumptions made regarding the future migration trends carried out under population projections.

Comparison with the results of similar study using 2010 WPP data reveals an interesting pattern where overall trends in influences of different demographic factors on population growth are the same however, levels vary significantly due to fast changes in fertility and mortality behaviours. A 2022 study by Dai, Shen and Cheng ¹⁴ evaluated nine population projections available for China (UN, IHME, IIASA, etc.) and concluded that the slowing of the country’s actual population growth rates from 2017 is earlier than most datasets projected. Therefore, the turning point of China’s population decline probably comes rapidly before these datasets expected between 2024 and 2034 providing guidance to modellers and forecasters to rigorously verify assumptions regarding fertility, mortality, and migration to improve the accuracy of population projections.