Why is rapid population growth a temporary phenomenon?
The demographic transition
Population growth is determined by births and deaths and every country has seen very substantial changes in both: In our overview on how health has changed over the long run you find the data on the dramatic decline of child mortality that has been achieved in all parts of the world. And in our coverage of fertility you find the data and research on how modern socio-economic changes – most importantly structural changes to the economy and a rise of the status and opportunities for women – contributed to a very substantial reduction of the number of children that women have.
But declining mortality rates and declining fertility rates alone would not explain why the population increases. If they happened at the same time the growth rate of the population would not change in this transition. What is crucial here is the timing at which mortality and fertility changes.
The model that explains why rapid population growth happens is called the ‘demographic transition’. It is shown in the schematic figure. It is a beautifully simple model that describes the observed pattern in countries around the world and is one of the great insights of demography.9
The demographic transition is a sequence of five stages:
Stage 1: high mortality and high birth rates. In the long time before rapid population growth the birth rate in a population is high, but since the death rate is also high we observe no or only very small population growth. This describes the reality through most of our history. Societies around the world remained in stage 1 for many millennia as the long-run perspective on extremely slow population growth highlighted. At this stage the population pyramid is broad at the base but since the mortality rate is high across all ages – and the risk of death is particularly high for children – the pyramid gets much narrower towards the top.
Stage 2:mortality falls but birth rates still high. In the second phase the health of the population slowly starts to improve and the death rate starts to fall. Since the health of the population has already improved, but fertility still remains as high as before, this is the stage of the transition at which the size of the population starts to grow rapidly. Historically it is the exceptional time at which the extended family with many (surviving) children is common.
Stage 3: mortality low and birth rates fall. Later the birth rate starts to fall and consequentially the rate at which the population grows begins to decline as well. Why the fertility rate falls is a question that we answer here. But to summarize the main points: When the mortality of children is not as high as it once was parents adapt to the healthier environment and choose to have fewer children; the economy is undergoing structural changes that makes children less economically valuable; and women are empowered socially and within partnerships and have fewer children than before.
Stage 4:mortality low and birth rates low. Rapid population growth comes to an end in stage 4 as the birth rate falls to a similar level as the already low mortality rate. The population pyramid is now box shaped; as the mortality rate at young ages is now very low the younger cohorts are now very similar in size and only at an old age the cohorts get smaller very rapidly.
Stage 5:mortality low and some evidence of rising fertility. The demographic transition describes changes over the course of socio-economic modernization. What happens at a very high level of development is not a question we can answer with certainty since only few societies have reached this stage. But we do have some good evidence – which we review here – that at very high levels of development fertility is rising again. Not to the very high levels of pre-modern times, but to a fertility rate that gets close to 2 children per woman. What level exactly the fertility rate will reach is crucial for the question of what happens to population growth in the long run. If the fertility rate stays below 2 children per woman then we will see a decline of the population size in the long run. If indeed the fertility rate will rise above 2 children per woman we will see a slow long-run increase of the population size.
Empirical evidence for the demographic transition
Rapid population growth is a temporary phenomenon
If fertility fell in lockstep with mortality we would not have seen an increase in the population at all. The demographic transition works through the asynchronous timing of the two fundamental demographic changes: The decline in the death rate is followed by the decline in birth rates.
This decline in the death rate followed by a decline in the birth rate is something we observe with great regularity and independent of the culture or religion of the population.
The chart presents the empirical evidence for the demographic transition for five very different countries in Europe, Latin America, Africa, and Asia. In all countries, we observed the pattern of the demographic transition, first a decline in mortality that starts the population boom and then a decline in fertility which brings the population boom to an end. The population boom is a temporary event.
In the past, the size of the population was stagnant because of high mortality. Now country after country are moving into a world in which the population is stagnant because of low fertility.
World Population Growth
by Max Roser, Hannah Ritchie, Esteban Ortiz-Ospina and Lucas Rodés-Guirao
https://ourworldindata.org/world-population-growth
Why is rapid population growth a temporary phenomenon?
The demographic transition
Population growth is determined by births and deaths and every country has seen very substantial changes in both: In our overview on how health has changed over the long run you find the data on the dramatic decline of child mortality that has been achieved in all parts of the world. And in our coverage of fertility you find the data and research on how modern socio-economic changes – most importantly structural changes to the economy and a rise of the status and opportunities for women – contributed to a very substantial reduction of the number of children that women have.
But declining mortality rates and declining fertility rates alone would not explain why the population increases. If they happened at the same time the growth rate of the population would not change in this transition. What is crucial here is the timing at which mortality and fertility changes.
The model that explains why rapid population growth happens is called the ‘demographic transition’. It is shown in the schematic figure. It is a beautifully simple model that describes the observed pattern in countries around the world and is one of the great insights of demography.9
The demographic transition is a sequence of five stages:
Empirical evidence for the demographic transition
Rapid population growth is a temporary phenomenon
If fertility fell in lockstep with mortality we would not have seen an increase in the population at all. The demographic transition works through the asynchronous timing of the two fundamental demographic changes: The decline in the death rate is followed by the decline in birth rates.
This decline in the death rate followed by a decline in the birth rate is something we observe with great regularity and independent of the culture or religion of the population.
The chart presents the empirical evidence for the demographic transition for five very different countries in Europe, Latin America, Africa, and Asia. In all countries, we observed the pattern of the demographic transition, first a decline in mortality that starts the population boom and then a decline in fertility which brings the population boom to an end. The population boom is a temporary event.
In the past, the size of the population was stagnant because of high mortality. Now country after country are moving into a world in which the population is stagnant because of low fertility.
人口転換
ウィキペディア
https://ja.wikipedia.org/wiki/人口転換
人口転換（demographic transition）とは、社会の近代化にともない、人口の自然増加の形態が多産多死型から多産少死型へ、さらに少産少死型へと変化することである。
人口増加は、出生数と死亡数の差である自然増加と都市間・国家間の人口移動による社会増加とを足し合わせた値により求められる。人口転換理論では、自然増加に着目して人口動態を議論する。
人口転換理論によれば、自然増加には3つの形態（多産多死型、多産少死型、少産少死型）があるという。衛生状態の改善や医療技術・医療水準の向上などにより、次のように段階的に形態が移行していく。
・第1段階…多産多死型
・第2段階…多産少死型
・第3段階…少産少死型
第1段階では、出生率も死亡率も高い多産多死の状態である。
ここに衛生状態の改善や医療技術・医療水準の向上といった社会的な要因が加わると、死亡率が徐々に減少し始める。これが第2段階の多産少死という出生率が高く、死亡率が低い状態である。出生率は高く維持されているため、人口が爆発的に増加する人口爆発が起こり、自然増加が非常に大きくなる。出生率が高く維持されるのは、高死亡率のために多くの子供を産んでおきたいという第1段階時代の慣例が残存しているためである。
ここで更に経済水準・識字率の向上や家族計画など避妊についての知識が浸透すると、出生率が徐々に減少を始める。これが第3段階の少産少死という出生率も死亡率も低い状態である。このため、少産少死の段階では人口がほとんど増加しなくなり、静止人口に近づく。
静止人口を過ぎると死亡率が出生率を超えるために人口が減少してどの段階にも当てはまらなくなり、少産多死という状態になる。
第３節 先進国の出生率の動向
内閣府
https://www8.cao.go.jp/shoushi/shoushika/whitepaper/measures/w-2004/pdf_h/pdf/g10h0300.pdf
人口転換理論
18世紀以降の欧米諸国では、経済発展により死亡率が低下し、19世紀後半からは出生率も低下し始め、1930年代には出生率、死亡率ともに低い社会が実現した。このようなプロセスを説明する理論として「人口転換理論」が登場した。現在では、様々な研究や議論があるが、伝統的に説明されている理論を簡単にまとめると、人口増加のペースは、経済社会の発展に伴い、「多産多死」（高出生・高死亡）から「多産少死」（高出生・低死亡）を経て、やがて「少産少死」（低出生・低死亡）に至るというものである。その背景をまとめると次のようになる。
まず、工業化が始まる前の伝統的農業社会では、飢饉、疫病、戦争等のために死亡率が高い状態にある。その一方で、農業が主体である社会であるために、労働力確保の観点から高い出生率が維持されている。このほかに、宗教や社会制度などによって高出生率が維持されることもある。その結果、近代化前の社会では死亡率と出生率が高く（多産多死）、大きな変動を保ちつつ、平均的には人口増加率は低い状態にある。
次に、工業化・都市化が進むと、人口増加の状況は変化する。所得水準の上昇、医学や公衆衛生の発達により、乳児死亡率などが低下することで、社会全体の死亡率が低下する。しかし、出生率は依然として高水準にある。その結果、高い出生率と低い死亡率の社会（多産少死）が実現し、人口は増加する。
その後、出生率も死亡率に追いつくように低下し、出生率、死亡率ともに低い社会（少産少死）が実現する。その背景として、出生数を減らしても家族・社会の存続が可能となること、子供の養育コストの増大、結婚・出産に対する価値観の変化、避妊など出生抑制技術の普及などを考えることができる。
わが国では、明治維新以前が多産多死、明治から昭和30年代半ばまでが多産少死、昭和30年代半ば以降が少産少死の段階であると考えられている。アジア、アフリカなどの地域では、第２次大戦後に死亡率が急速に低下した一方で、出生率が高い水準で推移しつづけてきた。しかし、国による違いはあるものの20世紀の末までに出生率低下が始まり、「少産少死」の段階に入りはじめたところもある。
「少産少死」の段階になると人口動態は安定するものと考えられていたが、最初に「少産少死」に達した欧米諸国では、人口置き換え水準よりも低くなるという一層の出生率低下がみられる。これは、「第二の人口転換」という言葉で呼ばれ、近年注目されている。この現象は、効果的な避妊法の普及、晩婚・晩産化の進展などがもたらしたものであるが、その背景には、結婚や家庭に対する個人や夫婦の価値観の変化があるとされている。わが国も、こうした「第二の人口転換」に至っている状況にある。