One Finite Planet

Paradox Resolved: Why The Population Explosion Is Ending and birth rates falling.

First Published:

Births per woman from a sample group of countries.

Surprisingly, humanity is at ‘peak child‘ and looks to be heading to population stability and perhaps even ‘leaving peak population‘ and correcting to a lower population.

We are still have an overpopulated planet, following the population explosion, but at least the explosion is ending, due to ‘births’ per woman falling from over 6 prior to 1900, to 4.7 by around 1950, and to around 2.3 in 2021.

So, why are birth-rates falling? There are countless studies of population trends, but in the end, most explanations for the fall in birth rates just don’t add up.

When birth rates fell, humanity was in a population explosion far beyond anything previously seen in our history. Many explanations assume the fall at a time it was essential for rates to fall, was just a lucky coincidence. Factors neither expected or planned to reduce population growth, luckily saved the day. But what really happened? This page, which is part of a journey to understand population, looks at three main types of explanations for the decline in birth rates coming just when it was needed.

  • Background
    • The explosions starts as an unfortunate side-effect of improved medicine.
    • The explosion ends due to decreased ‘births-per-woman’, another unintended side effect?
  • Explanations categories:
    • 0. The Fall In Birth-Rates Was Due To Planned Responses to solve overpopulation.
    • 1. Lucky Coincidence that “The Pill“, Education of Women, Women in the Workplace, etc led to a return to balance.
      • Either Lucky Timing and Rate To Give Balance, Or Just Ignorance Of History?
      • The Pill: How much of an explanation?
      • The Full List Of Factors.
      • Can We Explain The Luck: Are We Just The Lucky Ones?
    • 2. Birth Rates Adjust To the Environment, In Humans As In Nature.
  • Conclusion: The Paradox.


The explosions starts as an unfortunate side-effect of improved medicine.

Humanity has just experienced a population explosion. While there are various factors that enabled the explosion to continue, there is one single clear reason why the explosion happened. It was not increased birth rates, which actually fell during the population explosion. Birth rates fell, and at no time during the explosion reached pre-explosion birth-rates, even during ‘baby-booms’.

The difference between the time of population explosion, and the time before, is that previously over half of all children died before growing up to have their own families. Once we solved the huge historic problem of half of all children dying before becoming adults, almost all children then went on to have their own families.

There are people who explore theories such as how improved farming allowed supporting an increased population, and factors such of these explain how managed to sustain the increased population, but it not the cause of the increased population. Given that birth rates did not increase, it is clear that is the advances in medicine that caused the explosion.

Humans reduced infant mortality to end the tragic loss of life, and the population explosion was an unfortunate side effect.

The population explosion was an unfortunate, accidental side effect of saving children’s lives. It is an ‘accident’ in the sense that we did not pre-plan the population explosion, it was just a side effect that, without something bring the resulting growth rate to an end, could have ended civilization.

The explosion ends due to decreased ‘births-per-woman’, another side effect?

At peak 20th century growth rates of 2.1% annually, or 42 times the average of the previous 12,000 years. At that rate, the same increase in population as occurred over 12,000 years, would happen in just 200 years. Such a rate of increase would quickly completely overrun the planet if continued, but now births per woman has startlingly fell levels similar to those previous 12,000 years, for population growth back to the historic normal.

Births per woman, which has already began falling prior to 1960, in the 60 years to 2020, fell from over 5.0 to around 2.3. Why did the births per woman then fall? Was it another ‘accidental’ side affect of something else as with the start of the explosion?

Just in Time?

How many times in movies is the bomb diffused with just moments to spare before there would have been a disaster?

The ending of the population explosion in some ways seems just like those movies. In the 1970s, at the peak of population growth, many people were already predicting disaster unless something was done.

If, instead of this fall in birth-rates taking place, growth at the rate of the 1970s had continued, there would be 18 billion people today (2021), in place of the less than 8 billion people we actually have. If population had grown to 18 billion, what would the world now be like? Imagine CO2 levels now in a world with double the population. Plus accelerated deforestation to access resources for over twice the number of people, and all those mouths to feed.

How did the fall happen just in time?

0. The Fall In Birth Rates Was Due To Responses to solve overpopulation.

While a few countries, most notably China, did launch planned responses to end the population explosion, as most countries with a fall in birth rates did not launch any planned responses, this is not serious candidate for a global explanation. Planned responses do not explain with birth-fell across the globe without most countries having such responses, which allows ruling planned responses as an explanation, but it is still a contributing factor in birth-rates falling.

A key point is that birth-rates have fallen to ‘peak-child’/stable population levels, without a planned response from humanity.

So here is what has happened from deliberate planned responses that were specifically designed to reduce birth rates. While they cannot explain global trends, measures include:

While these measures are certainly not a full explanation for global trends the include countries where no measures were adopted, planned responses have contributed to the global reduction.

There is a full spectrum possible between trying to educate people on the advantages of less children, through to the one child policy of China which went as far as mandatory contraception and sterilization in some cases for women who already had two children. In between education and the more drastic measures are financial and employment based incentives and limitations.

Certainly there were place in the world that took steps some time after 1960 in order to reduce birth rates.

  • China introduced the one child policy in 1979 after around 10 years of an earlier two child policy.
  • Various countries introduced public campaigns to encourage parents to only have 2 children, including:
    • Vietnam (1960s), Hong Kong (1970), Singapore(1980).
  • Some countries including India (2019), have introduced incentives for less children, and others (UK, 2012) have contemplated such measures,
  • Other countries have had ‘family planning’ programs, and education programs encouraging parents to have less children.
    • see Bangladesh example in Hans Rosing video here.


The one child policy was introduced in China in 1979, when world population growth rate was still close to the 1972 peak, but there is another story, which requires considering the population pyramid for China in 1978. Looking at the pyramid, at first it looks like two child policy introduced 10 years earlier , partly in response to the great famine, was starting to work, The 0-4 year population was for the 1st time in a long time, smaller now than it was 5 years ago (the now 5-9 band). But, now consider that each five year band represents the children born to the population four bars, or around 20 years earlier. The children in the 0-4 band, were mostly born to the 20-24 band, and therefore, still represented an increase relative to their parents, and as each of the next two groups to become 20-24 in 5, and then 10 years, were even larger populations, on trend, the population would again be growing. They felt they needed more!

Hence the move to a ‘one child’ policy. The pyramid of 20 years later shows there was logic to moving to the even stricter ‘on child’ policy and even despite this, and the peak years (5-9, 10-14) were again peak years 20 years later, but the pyramid is no longer so “bottom heavy”. The world population in 1978 was 4.3 billion and increased 40% to almost 6 billion by 1998. The 31% increase in China from 0.97 in 1978 billion to 1.27 billion is below that world average. At world average, China would have reached. So how many less people than the world trend in China after ten years of 2 child policy followed by 20 years of 1 child policy?

world_increase_rate x china_in_78 - china_in_98 = (6/4.3)x.97-1.27 = 0.083 billion

Today, as the population pyramid of China has been transformed from that of 1978. The people over 50, were all born before ‘child policies’, and the top of the pyramid is still the traditional shape of from the 1960s and earlier. But from age 50 down, the shape is no longer a pyramid, but the straight sided shape of a stable population, and almost half, 48%, of the population are over 40.

Overall, from 1978 until now, the population of China grew by 48% from 0.97 billion to 1.44 billion, while the world population grew 81% from 4.3 billion to 7.79 billion. If China had increased population at the rate of the rest of the world, China would have 317 million extra people. Considering growth in China was over world average before the two child policy, perhaps China could kept growing beyond the world average? It is hard to project exactly what would have happened had China not introduced the policy. Would an extra 317 million people, or more, been able to survive? If such a population had been able to survive, would would the environment of China look like as a result, and how would having so many people have damaged the economy?

Just looking at averages, the fall in birth-rates since 1972 globally have spared the world needing to support at least 3.2 billion more people. The efforts by China have outperformed the rest of the world, and that contribution by China meant that the rest of the world had to find around 10% less reduction. Without China, there could be 317 million extra people on Earth today.

The result is significant, but only explains around 10% of the world reduction in population growth. Perhaps the policy of Bangladesh and other countries can account for another 5% of the impact, but that still leaves over 85% of the reduction in birth rates down to measures not targeted at reducing birth rates. Why effective the entire rest of the world also reduced birth-rates is down to conjecture, but what is clear is there was no world-wide deliberate program as in China.

It is not even in the same category as the way the population explosion started, where although the goal of eliminating child mortality clear was not targeted at increasing population growth, it is certain that more people alive must be a consequence. Measures such as ’empowerment of women’ do may logically be a reason for less children, but does not with certainly preclude women having the same number of children. As a cause of decrease in birth-rates, it is feasible, but untestable explanation.

The biggest conundrum with all explanations that did not deliberately seek to address birth-rates, is just how co-incidental it is that these events not only produced the lucky result of reducing birth-rates, but took place and had the effect just when we needed birth-rates to be reduced.

Worldwide, the birth-rate (Niger and some other countries in Africa excepted) birth-rates fell dramatically without intervention as implemented in China, or strong education programs as with Bangladesh. Why? Is this just luck? Or something more?

1. Lucky Coincidence Due To The Pill, Education, Working Women etc.

Either Lucky Timing and Rate To Give Balance, Or Just Ignorance Of History?

A common explanation is that a variety of factors have in combination been responsible for reducing birth-rates. Going through the factors, most of them were not planned to reduce population growth, specifically:

  • Education of Women, which started even before the population explosion, was not motivated by a need to reduce population.
  • Increased participation of women in the workplace did not happen in order to reduce population either.
  • Even the pill, a new contraceptive, was not invented primarily to reduce population growth.

The challenge to accepting that a variety of factors resulted in the reduction of birth-rates, is that it provides no explanation beyond luck for the reduction in birth-rates resulting in population balance, or for the factors responsible for the reduction just happening to occur at the right time to for the balance, neither to early, or too late.

Just imagine if all these factors, such as educating women, the invention of the pill, increased participation of women in the workforce happened before infant mortality was so dramatically reduced? In that case, the population would have crashed as births were previously at replacement level, so there would been significantly insufficient births! Or alternatively, if the delay between reducing infant mortality and these factors reducing births had taken longer, the peak explosion could have gone another 100 years resulting in 8x the current population.

This “variety of factors” requires the balance being achieved niether too late nor too early simply being a lucky coincidence.

Population History.

The requirement of coincidence can be overlooked, if it assumed the population growth seen at the time of the reduction in birth rates had been in place for hundreds of years. While an examination of the history of population quickly makes it clear that the growth had not been in place for hundreds of years, not everyone looks. Once you look, it also becomes clear that the rapid growth prior to the reduction in birth-rates was unsustainable.

This means, accepting the ‘variety of factors’ explanation, requires either:

  • Ignoring the history of population, and previous almost stable population.
  • Or, accepting the coincidence of the factors happening at the right time and rate.

“The Pill”: A Key Factor In The Balance, Or Maybe Not?

Did the availability of the “pill” cause the fall?

Note that the fall in birth rates from 6.0 in 1900, to 4.7 in 1950, is the similar to

There were many charts at one time showing the strong correlation between the availability of the pill in the USA and a strong correlation to decline birth rates. The correlation was stronger in earlier years but as birth rate decline was significant after market saturation reduced slowed increase in usage of the pill lead people moving their focus to other explanations. Note a similar correlation exists for the USA between the uptake of colour television and decline in birth rates, plus a similar timeline for the decline in birth rates occurred in some other countries where the contraceptive pill was restricted or the introduction delayed.

The pill was not invented by a company planning to solve the worlds population crisis. It was also not invented with a primary goal of reducing family sizes, although a reduction in family sizes on average is a logical outcome of the invention of the pill. Interestingly, prior to the population explosion, humans had almost the exact balance of children required to be in balance with parents, once infant mortality took its toll. This means that had the pill been available prior to the reduction of infant mortality, and the pill did achieve a reduction in children at that time, the human population would have declined following the introduction of the pill.

During the population explosion, there was certainly a market for the pill. Perhaps during this time there was an increased appetite to also reduce family sizes? How much the pill has changed when people have children compared to how many children, is hard to know.

Overall it is clear not all reduction was due to the pill, but the introduction did very likely accelerate the process.

Given the invention was not in response to population growth, then to whatever extend the pill has reduced family sizes and population growth, the pill becoming available at the right time was certainly lucky. Much earlier, and there was no reduction in family sizes needed, much later and overpopulation could be far more serious.

The Full List Of Factors.

Here is a list from ‘our world in data on factors linked to the change in the number of children women have?

Empowerment of women
Women’s labor force participation
Increasing well being and status of children
Increasing prosperity and structural transformation of the economy
Culture and norms
Family planning (Untargeted)
Religion and fertility
Family planning (Targeting having less children as in Bangladesh)
Coercive policy interventions
Fertility is first falling with development – and then rising with development


Note none of the top entries were measures targeting reduction of birth-rates. This section is targeted at those reasons, and yes, there are measures directly designed to target birth rates in the list, but these are discussed further in ‘planned responses’.

This is not my list, but the list from ‘our world in data’. I would change ’empowerment of women’ to ‘education and empowerment of women’, although when I follow the link, it does become clear they do associate ’empowerment’ and ‘education’.

These ‘explanations’, can be divided into three categories:

  1. Actions specifically intended to reduce birth-rates. (see planned responses)
    • Family planning (Targeting having less children as in Bangladesh), Contraception as part of targeted programs, government programs to reduce births.
  2. Changes/actions which target other outcomes but do also anticipate reducing birth-rates, or sometimes are targeting birth rate reductions.
    • Family planning (Untargeted), Religion and fertility, Contraception (simply providing a method for avoiding unwanted children together with sexual freedom)
  3. Changes/actions with no intent to change to birth-rates an not the intent, and are not a planned outcome, but may be a bonus.
    • eg, Women’s labour force participation, Increasing well being and status of children, Increasing prosperity and structural transformation of the economy, Culture and norms

As already covered, the vast majority of countries have not taken any steps intended to reduce birth-rates, so the first two classes of explanations have already been ruled out as a universal explanation. Coercive policy interventions as applied in China, family planning that also targets population growth, as practiced in Bangladesh, can have an impact, but there are too many counties with no such measures for these to explain it all. Improved contraception has also had an impact, but again simply does not explain the extent of the change.

This leave only class number three, factors not are not designed to reduce birth rates, as the only explanations capable of applying in most countries where birth rates have fallen.

It has nothing to do with sperm counts or the usual things that come to mind when discussing fertility.

Instead it is being driven by more women in education and work, as well as greater access to contraception, leading to women choosing to have fewer children.

In many ways, falling fertility rates are a success story.

BBC- Fertility rate: ‘Jaw-dropping’ global crash in children being born

This quote from the BBC represents a typical attempt to explain the falling birth-rates. A combination of factors, but with changes to society, not at all intended to reduce birth-rates at least partially responsible for a “success story”.

Birth-rates having been reduced by side-effects, puts the reduction of birth-rates into the same category as the reducing child mortality triggering population growth: a predicable but unplanned accident.

The implication is that if we had a society where, if religion or some other factor delayed the globalisation of education of women for years, or we had not invented the pill at the time we did, our society we would probably have collapsed due to overpopulation.

If, as is generally proposed, changes/actions which predominantly did not have reducing birth-rates as a primary goal, have played a critical role in preventing a catastrophe, then we have been saved at least in part by luck.

Summary: So, all explanations that are in not way linked to efforts to lower birth-rates still amount to luck that birth rates fell when needed, as above. While these explanation provide more detailed analysis, all of them share the concept that some change to society just happened to result in a reduction in birth-rates.

Can We Explain The Pure Luck: Are We Just The Lucky Ones?

If there is only one planet in this galaxy of over 100 billion stars with an advanced intelligent life form, what are the odds of being born on that planet? Incredibly low, but since only and advanced intelligent life form would contemplate those odds, the only people thinking about the odds, have already won that very lottery. In other words, unlikely as any event may be, if the event is required for our survival, that event must have happened, because we are here to think about it. So anything required for us to be alive, no matter how unlikely, happened in our past, because in order to think about what happened, we have to be on a planet where it did happen.

The same principle could be applied to the fall in birth rate. Perhaps it just happened randomly, despite it being incredibly unlikely. The only reason we are still able to contemplate those odds, is that it did happen, which enabled us to still live in a civilized society. All the people on planets were the birth rate did not fall, at a similar point in their society on their planet, died from overpopulation, and this is just another reason why we are not seeing other advanced civilizations.

Summary: It is possible that the reduction in birth rates just happened almost randomly, and just as there is no point in a lottery winner wondering ‘why me’, there is no point in wondering about birth rates, it is just chance. That we are on the planet where the lucky event happened, enables us to wonder about it all.

2. Birth-Rates Naturally Adjust To The Environment.

Birth-Rates Historically Have Resulted In An Amazing Stable Population.

Humans have existed for over 300,000 years with an amazingly stable population. Prior to 1650, the average annual rate of population growth was a mere 0.007% per annum.

For thousands of years prior to the recent population explosion, people had the approximately 6 children, which just happened to be the number historically required for stable population. Historically, over time the population, did ever so gradually rise, but humans were also improving the environment.

The need to have children is hard-wired into humanity, just as reproduction is hard-wired into all living things. Yet over millions of years, most plants and animals experience a population that can fluctuate, but in the big picture is amazingly stable. Might that hard-wiring be more sophisticated than we give credit, and the need to have children be tuned to have an appropriate number of children?

Consider our nearest relatives, chimpanzees and gorillas. They seem to have stable populations, without accidentally having population explosions that are unsustainable. Just perhaps, our basic instinct to have children, is also to have children in balance.

People in the past never lived in ecological balance with nature, they died in ecological balance with nature. It was utterly tragic!

Hans Rosling (see video, 19m)

The point by Hans, is that the birth rate only produced a balance once deaths were also considered. But it was in balance. Historically, humans produced the right amount of children such that, overall there was balance. Now consider, what if the system takes time to adjust, but in fact humans have now simply adjusted to this new lower death-rate, by naturally having a lower-birth rate?

Historical population levels reveal that human population historically grew very slowly. Growing so slowly requires almost exact balance between births and deaths.

Population levels did slowly rise as civilization evolved to increasingly displace nature further and further, and as human spread over the globe, but population with few rare exceptions, remained at the sustainable level for the society. Even at 1/10 the growth levels of the population explosion, in just the time of the Ancient Egypt, humans had time to completely overrun the planet. But instead, population was to stable for thousands of years.

When people first evolved from a common ancestor shared with chimpanzees, and before the invention of farming, like chimpanzees, humans would have had natural predators, and needed higher birth-rates to compensate. This means, that historical 6 children, would also have been a step down from the previous level when humans we preyed upon.

What if, the measures introduced in China were only required while people adjusted, and are no longer necessary? What if the explosion occurred because the change in infant mortality was so sudden that we humans needed time to adapt, but we are now adapting to our changed situation?

The attraction of this hypothesis is that, instead of requiring a belief that it was only coincidence that birth rates happened to fall the right amount, just when they needed to fall, it requires only a belief that it is our nature to find balance.

The one possible factor not explored in material I have found so far, is what if humans naturally reduce birth-rates when they need to?

Sure other factors affect birth rate, but perhaps regardless of other factors, humans will find a birth rate balance, at least provided nothing interferes to a level beyond our ability to adjust. History shows humans have been able to adjust birth-rates in the past, and by reaching peak child again, while the pill and other factors may have all helped, the fact that birth rates have fallen to the replacement rate suggests that human behaviour has also played a role.

On this basis, that fall in birth rate is in response to the fall in infant mortality. There is a lag, and modern society may make us slow at picking up cues, but if this is true countries with low infant mortality will find balance sooner or later.

Yes, there is correlation with many factors such as education of women, but these factors also have a strong correlation with reduction in infant mortality.

To me this seems possible. However, perhaps the instincts that while in the jungle or the savanna, could pick up signals to trigger people to having more or less children in order to maintain the balance, have difficulty adapting to pick up the signals in our modern city environments?

3. Nature: Instinct Targets Birth-Rates for a ‘Optimum’, Sustainable Population.

This explanation is based again on reproductive behaviour playing the final role in determining rate of reproduction, but instead of population stability, proposed the target is to march environmental carrying capactiy.

This proposal is explained in more detail in the ‘Optimum Population’, but in summary seeks to explain some details not covered by the assumption of an instinct for a stable population:

  • Population must grow following disasters or a decline in numbers, as for example has happened with whales.
  • In response to changes in climate, the ideal population can either increase or decrease, and species need the ability to adjust to changing circumstances.
  • Human population did increase with advances such as the invention of farming that provided for a larger population.
  • It is easier to explain people being able to sense that population feels appropriate, than being able to sense the correct number of children for population stability.

People now make decisions to limit their family size to 2 or 3 children, and in fact some people even make the decision to be be child-free. The choices people make go beyond While women being more educated or having an increased role in the workplace may factor into some of these decisions, the choices go beyond into other factors as well. To ignore the role of a change in how many children people want to have, seems to be ignoring something significant.

A key difference with the theory that nature targets a stable population and thus humanity will remain at ‘peak child’, is that if nature targets instead an ‘optimal’, or sustainable population, then birth-rates may continue to fall, and stabilises for some time at level that results in a population correction.

There is a lot to explore before declaring everything adds up. How the signals could work in society? Wouldn’t it be logical if people are responding to environment for people in crowded areas to have less children? So many questions that a dedicated exploration make sense.

But, it does seem that the fall in birth-rates compensating for the previous fall in child mortality, has better explanations than just coincidence.

Conclusion: The Paradox, and ‘Optimum Population’.

In summary:

  • Programs to reduce birth-rates played a role in ending the population explosion, but on all evidence contributed no more than at best around 15% of the reduction globally.
  • Explaining the other 85% of the reduction, as a a coincidence of several factors that just happen have a side-effect of reducing birth-rates, all occurring right at the time when humanity requires birth-rates to fall, doesn’t even apply in many countries that have seen birth rate fall.
  • It seems far more likely that the falling birth rates, even in countries where governments introduce measures designed to stop birth-rate falling, is explained by people naturally adjusting the number of children to the environment.

Update Aug 2021:

At the time I first explored this topic, that none of the existing answers I had found stood up to scrutiny was a paradox.

But further consideration, and particularly exploration of just how stable populations in nature are to not complete overrun the planet in the millions of years most organisms have been reproducing, has led me to believe ‘optimal population‘ to provide the explanation.



Table of Contents


Why Population growth even before the explosion?

Throughout history, although no other species on Earth has experienced such long term overall population growth, even before the recent population explosion, the human population kept slowly growing.

Yes, we recently had an unprecedented population explosion, driven by is hidden by by the near elimination of previously tragic infant mortality, but against the background of long term growth, many of us never didn’t even realise their was an explosion.

But what drove population growth even before the explosion? What will now happen as the explosion ends?

Read More »

Population Explosion: What happened to get to 8 Billion people?

Humanity has just experienced a population explosion. Whether you believe we are overpopulated or not, the facts are that we have just had a population explosion.

Over two centuries of population peaking at 50x higher than the long term population growth, and peaking at 100x higher during the baby boom.

Yet the explosion occurred with women having less babies than ever before, even during the baby boom, and is ending all by itself.

So what caused the boom, why a population explosion when birth rates are falling? Will our population grow until we are living life like battery hens?

Read More »

Peak Child: When, and what does it mean?

Globally, the when is now. Although locally, experiences differ by location.

Data now clearly confirms that globally, the world has returned to ‘peak child’, a precursor to a likely return to peak population, and the end of three centuries of a population explosion.

Return to peak child means we could either return to the gradual growth levels of prior to the industrial revolution, or become a mature species, and exist in balance with nature and other living things. Perhaps even stop displacing other species?

Read More »