Monday, April 11, 2016

The population problem: should the Pope tell people to stop breeding like rabbits?

Image by Rakka.

In this post, I argue that overpopulation is a complex problem that has to do with human choices at the level of single families. It is not impossible that such choices will eventually lead to a stabilization of the world population at a sustainable level. It has happened in some historical cases, such as in Japan during the Edo period.

The population question arises strong feelings everytime it is mentioned and there is a general feeling that people will keep reproducing like rabbits unless something drastic is done to stop them. This position often goes in parallel with criticism to religious leaders and to religions in general, accused of encouraging people to reproduce like rabbits. Or, at least, to hide the fact that reproducing like rabbits is bad for the planet.

But is it true that people tend to reproduce like rabbits? And would they stop if someone, let's say the pope, were to tell them to stop? Maybe, but things cannot be so simple. Let me show you an example: Japan during the Edo period.

The population of Japan during the Edo Period (uncorrected data as reported by the bBafuku government). It shows how it is perfectly possible to attain a stable population in an agricultural society, even without "top-down" rules and laws. (data source, see also this link)

Note how the population has remained relatively constant for at least 150 years. It is a fascinating story, discussed in detail in the book "Mabiki: Infanticide and Population Growth in Eastern Japan, 1660–1950" by Fabian Drixler. Here is an illustration from the book:

Another impressive set of data: the net reproduction rates in Japan remained around or below the replacement rate during the Edo period, keeping the population constant for, indeed, something like one century and a half. It is also impressive to note how the reproduction rate literally exploded afterward, bringing the Japanese population from the ca. 25 millions of the Edo period to the present level of around 125 million, five times larger. Note also how rapidly the reproduction rate collapsed after the 1950s; it is a stark example of what we call the "demographic transition."

As we can see from these data, human reproduction strategies are much more complex than what you would imagine if you limit yourself to the biblical commandment "grow and multiply". The Japanese did NOT reproduce like rabbits during the Edo period. It doesn't appear that they were forced to reduce their birthrate by the government or by religious credences. Some famines are reported in Japan during the Edo period, but they couldn't have been truly disastrous, otherwise you would see their effects in the population curve. The population remained stable, it seems, mainly by "bottom-up" strategies at the level of single women or single families: contraception and, when that was not enough, infanticide.

So, what led the Japanese families to choose (rather than being forced) to limit their reproduction rate? There is plenty of scientific literature on the strategies of reproduction of various species, including the human one. The basic idea is that, in all cases, parents have a choice on how to employ their limited resources. Either they invest in having a large number of offspring (the "r-strategy", also the "rabbit strategy") or they invest in caring for their young until they reach adulthood (the "K-strategy" or the "Elephant strategy"). The choice of the reproductive strategy depends on the situation.  Let me cite directly from a paper by Figueredo et al. (1)
...... all things being equal, species living in unstable (e.g., fluctuation in food availability) and unpredictable (e.g., high predation) environments tend to evolve clusters of “r-selected” traits associated with high reproductive rates, low parental investment, and relatively short intergeneration times. In contrast, species living in stable and predictable environmental conditions tend to evolve clusters of “K-selected” traits associated with low reproductive rates, high parental investment, and long intergeneration times.
Humans, clearly, are more like elephants than like rabbits. The number of children that a human female can give birth to is limited, and it is normally a good strategy for her to maximize the survival chances of fewer children, rather than trying to have as many as possible. So, for most of humankind's history a family - or a single woman - would examine its environment and make a rough estimate of what chances their (or her) children could have to survive and prosper. In conditions of limited resources and strong competition, it makes sense for parents to maximize the health and fitness of their children by having a small number of them. It seems to be what happened in Japan during the Edo period: facing limited resources in a limited island, people decided to limit the number of their offspring, applying the "K-strategy."

The opposite is true for periods of abundant resources and scarce competition. When the economy is growing, families may well project this growth to the future and estimate that their children will have plenty of opportunities, then it makes sense to have a larger number of them - hence to apply the "r-strategy". The dramatic growth of population during the past 1-2 centuries is the result of the increasing consumption of fossil fuels. Everywhere, and in Japan as well, people reacted by filling up what they saw as open slots for their children. But with the second half of the 20th century, economic growth slowed down and people started to perceive that the world was rapidly filling up and that the economy wasn't growing anymore. They may not have perceived the depletion of mineral resources, but the result was obvious anyway. It was the "demographic transition," normally related to increasing wealth, but that we may also see as the result of a perception of the future that was seen as less rosy than before.

There are other cases of human populations that remained stable for some periods, so we may conclude that humans do not - definitely - reproduce like rabbits; except in some very special are rare conditions of history. Humans are intelligent creatures and, within some limits, they choose how many children to have in such a way to maximize their survival probabilities. The human population will tend to grow in a condition of economic growth, but it should tend to stabilize in static economic conditions. So, if we were able to stabilize the economic system, avoiding major wars and the need of cannon fodder, then the human population may well stabilize by itself, without any need for a "top-down" intervention by governments (or maybe by the Pope). Unfortunately, between now and then, there is a little problem called "overshoot" and stabilization at a sustainable level may be anything but painless. But if stabilization was possible on the island of Japan during the 19th century, why can't it happen in the larger island that we call "Earth"?

See also a post of mine titled "The cuckoo that won't sing: sustainability and Japanese culture"

1. Aurelio José Figueredo, Geneva Vásquez, Barbara H. Brumbach, Stephanie M.R. Schneider, Jon A. Sefcek, Ilanit R. Tal, Dawn Hill, Christopher J. Wenner, W. Jake Jacobs, Consilience and Life History Theory: From genes to brain to reproductive strategy, Developmental Review, Volume 26, Issue 2, June 2006, Pages 243-275, ISSN 0273-2297,


  1. I wish I could be as optimistic as you are.

    Japan is really not a good example to use here. First, it's an island nation, with a lot of mountains and very little agricultural land (and mineral resources too) -- the idea of limits sinks in a lot more easily in that kind of environment. Second, people arguing that overpopulation constitutes a crisis to be dealt with urgently usually point out that a drastic cultural change is the only way to adequately address the problem, and that unthinkable measures such as mass abortions and infanticide will have to be implemented in the process.

    You bring up Japan, a society as far removed from the Abrahamic tradition as one can get on this planet, and how it used contraception and infanticide to control its population.

    But contraception and infanticide are precisely the very things that the Abrahamic religions refuse to compromise on and people such as me are criticizing figures like the pope for...

    Basically, you are saying that if we could make the Abrahamic religions in their current form disappear, then we might have a chance to solve the overpopulation crisis.

    Well, duh...

    But even that misses a very important aspect of the problem -- time. You have spent years playing with world models, not only that, but you have been popularizing the Seneca cliff idea. You know very well what the projected timing of collapse is (with all the uncertainties), thus you should also understand very well that any non-coercive, non-traumatic, gradual approaches to reducing the world's population will take a lot more time than what we have before it's too late. You even sort of mention that in passing toward the end.

    P.S. All humans are r-strategists, even the ones that have 10-15 kids and let the roll in the dirt all day. The demographic transition does not happen because people realize that there are ecological limits and voluntarily limit their fertility. Humans, as every other organism, maximize inclusive fitness. The catch is that they do not do so directly, because the vast majority of them have no clue what that even means, they maximize certain proxies for it that have historically correlated very well with inclusive fitness and accordingly behavior oriented towards their maximization has been selected for. Social status is the major one. In an industrialized economy, it becomes prohibitively expensive to maintain social status while having more than one or two kids, which is why most people stop at one or two, and the ones who don't tend to belong to groups where social status is achieved outside of the established norms of general society (gypsies, Amish, quiverfuls, etc.).

  2. Honestly, I don't see how Abrahamic religions can have so much importance as you say. Think of Italy. We are catholic; yet we have possibly the lowest birth rates in the world. Think of Ireland after the famine: they remained Catholic, but they drastically reduced their birth rates. I think there is something you say in Church, on sunday morning, and something you do the rest of the week

    1. Italy has a low birth rate because having kids in Italy is too expensive both financially and in terms of social capital (they are a hindrance if your goal is maximizing your social status).

      That is not the case in other parts of the world where a different set of cultural factors operates.

      It remains true that it would be much easier to solve the problem if the Abrahamic religions were not there arguing passionately against the very measures that will address it.

      Also, there is the question of worldviews. People in Europe in the 21st century may not be very religious, but their conception of the world is still firmly rooted in the Christian tradition -- the natural world is something external to, separate from and subordinate to what really matters, the world of humans (and according to some, souls/spirits). This is the single most important barrier to widespread understanding of the sustainability situation -- the inability to see the human civilization as integral part of the ecosystems of the planet.

  3. While I enjoyed and liked the example you provided, the problem I see with overpopulation is that it is ocurring in countries which in many regions don't even have the social and cultural cohesion that the Japanese had. Many countries in Africa as well as Afghanistan seem to be but collections of warring tribes with little governmental cohesion and constantly at war, suffering from many endemic diseases and an uncertain future (with quite a lot of help from foreign powers). With all this instability, it does not seem there is going to be a move to a lower fertility rates, especially when literacy levels are similar to those of Japan in the second part of the Edo period even nowadays. Hopefully it will change if politicians finally get their act together.

  4. I'd like to throw in an interesting piece of information.
    According to, the number of unintended pregnancies each year is about 80 Million - only in developing countries.
    The growth rate of the global population, i.e. the birth surplus, is about 76 Million, according to
    A lot if not all of those unwanted pregnancies could be avoided by access to contraceptive means, information and consulting.

  5. Merci Monsieur Bardi !

    Cet article est édifiant !

  6. Thank you for the article. Unlike in the japannese example we will have to mantain or decrease our numbers and energy consumption before.

    We only have to avoid our own Sengoku Jidai before getting there, and a hundred years of struggles (


  7. How does one really know what “overpopulation” is? (I am familiar with “carrying capacity of the planet” concepts and estimates, I ask the question mainly rhetorically to make a somewhat different point) :

    The population of Greater Tokyo is about 38 million people living in about 13,500 km2. Three Tokyos would make up the entire population of Japan of about 127 million people. This could fit into 3 x 13,500 km2 or 40,000 km2 or just over 10% of Japan’s total land area of 364,000 km2 leaving (purely in this thought experiment) the remaining 9 /10 of Japan empty of people. Could Japan support three Tokyos? “Only” two more than at present? Tokyo incidentally is a very livable city. (at least compared to many other megacities)

    The total land area of earth is about 510 million square km. and the current population is roughly 7400 million people which works out to about 14 people per square km. (forget the Himalayas and etc. etc. it is only a thought experiment !) The population density of Tokyo is 2,642 persons per square km. The entire population of planet earth could fit into just 200 Tokyos. And Isaac Asimov wrote about “Tantror” a fictional empire in the center of the Galaxy with about 45 billion human inhabitants first supported by underground farming and yeasts and later by having its food supplied by twenty nearby agricultural worlds. Energy was provided by geothermal “heat sinks”. Nice place to live? Maybe not.

    But Trantor is of course only fictional. Are 200 Tokyos spread throughout the planet with the rest of the planet being very sparsely populated and used mainly as a source of energy and food and material resources, also necessarily purely fictional? Do population density and spatial distribution matter ? The following article analyzes the energy and material flows of megacities. (much interesting analysis)

    But personally I find it rather heartening if we are able to conclude from the evidence and reasoning of this post that humans might adjust their population growth rates by themselves in response to a whole range of bottom- up factors rather than needing (or presumably needing) top down solutions to be imposed. Do governments ever get such things right? Eugenics is of course something else, (??) but it might be well to remember a bit of its history. Following below is just one quote from the article on this link. One that is in fact “pretty tame”. The entire article is well worth reading since eugenics is now mostly forgotten.

    “Eugenics would have been so much bizarre parlor talk had it not been for extensive financing by corporate philanthropies, specifically the Carnegie Institution, the Rockefeller Foundation and the Harriman railroad fortune. They were all in league with some of America's most respected scientists hailing from such prestigious universities as Stamford, Yale, Harvard, and Princeton. These academicians espoused race theory and race science, and then faked and twisted data to serve eugenics' racist aims”

    How easy it seems to be able to come up with a racist pseudoscience. How exactly did it happen and are there any lessons to be learned? (before we start to think too much about the optimal human population and how to achieve it)

    1. Very interesting article link Max. Thanks.
      It would be good to have the fully referenced account and see it supported by all historical documents on archive.

      From my little bit of British history I know something of Social Darwinism (not to be confused with scientific studies of evolutionary theory). Some late offshoots with the addition of cod-psycholgy and erroneous cognitive testing were supporting UK - particularly English - educational policy in my schooldays. Not so much 'racist' as in our case 1950s class politics - bonkers biology and bad science but persuasive for many.


  8. Had we but world enough and time
    This population were no crime.

    But we have neither world (resources) or time.

    On the bright side, Malthus will finally come to the fore.

    1. I'm not sure that's actually the case. Might I suggest a reading of Michael Mann?

  9. Hi Professor Bardi -

    great article, thank you. an observation from another angle. in my years in east Africa working in relief settings, the strategy that I saw often emerge was for families to have lots of children, and then direct scarce resource (such as food rations) to adolescent males or their young men. these fellows would then maximize that resource by trading, foraging, or often roaming very far in search of opportunities. because infant mortality rates were so high, this strategy seemed to result in high birth rates, high infant mortality due to malaria especially, with such a selection process resulting in males who then got extra or 'best' rations. aid agencies upset that particular dynamic by providing food rations and often medical care directed in particular at reducing infant mortality. an industry thence burgeoned... extrapolating then just a bit, I think that close boundary line that influences so much how certain populations in that more rural context coped revolved around 'swiddener' agriculture and the more permanent farms that had an annual cropping cycle. (the pastoralists another story). anyway, not to be reductionist, just positing very briefly a different angle on the issue to consider, and how our current relief structure (early on a good way for the US to dump subsidized surplus ag production for example) has intervened in local processes so much. a read or reread of Colin Turnbull's great works, the Forest People and the Mountain People is perhaps illustrative of some of this.
    all the best from the North Vancouver side of the world, as we watch the increasingly ominous weather events of the arctic unfold. interesting times we live in...

    1. Thanks for this comment and, yes, we tend to worsen problems by trying to solve them.

  10. Dear Professor ! I have not in front of my eyes, at this moment, the so loved graphics, but we have always known that higher birth rates come from poor people, underdeveloped countries. How comes that suddenly "The human population will tend to grow in a condition of economic growth".Religion has noting to do with such dynamics, poverty has .

    1. "We have always known that higher birth rates come from poor people, underdeveloped countries." Which is perfectly true for this specific moment in history. Many things are going to change soon.

    2. Ugo, let me ask you something. You point out the distinct possibility of civilization collapse in large part due to climate change. Is it not equally as possible that civilization decline while it transitions to a renewables economy over decades, which looks like it will happen, but remain intact while staving off the worst effects? Once the transition occurs, then things improve.

    3. The way I see it is that once we start the rapid decline that I call "The Seneca Cliff", then all bets are off. The models break down and we can't really say what's going to happen. We may all be boiled by heat, survive at the Northern and southern ends of the continents, and, yes, there is the possibility that the "dip" will not be so bad and that after a horrible period of a number of decades, civilization as we know it rebounds and transforms itself into something new, but maintains the links with the old world. A little like when Charlemagne thought he could restart the Roman Empire, as if there had not been the Middle Ages in between

    4. With all due respect, I think the boiling part is unlikely. I just tend to think civilization is more likely to collapse slowly, if at all, on a centennial to multi-centennial timescale. And there is the wilscarf of solar radiation management to consider. I find Michael Mann' take on climate science to be the most realistic. He says he's cautiously optimistic, but that we're playing with fire and need to be very careful.



Ugo Bardi is a member of the Club of Rome and the author of "Extracted: how the quest for mineral resources is plundering the Planet" (Chelsea Green 2014)