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 Bafuku 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, http://dx.doi.org/10.1016/j.dr.2006.02.002

The story of Rabbit Island

This post had started as a review of the book "Too Smart for Our Own Good" by Craig Dilworth. After having worked on it for a while, however, I found that I could add nothing to the excellent review already written by George Mobus. So, I thought that I could rather express my feelings in terms of a little story. If you have studied population dynamics, you'll recognize that what I wrote is a fictionalized version of the Lotka-Volterra model of foxes and rabbits interaction. I think it is the bottom line of Dilworth's thesis: humans are smart at inventing technological toys just as foxes are smart at catching rabbits. The results, however, are not necessarily good. It doesn't matter if you are a fox or a human being: you are too smart for your own good. (Image above from Jokeroo)

The Story of Rabbit Island
by Ugo Bardi

It is said that foxes came to the Island long, long ago. Some say that the first fox couple arrived on a raft from a place beyond the horizon. Others say, instead, that foxes were created here by the Fox-God, and others still that they had always been here, from the day when the Gods raised the Island out of the waters of the infinite Ocean.

Wherever they came from, the first fox couple found the Island rich in grass, trees, water, and many, many rabbits. And the foxes were smart and strong and they grew by chasing rabbits and killing them in great numbers. Many young foxes were born and that was -  some foxes said -  the way things had to be.

In time, foxes grew even more in numbers and some foxes started saying that rabbits had become difficult to find. A story is told of an old fox, that many saw as wiser than most, who gathered the whole fox folk and spoke to them. "Fellow foxes," the old fox said, "we have been growing so much in numbers that soon there won't be enough rabbits to feed our youngsters; and they will starve. We shouldn't kill so many rabbits as we have been doing so far, and we shouldn't have such large litters, either."

But some foxes said that there was no such a thing as too much killing of rabbits. They said that there were still plenty of rabbits around; it was just a question of looking harder. If some young foxes were starving, they said, it was because they had become lazy. They had to be taught how to run faster and to be smarter. In this way, foxes would still be able to catch as many rabbits as they needed. And they laughed at the old fox and they returned to chasing rabbits.

And then, the Great Die-Off came. I have been telling this story many times and it still scares me, even though I am the oldest rabbit of the Island. But I have to tell to you this story, young rabbits. I have heard it from my father, who heard it from his father, who heard it from the father of his father, and so on in a chain that arrives to one of the few rabbits who survived the Great Die-Off. And, believe me, young rabbits, it was a terrible time, for the Island was full of foxes. Rabbits died in large numbers and there was no way for them to escape. It is said that just a few of them could hide in the darkest places of the forest; in thornbushes and in mazes of tree roots, praying the Rabbit-God that they could be spared from the fury of the foxes.

And the Rabbit-God must have heard their prayers because they were not found by the foxes. After some time, they dared to come out of their hideouts and they found that there was no fox to be seen alive anywhere on the island; only their bones were left; strewn all over the plains. Once, there were many, many more of these bones, but you may have had a chance, young rabbits, to see some of the few that remain.

So, this story has a happy ending. After the Great Die Off was over, we rabbits had the Island all for ourselves. And we have had good grass to eat and good times to grow and multiply, which some say is what the Rabbit God told us to do. Yet, sometimes I think that this story may not have such a happy ending after all.

You know that there are now many, many rabbits living in the island; so many that fields seem at times to be white and brown rather than green. And that cannot be good. Some of the wise rabbits have been telling us that we shouldn't let our numbers grow so much, because grass can't regrow fast enough to feed so many of us. But others have said that grass is not the problem. Young rabbits have become lazy, they say, and they only complain so much because they can't always find grass at paw length. That's not the way a good rabbit should be: they must learn to find their food, even at the cost of walking far away, where there is still plenty of grass.

It may be that there is still enough grass for all of us, somewhere, although I doubt it. But what makes me afraid the most is what I have been hearing lately. You may have heard the same rumors: that some rabbits have disappeared and nothing was heard of them any more - not even their bones could be found. And you may have heard of some who have been telling of grey shapes they saw hiding in the forest. And, at nightfall, some have been telling of bright, yellow eyes looking at them from the darkness. Could it be, God forbid, that the foxes are back?

If that is true, nobody can say if some foxes had survived the Great Die-Off, or if some of them came again on a raft from beyond the horizon. The only thing I can tell is that perhaps we should not have grown in numbers so much, because rabbits make good food for foxes and some old rabbit folks had alerted us about that; long ago, but nobody listened to them. And now it is too late. I am an old rabbit now, so I won't see what the future has in store; but you will, young rabbits. So, it is time for you to go to sleep. Sleep well and don't look at the forest.