Cassandra has moved. Ugo Bardi publishes now on a new site called "The Seneca Effect."

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 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,


Ugo Bardi is a member of the Club of Rome, faculty member of the University of Florence, and the author of "Extracted" (Chelsea Green 2014), "The Seneca Effect" (Springer 2017), and Before the Collapse (Springer 2019)