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

Saturday, December 6, 2014

The Limits to Growth Revisited: some comments

Recently, I received a set of comments on my 2011 book, "The Limits to Growth Revisited". I am publishing them here with some minimal edits; with the permission of the author. 

by Max Kummerow

Dear Dr. Bardi,

I've just finished reading limits to growth revisited, a great pleasure. The logistic function adds greatly to the original Malthus exponential population, linear food, and the predator-prey, capital-resource comparison makes sense, although the "prey" can't regenerate, so we only get one cycle with a non-renewable resource. So humans try to find a new resource.

I was an undergraduate chemistry major who ended up in the social sciences, eventually dabbling a little in econometrics. I read some philosophy of science and have a few expansions of your comments on modeling to suggest:

1. The "all models are false" quote comes, according to Peter Kennedy's "Guide to Econometrics" (fifth edition, I think) from G.P. Box, the giant of time series statistics. The full quote is: "All models are false, some are useful." The reason all models are false is that to be useful a model must be a simplification of the real system, which is infinitely complex in all real cases. So all models suffer from omitted variables bias. Useful models reproduce important behaviors and offer insights about the system.

2. Hugh Mellor, a Cambridge U. philosopher of science commented that all models fail to mention "auxiliary conditions" or "side conditions" (or omitted variables) that are assumed not to vary. We assume, for example, that the sun will rise and no nuclear war will occur. And so on. Sometimes these omitted variables do change and so they matter.

3. Nassim Taleb noted that extremely improbable events do occur and can have big impacts--the Black Swan idea. But since they are so rare, the probability of these events can't be estimated. The black swan phases come from the example of inductive logic that observing any number of white swans cannot finally prove the generalization all swans are white, but one black swan can disprove it. This led to Popper's falsification logic where we try to confront null hypotheses with data that might disprove them.

4. However, in a complex system, any counterexample that might seem to require rejecting a null hypothesis, can be rationalized as due to some confounding variable, leaving the generalization unrefuted. So falsificationism fails to be a panacea of inductive logic.

5. David Orrell, a former climate modeler, has written about complexity, arguing that there will always be inability to confirm complex models (due to limited data, measurement issues, misspecification, functional form, etc.), so that a reasonable person could choose to doubt. Especially if funded by an energy company and ideologically inclined to doubt. But, Orrell says, even though we can't specify and prove a complete model, we almost always know enough to choose better and worse policies. (Best to emit less carbon and slow population growth.) 

6. My time series teacher, Michael McAleer, formerly at the University of Western Australia, has written a book (I believe Keutzenkamp is the co-author) on simplicity in models. Their idea is that there is a kind of optimum degree of model complexity such that too much complexity makes the model less useful, in principle, because of the inability to really verify complex mathematical interrelationships. For example, adding terms leads to multiplication of interaction effects, even in linear models.

7. George Soros talks about "reflexivity" echoing his mentor Karl Popper, who pointed out "the poverty of historicism" meaning that one can't forecast the future in principle, due, in addition to complexity and chaos, to the fact that people can decide to act differently based on the forecast. So if everyone believed climate change will end civilization, we could easily solve the problem, whereas if everyone denies climate change it will surely end civilization. Inherent indeterminacy.

8. This is related to the idea of "free will" which a philosopher told me is not a good way to think about decisions. We always have some choice and some constraints--limited information and limited ability to process information. So we should be trying to improve our decision making capability to get better decisions and always realize our limits. The "ultimate resource" is also a limited resource--very important point.

9. Game theory provides formal confirmation of Hardin's tragedy of the commons--there are games where rational individual decisions lead to the emergent property of collectively irrational outcomes (prisoner's dilemma, for example). 

10. Don McCloskey (now Dierdre) classified quantitative models in economics as merely another form of rhetoric, rather than any actual proof or absolute confirmation. Econometricians with different ideologies come up with different models from the same data. McAleer has an example where two models fit the data about equally well, but lead to opposite policy conclusions. (Sherlock Holmes is in the title of that paper.)  LTG succeeded as rhetoric and so did the refutations.

Getting the philosophy of science foundations in place is a key to good models. 

Final point: I think every discussion of fixing the world should emphasize the key role of demography. Population growth and economic growth are both fundamental drivers of ecological damage. About 45% of the world has transitioned to below replacement fertility levels (so it isn't that hard to do), but the wide divergence of fertility rates (from <1 to >7 between countries) and the high fertility of religious fundamentalists (See Kaufman, Shall the Religious Inherit the Earth) means fertility could rise as below replacement groups exponentially decline and high fertility groups exponentially increase. Also drives immigration, of course. The population topic was driven off the agenda along with the LTG topic. It is amazing how universal the faith in growth as a solution to scarcity is, when in fact, it is growth that creates scarcity.  

Max Kummerow


  1. A collection of decidedly brilliant remarks by Max Kummerow. I cannot find flaw in a single one of them.

    Garret Hardin's "tragedy of the commons" is one of the principal stories of our time. Garret Hardin himself understood the disturbing fact in which, if primarily those individuals who were conscientious about reducing human population had fewer children than others, then in the long run the human population would be made up almost entirely of those individuals whose parents were not conscientious about having fewer children. This is why Hardin quit his membership in "Zero Population Growth".

    The aggregate human gene pool is one of many commons about which people should be concerned. Currently it is primarily fecundity which is being selected for among human populations. Resistance to disease and general hardiness were selected for until recently, maybe the last 100 years or so. However, modern medicine has equaled the playing field, and sickness usually does not mean death or a reduction of fertility until old age has set in. This spells trouble for the human gene pool in the long run. Intelligence is also not being selected for. In fact, it might be being selected against, which is quite disturbing.

    This matter of the erosion of the overall fitness of the human gene pool leads to a big question - is eugenics the only answer now and in the future for maintaining or improving the fitness of the human gene pool? If so, then we could very well be headed for extinction in the not too distant future, even without taking into consideration various ecological factors like global climate change, loss of topsoil, acidification of the oceans, loss of biodiversity or resource depletion.

    Eugenics on a scale that would be significant would require global cooperation among people that is exceedingly unlikely.

    1. I wish that these eugenics arguments would stop being put forth by such a large number of otherwise intelligent and honest people.

      Inheritability of intelligence is a real phenomenon, yes, but rapidly regresses to the mean. For instance: (

      "If one starts with two parents whose IQs are 160 and looks at the average IQs across generations the speed of the regression to the mean is quite fast.

      Parents 160, 160
      Children average 136 (assume these mate with a 136)
      Grandchildren average 122 (assume these mate with a 122)
      Greatgrandchildren average 113 (assume these mate with a 113)"

      I've seen the average regression quoted around ~2.5 generations or so.

      On the other hand, early childhood diet, engagement and stress factors seem to combine to roughly a 7-10 pt increase. And of course there is the Flynn effect (

      "Ulric Neisser estimated that using the IQ values of 1997 the average IQ of the United States in 1932, according to the first Stanford–Binet Intelligence Scales standardization sample, was 80. "

      All in all, you can't create a smarter population by focusing on genes, but you can create it by focusing on lifestyle and development. This is leaving aside the point that most of the world's problems are being caused by "intelligent" people bent on creating infinite growth, so I'm not even sure the focus on IQ is relevant to our issues at all.

      The point about hardiness is harder to refute but again, the greater threats are sociological instead of biological. We are rapidly moving towards a period of massive anti-biotic resistance not because of genetic issues but misuse due to cultural laziness. This will lead to an increase in diseases that affects even the most hardy. Similarly, large pandemics tend to affect those with strong immune systems more than those that don't, due to cytokine storms. If there is any massive threat to humanity as a species, it is animal breeding, vaccination development and quarantine practices that encourage viral genetic recombination at an unimaginable rate, then make our society unable to handle the outcome. If humanity does get "feeble" then diseases will rapidly cause greater vigor, but true plague is either bacterial (bubonic) or cytokine storm (smallpox, spanish flu, ebola) in nature where the immune system is generally inadequate.

      If genetic theory and practice has taught us anything, it is that the wider the genetic pool the greater the fitness of the population, and the narrower, the worse it is. Thus, I would argue that our ability to allow a greater variety of people to live and reproduce is actually beneficial to long term potential of human evolution.

  2. Traducción al español 1
    Recientemente, recibí una serie de comentarios en mi libro de 2011, "Los límites del crecimiento Revisited". Que publico aquí con algunas modificaciones mínimas; con el permiso del autor.

    por Max Kummerow

    Estimado Dr. Bardi,

    Acabo de terminar la lectura de los “Límites del Crecimiento Revisado”, un gran placer. La función logística añade mejora en gran medida a la curva población exponencial original de Malthus exponencial, comida lineal, y la comparación predador-presa , capital –recursos tiene sentido , a pesar de la "presa" no puede regenerarse, por lo que sólo es posible conseguir un ciclo con un recurso no renovable . Así que los seres humanos tratan de encontrar un nuevo recurso.

    Yo era un estudiante de química de pregrado que terminó en las ciencias sociales, con el tiempo he incursionando un poco en econometría. He leído alguna filosofía de la ciencia y le remito unas reflexiones de sus comentarios sobre el modelado de sugerir:

    1. La frase "todos los modelos son falsos" viene, de acuerdo con la "Guía a la Econometría" de Peter Kennedy (quinta edición, creo) de GP Box, el gigante de las estadísticas de series de tiempo. La cita completa es: "Todos los modelos son falsos, algunos son útiles". La razón todos los modelos son falsos es que para ser útil un modelo debe ser una simplificación del sistema real, que es infinitamente compleja en todos los casos reales. Así todos los modelos sufren de sesgo omitido variables. Modelos de utilidad reproducen comportamientos importantes y ofrecen puntos de vista sobre el sistema.

    2. Hugh Mellor, un filósofo de Cambridge U. de la ciencia, comentó que todos los modelos se olvidan de mencionar "condiciones auxiliares" o "condiciones secundarias" (o variables omitidas) que se supone que no van a variar. Asumimos, por ejemplo, que el sol saldrá y que la guerra nuclear no se producirá, etc. A veces, estas variables omitidas cambian y lo tienen importancia.
    3. Nassim Taleb señaló que eventos extremadamente improbables ocurren y pueden tener grandes impactos - la idea del Cisne Negro. Pero ya que son tan raros, la probabilidad de estos eventos no se puede calcular. Las fases cisne negro provienen del ejemplo de la lógica inductiva de que la observación de cualquier número de cisnes blancos no puede finalmente demostrar la generalización todos los cisnes son blancos, pero un cisne negro puede refutarlo. Esto llevó a la lógica falsificación de Popper donde tratamos de confrontar hipótesis nulas con datos que podrían refutar a estas.

  3. Traducción al español 2
    4. Sin embargo, en un sistema complejo, cualquier contraejemplo que podría parecer a exigir rechazar una hipótesis nula, se puede racionalizar como debido a alguna variable de confusión, dejando la generalización sin refutar. Así el falsacionismo deja de ser una panacea de la lógica inductiva.

    5. David Orrell, ex modelador climático, ha escrito acerca de la complejidad, con el argumento de que habrá siempre incapacidad para confirmar los modelos complejos (debido a la escasez de datos, cuestiones de medida, errores de forma funcional, etc.), de manera que una persona razonable podría elegir el dudar. Especialmente si está financiada por una compañía de energía e ideológicamente inclinada a dudar. Pero, Orrell dice, a pesar de que no podemos especificar y probar un modelo completo, casi siempre sabemos lo suficiente para elegir las políticas mejores y peores. (Mejor para emitir menos carbono y el crecimiento lento de la población).

    6. Mi profesor de series de tiempo, Michael McAleer, anteriormente en la Universidad de Australia Occidental, ha escrito un libro (creo Keutzenkamp es el co-autor) sobre la simplicidad de los modelos. Su idea es que hay un tipo de grado óptimo de la complejidad del modelo de tal manera que demasiada complejidad hace que el modelo menos útil, en principio, debido a la incapacidad para verificar realmente interrelaciones matemáticas complejas. Por ejemplo, la adición de términos conduce a la multiplicación de los efectos de interacción, incluso en modelos lineales.

    7. George Soros habla de "reflexividad" haciéndose eco de su mentor Karl Popper, quien señaló "la pobreza del historicismo", que significa que no se puede predecir el futuro, en principio, debido, además de la complejidad y el caos, al hecho de que las personas puede decidir actuar de manera diferente sobre la base de la previsión. Así que si todo el mundo cree que el cambio climático va a terminar con la civilización, podríamos resolver fácilmente el problema, mientras que si todo el mundo niega el cambio climático seguramente acabar con la civilización. Indeterminación inherente.

    8. Esto se relaciona con la idea de "libre albedrío", que un filósofo me dijo que no es una buena manera de pensar acerca de las decisiones. Siempre tenemos alguna opción y algunas limitaciones - información limitada y limitada capacidad para procesar la información. Así que deberíamos estar tratando de mejorar nuestra capacidad de toma de decisiones para obtener mejores decisiones y siempre realizar nuestros límites. El "último recurso" es también un recurso limitado - punto muy importante.

    9. La teoría de juegos proporciona la confirmación oficial de la tragedia de los bienes comunes de Hardin - Hay juegos donde las decisiones individuales racionales conducen a la propiedad emergente de los resultados colectivamente irracionales (dilema del prisionero, por ejemplo).

  4. Traducción al español 3
    10. Don McCloskey (ahora Dierdre) clasificó los modelos cuantitativos en economía como simplemente otra forma de retórica, más que ninguna prueba real o confirmación absoluta. Los econometristas con diferentes ideologías vienen con diferentes modelos de los mismos datos. McAleer tiene un ejemplo en el que dos modelos se ajustan sobre los datos igual de bien, pero conducen a conclusiones políticas opuestas. (Sherlock Holmes está en el título de ese documento.) LTG sucedió como retórica y lo mismo hizo las refutaciones.

    Obtener la filosofía de las basesde las ciencias en su lugar es la clave de los buenos modelos.

    Punto final: Creo que cada discusión sobre arreglar el mundo debe hacer hincapié en el papel clave de la demografía. El crecimiento demográfico y el crecimiento económico son dos motores fundamentales del daño ecológico. Aproximadamente el 45% del mundo ha quedado por debajo de los niveles de fecundidad de reemplazo (por lo que no es tan difícil de hacer), pero la gran divergencia de las tasas de fecundidad (de <1 a> 7 entre países) y la alta fertilidad de los fundamentalistas religiosos (ver Kaufman, ¿Debe la religiosidad heredar religiosa la Tierra) significa fecundidad podría aumentar en la medida que continuación grupos de reemplazo exponencialmente disminuyen y los grupos de alto fertilidad aumentan exponencialmente. También es impulsada por la inmigración, por supuesto. El tema de población fue conducido fuera de la agenda junto con el tema LTG. Es sorprendente cuán universal la fe en el crecimiento como una solución a la escasez es, cuando en realidad, es el crecimiento que crea escasez.

  5. Graham Turner from Australia's CSIRO has also published, officially for CSIRO in 2008, a 30 year revisit of The Limits To Growth, with 30 years of actual data to recalibrate the original model. Unofficially, in his own personal capacity, he published a subsequent 2012 update, which is available here:

    I cannot find Graham Turner now. His CSIRO email address is no longer active since 2013. I assume he has cut ties and gone to his family lifeboat solution.

  6. > intelligence ... regression toward the mean

    Noted also by Tom Paine; I quoted him at some length several places, e.g.

    as well as quoting Jefferson on education



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)