About Rationally Speaking

Rationally Speaking is a blog maintained by Prof. Massimo Pigliucci, a philosopher at the City University of New York. The blog reflects the Enlightenment figure Marquis de Condorcet's idea of what a public intellectual (yes, we know, that's such a bad word) ought to be: someone who devotes himself to "the tracking down of prejudices in the hiding places where priests, the schools, the government, and all long-established institutions had gathered and protected them." You're welcome. Please notice that the contents of this blog can be reprinted under the standard Creative Commons license.

Wednesday, October 26, 2005

Rational decisions vs. biological instinct

I just got out of a seminar in my department by a visiting scholar, James Brown from the University of New Mexico. Bear with me for a couple of paragraphs, when we pass the basic biology it does get interesting for human beings. Brown talked about a well-known relationship that ecologists have established between the per capita use of energy and the number of offspring that animals produce. It turns out that such relationship is remarkably constant across large groups of animals (like, mammals, or primates), and it is a negative one: the higher the per capita use of energy, the lower the number of offspring.

This isn't surprising within the context of ecological theory. It has been known for some time that there are two "strategies" that species can adopt in nature: either they produce many offspring, but investing little in each of them (a "shotgun" approach to pass one's genes to the next generation), or they invest a lot of energy in few "high-quality" offspring that are more likely to compete in life. In reality, these are two extremes of a continuum, and the relationship discussed by Brown can be seen as the result of the fact that any particular species of animals finds itself somewhere along such continuum.

Now, things become particularly interesting when we get to humans. See, the same exact relationship holds for humanity at large (i.e., when one examines different countries, cultures, and ways to make a living), and in fact it holds even within one society (like the US), over time (i.e., during the past century or so, per capita consumption of energy has gone up steadily in the American population, and accordingly the number of children per woman has gone down).

What is astonishing, in fact, is that the mathematical equation that describes the relationship between per capita energy and offspring in humans is exactly the same as that for other primates, and it is parallel to that of the rest of mammals (primates produce fewer offspring per same amount of energy when compared to other mammals). Indeed, humans are simply at one end of the primate curve (at the extreme end of high consumption, of course!).

This is a phenomenon well known in sociology, and it is usually attributed to conscious decisions by women or couples, to education, to culture. But Brown's results (and this is my inference, he didn't say this in the seminar) are much easier to explain if we simply assume that humans follow the same instincts as other primates, and then rationalize their decisions consciously. the implications for the debate on biological influences on human behavior and free will are staggering...


  1. Nothing to do with the post (for now), but since you mentioned a seminar in your university...

    Are you going to attend this really cool looking event I've heard of that starts tomorrow in Amherst/Buffalo? The "Council for Secular Humanism's 25th Anniversary International Academy of Humanism World Congress".

    If you do, please consider sending us poor people far away some news of it! :-)


  2. J, no I won't be able to make it to Buffalo. Too busy with enough travel for the foreseeable future (and for the future I can't foresee, who knows... :)

  3. I believe you are referring to what biologists call the "r" and "K" reproductive strategies. In my understanding "r" organisms produce many offspring, investing little energy in each, whilst "K" organisms invest more energy into a smaller number of offspring. The driving factor is believed to be the stability of the organism's environment.

    But, I'm not sure how this correlates to the energy use of the parents (versus energy invested in offspring). Also, from what I have read, Gorillas are even more "K" oriented than humans - capable of producing only one offspring every 3 to 5 years (this is from memory, so please correct me if I am wrong).

    Thus I am skeptical that increased energy use (in industrial terms) can actually change or move a subset of individuals of a species along the "r"/"K" curve instinctively? Natural selection pushes some organisms to be more complex thus requiring more energy to build bodies in the next generation, thus pushing them towards the "K" strategy, but does an excess of energy use by the resulting organism then (in the absence of other selection forces) continue the K-ward push?

    This seems to be what you are implying. But I don't know if this correlation is coincidence when it pertains to humans and human society or is it in fact driven by innate biological forces. Certainly with humans its dangerous (as you have written about) to play the nature vs. nurture game. Obviously for some people endowed with certain religious beliefs and increase in energy consumption per capita does not lead to fewer offspring, but more (I recall a family recently on the Today show with a huge number of children).

    One possible experiment would be to give a subset of chimps (or other primates) a higher per capita energy use (whatever that means to a chimp) and see if that subset reproduces at a lesser rate than another subset with otherwise equal environmental factors. Would the same experimental results hold true for dogs, horses, birds, etc.

  4. Alan,

    good points. I'm not pushing the "nature over nurture" angle, just raising the question. Sociological studies have always assumed that the biology must be negligible here because of obvious cultural influences, but perhaps there is an important role that biology plays in the trend (I have a hard time looking at graphs like those that Brown presented in his talk and think they are the result of coincidences).

    Good point, of course, about religious fundamentalists using a lot of energy and still reproducing like hell (or is it heaven? :) In fact, Brown's graph showed a few outliers in the human populations. They were all oil-rich Arab countries...

  5. Here is what E.O Wilson said in a SciAm article last year:

    "The freeing of women socially and economically results in fewer children. Reduced reproduction by female choice can be thought a fortunate, indeed almost miraculous, gift of human nature to future generations. It could have gone the other way: women, more prosperous and less shackled, could have chosen the satisfactions of a larger brood. They did the opposite. They opted for a smaller number of quality children, who can be raised with better health and education, over a larger family. They simultaneously chose better, more secure lives for themselves. The tendency appears to be very widespread, if not universal. Its importance cannot be overstated. Social commentators often remark that humanity is endangered by its own instincts, such as tribalism, aggression, and personal greed. Demographers of the future will, I believe, point out that on the other hand humanity was saved by this one quirk in the maternal instinct."

    This echoes (at least superficially for the purposes of the article) the sentiment that this pheonomena is cultural.

    An interesting case study is the U.S. vs. Europe. It is said the U.S. has the greatest per capita energy expidenture in the world, yet birthrates are signficantly lower in Europe. I would say that by and large women have more equality in Europe but I don't know that for a fact.

    Of course historically liberty and per capita energy use (resulting from increased economic freedoms) have gone relatively hand in hand, so an accurate analysis may be difficult.

    Just more food for thought...

  6. Let me start by stating that I am not an expert in biology or sociology. I’m just a lay person who enjoys a good discussion and has love of learning the way others of my ilk have a love of sports.

    That being said, it seems to me that the truth in the nurture vs. nature debate lies somewhere on a spectrum of possibilities. I also think that it may differ from person to person (i.e. for some, their “nature” trumps their “nurture” and vice versa.)

    To use an analogy, think of two baseball players. Player A is born with marginal ability but has the opportunity to stat playing at an early age and is able to build a wealth of experience in the sport that more than compensates. Player B, on the other hand, is born with a wealth of ability, but due to social circumstances (say, his parents are unable to provide him with health insurance) he is not able to formally play until much later than Player A. As a result, when the recruiters come, Player A gets the scholarship because his experience has made him the better player. But, if Player B had the same opportunities as Player A, he might be an even better player.

    (Reading that back, I realize that sounds like every high school jock sob story you’ve ever heard, but I really am trying to make a point here so quit giggling.)

    I guess what I’m trying to say is: Your genetics, your nature, what have you, can only get the ball (your life) rolling. What happens after that probably does have a lot to do your experiences.


  7. Noah, I agree with you completely. Nature places you somewhere in a probabilty distribution of whatever innate predispostion we are talking about. Then Nuture allows you to move a few deviations either way from your orignal genetic disposition. Of course, thats a very simplistic explanation -- I'm sure the reality is very convoluted and complex with multiple combinations of genes playing complex roles in even the simplest of traits. And then some traits being more resistant to environmental change than others. In fact the whole nature/nurture debate may be a bit of an artificial construct because the dyanmics are so intertwined. For an interesting insight into this topic read the excellent "Genome" by Matt Ridely where nature/nurture is a bit of a sub theme.

    Anyway, I never intended that to be the discussion here. I only questioned whether the original premise was subject to instinctive behaivor, I had a hard time seeing a mechanism for it in the traditional r vs K sense (that is energy use vs. engergy devoted to child rearing)

  8. Alan (et al.),

    the nature-nurture issue is not only a continuum, but one that is characterized by complex interactions between the two components.

    For those who haven't noticed, this is actually my technical area of expertise (see the link to "Phenotypic Plasticity: Beyond Nature and Nurture" on the header of the blog).

    In this particular instance, the fact that humans fall on the continuation of the primate regression line (n. of offspring vs. energy per capita) seems to indicate a biological component, but there was a lot of statistically "unexplained" variation around that line, which of course could be where culture comes in. Unfortunately, these being humans, it's hard to conduct experiments with proper controls...

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  10. As an inquisitive geologist, I say, examine humans first as animals, with culture then serving or perverting our proper animal functions. In discussions of human behavior, I yearn for a big step back: I don't see that what we call Nature has a motive or purpose, but unfolds along the mathematics of matter: it is an emergent quality of matter; life is a behavior of matter. Until conditions that prexist humans are examined, we have no basis for deciding nature-nurture questions.


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