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It is generally accepted that the only two possible causes of human behavior are nature (the biology a baby is born with) and nurture (the environment in which it grows afterwards). Strictly speaking the mother's womb is part of the environment and biology is defined only by the genetic makeup of the individual, but this observation does not affect the discussion that follows, so I shall push it aside.

It is generally believed that both nature and nurture play an important role. In this article I will argue that the effect of nature is much less than generally assumed. I will focus on intelligence, even though the argument covers the whole range of typically human behavior. The argument has two parts: First I will weaken the case for nature by showing that intrinsic methodological problems of statistical studies will inflate the apparent role of nature, and secondly I will argue for the view that human babies are not born intelligent but learn intelligence through the interaction with society.

The best objective way to measure the relative strength of these causes is to do statistical studies using identical twins (and thus possessing the same biology) that were separated soon after birth and grew up in adaptive families. Suppose then we have a general population of children which are adopted in families living in some society and thus growing within a fixed distribution of environments. We shall measure the adult intelligence of two subgroups of that population: In the first subgroup we shall put children who are identical wins and in the second all others less genetically related children. We shall assume that the distribution of environments is identical. Different children get to live in different environments (given the kind of parents that adopt them, where they live, and so on), but the distribution of such environments is identical for both subgroups. In other words we assume that parents don't get to choose whom to adopt, but that babies are randomly distributed in the various environments. This is not in fact what happens in reality, but it is close enough for our argument, and in any case any such uncertainty can be approximately factored in before producing statistical results.

Now we define a series of intelligence tests and apply them to all children when the reach adulthood. If then we find that the test scores among twins correlate much stronger than among the population at large then we have a measure about how important biology is for that particular kind of intelligence test. I don't know how carefully such studies were made, but I understand the general result is that biology was proven to be quite important, perhaps as important as environment factors. Now one can further divide the population to find out what kinds of biological differences are more causally effective. One might use parameters related to race such as skin color or facial features or geographic ancestry, but also any other parameter one likes including height, blood type, or whatever. Given a large sample perhaps many statistically significant differences will be observed. Finally, similar studies could be made for other societies in order to find out how the distribution of environment affects the result. So, for example, how the results are affected by societies with good publicly available education, or with high degrees of religiosity, or low degree of criminality, or high degree of social protection and so on. Such inter-societal studies will sharpen our confidence about the relative strength of biology versus environment.

I wish to suggest the argument that all such studies suffer from a type of methodological problem which will increase the apparent importance of nature. Moreover this problem is impossible to factor out since it is part of what one tries to measure. The problem in short is that one's biology is to a significant degree part of one's environment. I will mention two examples:

1. How one looks affects the way important people in one's environment (parents, teachers, friends) behave towards one. Thus smart-looking children will elicit more encouraging and motivational behavior which in turn will affect the nurturing of one's intelligence. Similarly if boys are expected to do better at math than girls this will affect measurable differences in the scores of mathematical tests. If strongly built children are expected to do worse at math this too might have a causal effect. More beautiful children which might elicit more attention – perhaps teachers will unconsciously care more for the more beautiful in their care. Also racist sentiments and preconceptions are global and impossible to completely remove from a child's environment.

This effect of biology, namely how one's looks affect the behavior of people in one's environment, will go unnoticed in the studies based on identical twins and will increase the apparent causal power of biology. In particular it will strengthen the correlation of test results between twins for reasons that have nothing to do with the kind of brain they were born with and thus with their biologically given capacity for intelligence. In short the correlations observed among twins may be the effect of both the biology of their brain and of how appearance happens to affect their environment. It's difficult to think of how to experimentally separate the two sub-effects, and probably impossible to do so. The best one can probably do is to make an educational guess about how a child's looks affects its environment and see to what degree it accounts for the correlations observed among twins.

2. Factual knowledge may affect a child's self-confidence and thus its motivation to succeed. Thus, for example, a black child in the US learning about the history of slavery will feel inferior. A Jewish child learning that Jewish people have won per capita several times more Nobel prices than any other group will feel superior. A girl observing how few great mathematicians are women will feel inferior. This effect too will tend to increase the appearance of biology's causal power. This effect too is impossible to factor out from the respective studies.

I would like to suggest a series of arguments why environment has a much larger causal effect than biology on the production of intelligence (intelligent behavior) in human adults. I will argue for three sub-hypotheses: First, that the evolution of human intelligence is the product not of biological evolution of the brain, but of the symbiotic evolution of human brain and society as a unit. Secondly, that babies are born with hardly any intelligence hardwired, but with a large capacity for acquiring intelligence by interacting with society. And thirdly that the large differences of intelligence we observe among individual adults is caused mainly by that interaction with society and much less by genetically caused differences among babies as far as their brains capacity to learn.

1. There is no doubt that intelligence is a product of natural evolution, after all the human species is far more intelligent than any other. (I assume that the reader agrees that the human species is the product of a particular physical event we call “natural evolution” including in the sociobiological sense. As a theist I believe that event was guided by God but in a way that does not violate the physical closure of the physical phenomena, here an explanation about how this might work.)

Now we understand quite well how Darwinian evolution works in biology. One principle we know is that natural evolution produces adaptive changes in the “cheaper” way possible, namely in a way that requires the fewest changes in the genetic sequence. We know this because we know the probabilistic manner by which the Darwinian mechanism works, but there is plenty of evidence of the same. Thus we find that even though we are much more intelligent than chimpanzees our genetic makeup is to a very large degree (some 99%) identical to theirs. And since there are many other biological differences between humans and chimpanzees it follows that only a portion, possibly a small portion, of the different 1% accounts for the physiology or genotype of the human brain. Thus a huge adaptive advantage of our species has been achieved by a comparatively tiny change in the genome.

Given this principle we might deduce that Darwinian evolution will not produce an intelligent species by evolving the large number of genes which will produce an intelligent brain (with the respective structures hard-wired into the brain of babies) but much more economically by evolving genes which will produce an “intelligence-ready” brain for babies to be born with. This brain will learn intelligence (and hard-wire the respective structures) by interacting with a society in which intelligence is already present. Such a result is not only evolutionary cheaper but also of greater worth: Such a brain is much more adaptive in the sense that it can react faster to changes in the environment since it produces functional intelligence in adults by sociobiological mechanisms which work much faster and with greater flexibility than genetic evolution. On this view the intelligence of the human species is not a case of evolution of the human genome alone, but is a case of symbiotic evolution between genome and society. Biological evolution which produced brains with greater capacity for intelligence acquisition allowed the realization of more intelligent societies, and more intelligent societies accelerated biological evolution, given that survival would tend to select those genes that produced more “intelligence-ready” baby brains (as well as those societies that realized not only a more intelligent organization but a better upbringing of their young).

Now the above view is not really controversial. We do know that baby brains evolve intelligent behavior by hard-wiring structures in their brain as the result of their interaction with the environment. This capacity of greatly advancing intelligence by interaction with the environment is a unique feature of human brains. For example despite much effort of giving chimpanzees the appropriate environment we haven't been able to produce any increase of intelligence in their brain comparable even to a small degree with what human brains manage even in the worse social environments. Given then that “capacity to learn intelligence” is a unique biologically produced feature of the human brain it is to be expected that some children are born with more or with less of this capacity. The third and perhaps more critical sub-hypothesis we are investigating is to what degree that difference in that genetic make-up of children causes the differences we observe in adult intelligence. I believe such differences have minimal effect at least for the vast proportion of individual cases (the rare cases at both extremes may be an exception), and thus that the correct view is that babies are born with basically the same capacity to learn intelligence. Which implies that environmental factors almost exclusively cause the observed differences in adult intelligence in the great majority of the population.

This view may be confused with the “tabula rasa” view for the baby brain, namely that the baby's brain is in a blank state and learns intelligence by analyzing the impressions of its environment. My view is similar but distinct, namely I hold that there is indeed a hard-wired evolution of human intelligence but that it is realized outside the genome (and thus outside of the physiology of the newborn's brain) and within human societies. The baby's brain does not reinvent intelligence by processing any environmental inputs, but absorbs intelligence specifically from society's store. The most effective way for a child to absorb intelligence is by way of a process of discovery within the respective learning environment, but that's a separate issue.

Finally I am only referring to those cognitive faculties which we associate specifically with human intelligence, such as reason, creativity, morality, etc - and not to those faculties we share with other higher animals, such as three-dimensional vision, the capacity to resolve sounds or smells, etc. The latter are probably hard-wired to a significant degree in the newborn's brain, a hypothesis that is again what one would expect given our knowledge of the evolution of the species, namely that these features evolved slowly by Darwinian evolution during hundreds of millions of years with little or no sociobiological interaction.

2. As we saw before statistical studies based on twins that appear to show that babies are born with significantly more or less capacity for intelligence acquisition suffer from methodological flaws which increase the appearance of biological importance. There is also anecdotal evidence of parents who swear that they gave their children the same king of environment but some children were gifted in ways that others weren't. The fact that scientific studies fail to convincingly show that there is such an effect renders such anecdotal testimonies irrelevant. What probably happens is that parents are not aware about the different environments their children actually grow in. For example child psychologists have long explained how different the family environments are for the first- and second-born child.

3. The scientific evidence is sketchy but there have been a few cases where human babies who were lost in the woods but survived to be found and be brought back to society many years later (so-called feral children). Despite the best care it proved impossible for such people to develop normal intelligence, evidencing that normal intelligence must be learned in the first years of life and that having lost contact with society the human brain after a few years also loses the biological capacity to acquire intelligence. This phenomenon fits better the view that intelligence is a symbiotic feature. The human species is far more intelligent than the chimpanzees, but if the window of opportunity during the first years of life is lost then the human brain starts to resemble a chimpanzee's.

4. The view that humans are born merely with intelligence-ready brains explains why we possess cognitive capacities which cannot be explained by evolutionary pressures, such as the capacity for discovering deep mathematical truths, for composing great music, etc. Such have not served any adaptive purposes whatsoever, but are some of the most striking features of human intelligent behavior. The view that human intelligence is the symbiotic relationship between brain and society and that baby brain are basically flexible grounds for learning intelligence easily explains such phenomena.

5. For an intelligence-ready human brain to learn intelligence by interacting with the society in which it grows the means for efficient communication is a critical feature. Structured or productive language (i.e. language in which a limited number of words can be combined to produced a large range of meaning) is an exclusively human feature which appears to have started about 80.000 years ago and has reached full development about 5.000 years ago. This correlates well with an accelerating development of intelligence with the production of sophisticated tools (50.000 years ago), cave paintings (30.000 years ago), agriculture (12.000 years ago), urban civilization (5.000 years ago). And surely enough structured language appears in its basic features to be hardwired in the brain (see for example Chomsky's theory of universal grammar), which is to be expected given its critical adaptive value and the time available for Darwinian evolution to have had an effect (some 3.000 generations). The importance of communication is also evidenced by the fact that great jumps in civilization and knowledge came after the discovery of a new communication technology – structured speech (the start of human intelligence), writing (the start of history and of complex societies), long distance travel (commerce and wealth to invest in civilization), printing press (the start of renaissance/enlightenment), internet (today's world-shaping revolution). The critical role of communication supports the view that baby brains are basically blank grounds for learning.

A related problem here is this: At about the same time we hold that structured language started to evolve (that is at about 80.000 years ago) humanity left its home ground of Africa to migrate all over the world to live more or less independent from each other. This historical fact produces some tension with the biological fact that the same basic linguistic grammar is hardwired in the genome of all humans. One possible answer is that the 80.000 figure for the beginning of language is wrong, and that language begun much earlier perhaps concurrently with the appearance of homo sapiens about 200.000 years ago and when humanity left its home ground in Africa at about 80.000 years ago language was already quite advanced and its basic grammar already fixed in the human genome. There are other possible answers, such as that there have been sufficient social interaction between groups in the last 80.000 to coordinate the evolution of a common basic grammar, or that for some strange reason only one basic grammar could evolve – but this appear to be less plausible answers. If the hypothesis of a much earlier evolution of language is correct then the implications are significant. Language is not just one more intelligent thing we do. On the symbiotic model of the natural evolution of human intelligence I here propound, the structure of human language is the very foundation of that evolution, and also circumscribes its development. This in turn may be part of the explanation why no measurable differences in average capacity for intelligence in different geographical groups of humans has been demonstrated.

6. Cognitive capacity clearly correlates with brain size, which makes sense since bigger brains will have more neurons and thus greater capacity for learning whatever they wish. Hominid brain size has significantly increased before reaching the modern human species of homo sapiens at about 200.000 years ago. But after that point we encounter two anomalies: The biologically extremely similar subspecies of Neanderthals had a significantly larger brain and yet it went extinct (with parts of its genes being absorbed into the homo sapiens genome). And secondly human brain size of homo sapiens has decreased in the last 30.000 years. The hypothesis that intelligent behavior is the result of a symbiotic evolution between brain biology and society offers plausible explanations for these two anomalies:

a) Neanderthal brain may well have had larger capacity for learning but its symbiotic companion, the intelligence realized in society, may have lagged behind the societies of its close kin of homo sapiens. In other words Neanderthals were perhaps born with greater capacity for intelligence but in less intelligent societies and thus their capacity for intelligence was wasted. Why their society was less intelligent is a separate question, and one answer is that by chance. For example homo sapiens societies might by chance have developed cultural norms which were more conducive to efficient communication and thus to the health of the symbiotic evolution of intelligence.

b) About 30.000 years the symbiotic evolution of intelligence may have reached a state in which the intelligence realized in society became so advanced and enduring that the capacity of brains to learn intelligence (and thus their size) became less adaptive. As it were the center of gravity of human intelligence moved outside of brains into the society. Thus the capacity of brains to learn intelligence became less important and was allowed decrease in fitness (decrease of fitness of an organ that becomes less adaptively important is a common feature of biological evolution). Beside its size there may be many other biological features that increase a brain's capacity for learning intelligence (perhaps brain chemistry), which may also have decreased in fitness in the last 30.000 years, but which we haven't noticed because these features don't leave behind obvious pieces of evidence as craniums. Still prehistoric DNA evidence as well as a better understanding of the genetic basis of what makes human brain have more capacity for intelligence may provide us with additional evidence.

7. We are comparing two hypotheses: Intelligence is mainly caused by biology (the B-hypothesis) and intelligence is mainly caused by the environment and specifically by the child's interaction with society (the S-hypothesis). These two hypotheses produce different different dynamics of advancement under the effect of progress in the environmental factors. Obviously biology cannot keep pace with the advancement of the environment (the environment today has advanced greatly compared to the past, whereas biology clearly hasn't to any comparable degree, and couldn't possibly given the mechanisms of Darwinian evolution). Under the B-hypothesis we'd expect a relatively slow advancement, but what we observe is a fast advancement of intelligence. For example average IQ scores have advanced significantly only in the last generation, whereas the biological evolution that is possible in one generation is virtually nil.

8. Historical events such as “golden ages” in which we observe an explosion of the advance of knowledge through the work of geniuses is what one would expect to happen if intelligence was first and foremost an effect of environment (the S-hypothesis). On the contrary if intelligence was mainly caused by biological nature (the B-hypothesis) such facts are more difficult to explain. On the B-hypothesis one would expect a much more continuous advance of human knowledge with no sudden peaks in it.

9. But what about the phenomena of peak intelligence, the kind of intelligence that characterizes geniuses? Outside the rare and geographically and historically very restricted cases of “golden ages” there is no clear evidence that such individuals have become measurably smarter or more populous in the last 3.000 years or so in which we have sufficient evidence to judge one way or the other. Let us not be confused by the success of modern science and technology. Scientists and engineers do marvelous stuff today by building on the work of others, not by being smarter than thinkers thousands of years ago. Perhaps a better evidence might be found in the case of solitary geniuses such as in the fine arts (and to a smaller degree in theology, and even smaller in philosophy), and here I see no evidence of improvement. It's not at all evident that today we have a larger proportion of great artists, theologians, or philosophers compared to 3.000 years ago, despite the fact of much advancement in the environment including universal access to general education, much higher proportion of access to higher education, cheapness of educational materials, etc. It looks like the increase of intelligence in society has increased the average intelligent behavior of individual adults but not the incidence of peak intelligence. Suppose this assumption is right; what follows for the hypothesis we are studying, namely that babies are not born intelligent but learn it by interacting with an intelligent society?

The distribution of intelligence in a population is bell shaped. We know that the average intelligence has increased in historical times (IQ scores increased in the last generation alone). Let's define some absolute (fixed) genius level G at the far right of the curve. Under the B-hypothesis of a biologically primed intelligence one would expect the bell curve to move to the right representing a general increase in biological fitness, which would lead to a very significant increase of the proportion of people at the G-level which is not what we observe. What are the dynamics of the alternative hypothesis that intelligence is primed by society, the S-hypothesis? There are many ways a society can prime intelligence, and thus there cannot be a fixed answer. The biggest difference is probably universal access to education, and this would be more effective for those children born with lesser capacity for intelligence, for children with greater capacity are more probable to advance even at the absence of good education. Thus in this case the dynamics would be such that while the bell curve moves to the right its left side (that of the less gifted) would be moved with greater speed thus producing a narrower curve, a more egalitarian kind of distribution with little or no effect to the far right G-level point. In fact education primed to the less gifted (which is probably the most useful one for society) may even restrict the development of peak intelligence. Thus what we observe to be the case about peak intelligence fits better the S-hypothesis than the B-hypothesis.

What changes in the environment cause the phenomenon of golden ages and the increase of the number of people with genius level intelligence is beyond the scope of this article, but what appears to be necessary characteristics of such societies is wealth (of the kind that allows more time for the higher functions of the intellect) and competitiveness (of the kind that motivates people to demonstrate excellence in such factors). Incidentally in this context “society” should not be understood as “society at large” but only as the presence of an stable environment available to a significant number of people. Thus, for example, monasteries in the Middle Ages may have provided such a golden environment within a general society which was anything but fertile ground for the development of intelligence.

I trust I have successfully demonstrated first that the one good argument in favor of the B-hypothesis, namely studies about identical twins, suffers from an intrinsic methodological problem which is next to impossible to quantify because any quantification depends on knowing the truth about what one is investigating, and secondly that all we know about the mechanics of sociobiological evolution as well about the facts of humanity's history, fit the S-hypothesis much better then the B-hypothesis. We have good reason to believe that human flourishing depends to a much larger degree on nurture than on nature.

Dianelos Georgoudis, Volos, 3/23/2017