Descartes (1596-1650) responding to Montaigne's "Essays" on abilities of
animals, argues that automatic ability in animals does not show reason; instead
it shows none!
"nature makes them behave as they do according to the disposition of their
organs; just as a clock, composed only of wheels and weights and springs can
count the hours and measure the time more accurately than we can with all our
intelligence. Descartes, R. (1641) Discourse on Methods, Fifth Part.
While Descartes' argument comes out of a cultural, scientific, linguistic, and
religious environment quite remote from our own, it provides a high contrast
background against which to evaluate human and animal abilities.
This suggests that we cannot assess the "intelligence" of an organism by
observing it in its natural environment for which we suppose it is already
well-adapted. The question is whether or how much of this adaptation is due to
the "intelligence" of the individual rather than inheritance from its ancestors
(ontogeny vs phylogeny, nurture vs nature)
We can only evaluate this by evaluating individual organism's ability to adapt
to novel environments, either experimentally, natural observation, or pure
Even this is not straightforward since a complex adaptive behavior may have
components of different origins --even in members of the same species. (Human
language may be an example of this, especially comparing early vs. later
acquisition. See language notes.)
Finally, even this definition ignores the problems of specifying WHAT
environments should be considered.
This of course is consistent with the extreme neotenous condition of humans.
Recall our discussion of the ontogeny of forelimbs-- also very unspecialized.
Intelligence results from a set of abilities, including multipurpose
flexibility but also specific language skills, spatial and other
representational ability, memory capacities, motor skills, social skills, etc.
that indeed are inherited to some extent.
Apparently some behaviorist psychologists thought so in that they believed the
general principles of behavior could be discovered using almost any species and
that the only differences were in sensory-motor processes and perhaps size of
memory. Although much of what they had to say about behavior in
general seems now misguided, there is the strong possibility that we shall find
that the idea of "intelligence" is an intuitive concept better understood in
terms of differential component processes including sensori-motor and memory
processes, processing rates, motivation, and the self-organizing
characteristics of the brain as it develops in particular environments.
For now, it seems reasonable to take "intelligence" as characterized above.
I suppose we should evaluate the role of the brain itself in intelligent
behavior. Passingham (1980) reviews the role of naturally occurring lesions
and experimentally lesioned animals in understanding the role of various brain
regions in behavior. Intact mammal brains are required for intelligent
behavior (However Karl Lashley (1929?) demonstrated animal brains could sustain
a surprising amount of damage and maintain performance and there are a few
amazing clinical cases of humans with serious brain tissue deficit maintaining
near normal intellectual functioning.)
One implication of these findings is that motivation, interest and planning
play a large role in adaptive behavior. Clinically, the famous case of Phineas
Gage (MacLean, 1990) who had an iron bar pass through his left frontal cortex
from below his eye and exiting through his skull demonstrated how planning and
interests in the future depend on forebrain structure. Gage was a
conscientious railway construction foreman until the accident; afterward he
became an "indulgent, profane, capricious, vacillating" person who drifted from
job to job until he died. It appeared that the frontal lobes damaged in Gage
play a major role in control of temperament, social interactions, and planning.
In the words of his physician, "Gage was no longer Gage." (MacLean (1990).
This was experimentally demonstrated in chimpanzees, research which led to the
use of pre-frontal lobotomy in the 1940-1960s as a psychiatric treatment for
serious anxiety or other? disorders.
There remains much controversy about this issue--see Gould, 1981, Mismeasure
of Man). However, even Darwin believed it was important, noting for
example that more intelligent insects had larger ganglia and that domestic
animals had smaller brains than their wild counterparts. Taken together, this
suggests that intelligence may in part depend on brain size but that size
itself needs analysis and can reflect environmental circumstances.
Harry Harlow (190x-198x) pioneered methods of evaluating "learning set" ability
across species using his Wisconsin General Testing Apparatus (WGTA).
Passingham (1982) summarizes much of the research in the figure below, showing
performance of various species on a two-choice visual discrimination task in
which the organism picks one of two objects with a hidden reward under it.
Essentially the subject must learn a rule --that the food is under a certain
type of object despite other changes in position or features of the
objects--and follow that rule in choosing.
Sketched from Fig. 5.8 in Passingham (1980).
Passingham concludes from these data that when mammals are ranked in terms of
their improvement over a series of these problems, their rank is predicted by
Jerison's (1973) measure of surplus brain cells.
One can safely assume that except for the most fundamental aspects of
individuals--number of limbs, etc., there will be variation on every
characteristic including "intelligence." Of course the details will depend on
how it is assessed. Moreover if it is a complex of other factors working
synergistically, then measurement will even be more problematical and variation
likely to be greater.
Passingham is very cautious in assessing the data available to him and
concludes the relationship between brain size and intelligence is very slight
at best. Nevertheless, only a slight advantage (correlation) may have
considerable evolutionary significance in explaining the rapid increase in
hominid brains over the past 2-3 million years.
Greater intelligence would be particular important in this period of
considerable climatic variation --variation that would place a premium on
intelligence. (See Calvin's paper on intelligence.)
"When we say intelligent rather than clever we are often implying a substantial
amount of looking ahead. " Calvin p.23
A recent review by Wickett, et al. (1995?) reports that most correlations based
on head size range between r = .10 to r = .30, with an n-weighted mean of r =
Using new methods, four recent studies of this relationship for the first time
obtained estimates of brain size from high quality magnetic resonance imaging
(MRI), instead of using external cranial dimensions. All four studies show
about twice the correlation with intelligence than does head size. Willerman
et al. (1991) report an estimated correlation of r = .35 (N = 40); Andreasen et
al. (1993) found a correlation of r= .38 (N = 67); Raz et al (in press) found a
correlation of r = .43 (N = 29); and Wickett et al. (in press) report a
correlation of r = .395 (N = 40, all females). These are all statistically
It appears that there is a small but reliable relationship between
intelligence and brain size. This small difference has no importance in
assessing individual intelligence --considering all the other factors involved.
It does have evolutionary significance since a very small fitness advantage of
a characteristic can, over generations, can have a dramatic effect --like the
tripling of the hominid brain over the last 2 million years.
Hamilton (1935) reported that rats selected for 12 generations to be either
"maze-bright" or "maze-dull" differed by about 2.5 standard deviations in brain
weight. Within unselected control rats there was a correlation of r = .25
between maze ability and brain weight. Recently Anderson (1993) reported data
on rats in which several cognitive tasks were given and a general factor
extracted, and brain weights were obtained. The correlation between this
general factor and brain weights in these rats was r = .48.
These animals studies only support the general idea that brain size is related
to certain abilities. However keep in mind what Darwin observed, that the
early experiences themselves can influence brain size. Indeed, it is
conceivable that some increments in brain size occur from the extra post-natal
experience and nutrition that the "premature" human brain gets as an indirect
consequence of its neotenous condition.
This allows greater interconnectivity among individual neurons.
These cells essentially take up the space among neurons, having served to guide
the placement of neurons during early development. here is an increasing
glial/neuron ratio as brain weight increases.
Post mortem analysis of Einstein's brain by Schiebel (19xx, video) reportedly
indicates an usually large number of glial cells!
effects of prefrontal lesions on motivation
Passingham notes that relative brain size predicts well the responsiveness
("curiosity") of zoo animals to novel objects over time. Suppose animals just
get bored after exhausting their repertoire of responses to the object. Perhaps
brain size indirectly reflects the variety of responses available to various
species. Primates have the obvious handling advantage over carnivores, for
example. And humans have a linguistic repertoire far exceeding that of any
Responsive of zoo animals to novel objects
Sketched after Fig. 5.10, Passingham (1980)
Humphries (1976) suggested human intelligence may have come from the large
brain needed to engage in complex social relationships. Support for this comes
from a study by Dunbar (199x) showing that typical group size is perhaps the
best predictor of brain size in primates.
Obtaining food plays a double role.
In phylogeny of a species, it drives evolution of the necessary mental
capacities for obtaining food as well as in ontogeny of an individual,
nutrition plays a complex role in maximizing the intelligence of the organism.
(See diagram below adapted from Brown and Pollitt (1996) on the complex effects
Adequate nutrition is also fundamental to fertility thus linking foraging to
the spread of "intelligence" in the population.
The important point here is that there are a number of reasons to suppose
social animals would be more intelligent. However not all of these involve
imitation which seems to be quite rare in animals and even large primates.
Other social factors, e.g stimulus or response enhancement, rather than
imitation are much more common. Imitation requires complex sensorimotor and
cognitive processes along with the social dimension. (See Bryne notes and book and Tomasello, 1990, for
Like imitation, these play a role in accumulated wisdom --the acquisition of
acquired traits and knowledge without going through heredity.
Organisms may be very skillful in dealing with specific objects or situations
but not be able to generalize those skills to other situations. This raises
questions about the very nature of intelligence. Many humans, for example, can
solve a certain logical problem with some variables but not others--even though
the logic of the problem is identical. (cf. Rumbaugh, 1988, Cheney-Seyfarth,
1990: Cosmides, 1989)
Much of the political and social concern about intelligence in humans revolves
around concerns of differences among various ethnic or racial groups of humans.
Reading statements from the 19th century by Galton and especially Spencer and
then seeing how these ideas were transformed into the political horrors of Nazi
Germany explains much of the disfavor the idea of hereditary intelligence
engenders. (The Soviet Union banned the use of such tests along with most of
the field of genetics from about 1920 until the 1950s.)
In the United States, testing was connected with the rise of eugenics ("good
genes") movement that advocated sterilization for mentally retarded and
Some early efforts involved measuring brain size; Francis Galton unsuccessfully
tried to use sensorimotor measures (e.g. reaction times) to predict success in
life --presumably reflecting increased adaptive capabilities. The general idea
is to establish a metric for assessing an organism's adaptibility (i. e.
intelligence) and show that it predicts some real-life "adaptability."
Binet in the early 1900s in France developed a test to evaluate children's
potential for schooling. It assessed the basic skills of children that
schoolteachers believed necessary for further learning. Basic number skills,
language abilities, and logical assessment were prominent. From this developed
the idea of group tests, norms, and mental age. Piaget, trained as a
biologist, worked in Binet's lab and began to apply his ideas on biological
epistemology (growth of knowledge) to humans.
Many in England, e.g. Spearman, Fisher, worked on the mathematical development
of testing that we have today. Spearman was responsible for the idea of
general intelligence as a "common factor--g--" to all performance--not seen
directly but revealed in all adaptive behavior. The technique of "factor
analysis" was devised to extract the common "factor" in a range of tests. Some
speculated "g" reflected some general brain capacity such as size or speed.
In the USA, Binet's test was translated and revised for use here by
psychologists at Stanford--resulting in the widely used Stanford-Binet test by
L. Terman. R. M. Yerkes and other also devised a version of the Binet, "A
Point Scale for Measuring Mental Ability, 1916" These tests were used in
mobilzation for WWI--with minimal success. Nevertheless, they became important
factors in clinical, school, and industrial settings.
While scores on these tests may predict success in schools, many critics point
out that language skills and cultural biases make inferences about heredity on
the basis of these tests very risky.
Galton was a cousin of Charles Darwin and was one of the first to attempt
measurement of intelligence; he was also a founder of the widespread eugenics
movement to improve the human race. By 1931, thirty states in the US had
passed sterilization laws.
"I propose to show in this book that a man's natural abilities are derived by
inheritance, under exactly the same limitations as are the form and physical
features of the whole organic world. Consequently, as it is is easy,
notwithstanding those limitations, to obtain by careful selection a permanent
breed of dogs or horses gifted with peculiar powers of running, or doing
anything else, so it would be quite practicable to produce a highly gifted race
of men by judicious marriages during several consecutive marriages."
Galton, F. (1869). Hereditary genius, an inquiry into its laws and
(See Chevalier-Skolnikoff, S. (1977). A Piagetian model for describing and
comparing socialization in monkey, ape, and human infants. [136-166] )
We saw how complex and indirect the relationship between nutrition and
intelligence can be. The relationship between genes and intelligence must be
more so. Genes determine enzymes that regulate and structure biological
development; at every stage, one's environment plays an important, necessary
role. See the table below outlining these processes.
PROCESSES PRODUCTS ENVIRONMENTAL
form genotype at regulatory genes amino acids, etc.
conception structural genes
enzyme production enyzmes nutrients (proteins,
regulated biochemical cell development and nutrients including
reactions metabolism oxygen etc prenatally
via the placenta (etc
here can have negative
effects, e.g. viruses,
organized development of physiological structures nutrients and
body incl. c.n.s., including neural sensory-motor feedback
endocrine systems, etc. circuits and sense effects on neural
receptors development esp. late
fetal and early neonatal
c.n.s commands, sensory Behavior sensory input nutrients
Most IQ tests evaluate a narrow range of abilities that are influenced by a
complex of biological and environmental factors. Even if there are small
differences attributable specifically to gene differences, these are not large
relative to environmental factors and never are independent of them. They seem
irrelevant to comparisons among primate species and add little to our
understanding human nature beyond the assumption that variablity must have
existed in order for natural selection to function.
Data from Gould (1977), Passingham (1980) and reflection suggest a complex of
interrelated characteristics are observed in intelligent creatures. Many of
these derive from the neotenous condition of primates but can be expected to
reflect "intelligence" in other species who happen to have similar
The puppetmeister metaphor is useful here; how may strings and how often must
you pull them gives a rough indication of the sophistication of the system.
Generally speaking, more degrees of freedom require greater information flow
per unit time.
The most complex example is probably speech, where Darley, F. L., Aronson, A.
E., & Brown, J. R. (1975). Motor speech disorders . Philadelphia:
Saunders estimate 14,000 muscles must be controlled and at a very rapid rate!
Vision requires a sophisticated nervous system that can be used for
"intelligent" actions--vision thus is perhaps a useful preadaption for
intelligence. It may be particularly important enabling the use of visual
imagery in problem solving. See Piaget's "figurative intelligence" and
Kohler's "insight", etc.
The outline of primate intelligence that Passingham (1980) sketched remains
reasonably accurate today, although Bryne (1995) points out some
limitations. There is additional evidence on the small
relationship between brain size and intelligence; there is also much more
evidence on social factors in intelligence. Finally, while there is much more
information about the role of genes in behavior, none of these relationships
appears at all direct nor independent of many aspects of an individual's
Thus brain size may be a function of specific regulatory genes, it is also a
function of nutrition and experience -especially early experiences. Natural
selection can operate directly only on the regulatory gene component of the
phenotype brain, though other genes relating to parenting and accelerated
gestation--relevant to nutrition and neonatal experiences-- may also be
(Recall the sketch on multiple effects of
genes on the phenotype in my evolution notes. Plug the above
variables into that sketch. You also might want to read Calvin's recent
Scientific American article (Oct., 1994) on "The emergence