GENDER DEPENDENCE AND
ASYMMETRY OF BRAIN AND MIND GROWTH
Dr. Herman T. Epstein
Developmental gender differences in human brain
and mind growth have been found in brain weight, head circumference, EEG,
laterality, and sensitivity to non-verbal cues. Most differences appear during
the age spans of rapid brain growth: 2-4, 6-8, 10-12, and 14-16 years.
Brain weight gender differences are found
most strikingly during the last two brain growth spurts: in age spans 10-12
years and 14-16 years. Although both genders have spurts in brain growth during
those spans, during the first one females have about twice as much brain weight
increase, while the situation reverses during the 14-16 year span. A similar
difference is found in the data on head circumference and on the percentage of
energy in the alpha frequencies. Non-verbal cues assayed by the PONS test show
a sharp increase of female lead during the 10-12 year period.
Females have a symmetrizing brain growth during
the first of those two age spans while males are having relatively less
increase. During the last spurt, males have a more asymmetrizing brain growth
while females are having relatively less increase. As a result, female brains
end up much more symmetrical while male brains end up relatively more
of gender differences in human brain structures and functions is constrained by
the same factors affecting general brain research today: brain complexity is so
great that attempts at reductionist portrayal is simply not yet achievable.
That progress is being made can be illustrated by a fairly recent study of
neuronal density  that found a gender difference with potentially
interesting ramifications. Fortunately, less reductionist gender differences in
human brain organization and functioning have been described in enough
not-so-recent reports that highly worthwhile novel inferences can begin to be
drawn from them.
PET scans were used  to display functional
differences between adult females and adult males because the brain regions
activated by various inputs were processed mainly in different brain
 to uncover gender
differences in the brain regions involved in reacting to the same language
inputs. Memory differences have been found using MRI . Taken together,
these studies demonstrate significant gender differences in both brain
organization and brain functions. More recent MRI studies have found gender
differences in some structures [21, 22] that do little more than confirm
results of older studies to be described.
Such reports [23 ]invoked the possible effects
of upbringing and other environmental factors as sources of gender differences
in brain function without referring to the kinds of data sketched below showing
significant developmental gender differences in brain anatomy.
Significant cerebral lateralization and related anatomical and functional
gender differences during development were long ago convincingly demonstrated
and discussed in a review .
A fairly common belief is that differences
between brains of males and females appear mainly at puberty when
concentrations of the gender-related hormones begin to increase significantly.
This belief is not entirely valid for both anatomical and functional reasons.
Functional reasons are readily noted in
children as young as 1 to 3 years, by which ages most females tend to prefer
toys such as dolls and doll-houses while most males show an extremely marked
interest in trucks and other such moving toys. This is far earlier than
significant puberty-related hormonal changes and seems not to depend only on
A relevant finding is that a major anatomical
difference appears between 10 and 12 years during which period a rapid brain
growth spurt is found in both genders [4,5,6]. However, the amount of new brain
growth in females is about twice as much as in males. After puberty, during the
14-16 year brain growth spurt, males have at least twice as much brain growth
as females. The net result is that female brain/body ratio exceeds that of
males: it takes relatively more brain to be a functioning female.
There is also a large functional gender
difference in sensitivity to non-verbal cues starting at the youngest age
measured: 6 years. Using the PONS test (Profile Of Nonverbal Sensitivity), it
was found  that females test almost a full standard deviation higher than
males between ages 6 and 10 years. The female's large lead on the PONS test
doubles during the 10-12 year brain growth spurt. Later, the males gain
back but only to the pre-age-10 year level of difference.
The anatomical difference appearing during the
10-12 year period serves to support the functional difference on the PONS test
and could be explained as having been selected for to enable females to become
more sensitive to the most non-verbal of all humans: their imminent newborn
The greater brain growth in females during the
10-12 year age period might be the result of a chromosome effect: females have
two x-chromosomes while males have one x plus one y chromosome. If the factors
responsible for increasing brain growth at the 10-12 year period depend
positively on the x chromosome and less strongly or negatively on the y
chromosome, females will have more brain growth. Then, if the factors
regulating brain growth during the 14-16 year period are connected
predominantly with the y chromosome, males should have far more brain growth
around age 15 years than females who have no y chromosome. That sex chromosome
genes contribute directly to sex differences in the brain has been shown for
In any event, the markedly greater increase in
female brain weight during the 10-12 year period should generate investigations
into the functional consequences of that greater increase. Perhaps schooling
should incorporate some gender-related differences due to the greater brain
development of the females which is likely to be located in particular brain
regions involved in characteristic functions.
There are still other gender-connected brain
differences. One of the lesser-known ones was discovered while studying humans
in the observation that
most right-handed males have a slight protrusion of the right front cerebrum
and forehead compared to their left forehead while female foreheads are much
more nearly symmetrical in that respect. The article also pointed out that, for
reasons we will not discuss here, the left rear side (occipital) brain growth
correlates with a greater protrusion of the right frontal side of the brain and
forehead, while right rear side (occipital) brain growth correlates with a
protrusion of the left frontal side of the brain and forehead. This finding
thereby ties the forehead protrusion to the brain growth symmetry or
EEG data  show a tendency for some
predominantly right-sided brain development around ages 4 and 11 years, with
left-sided predominance and development around ages 7 and 15 years. These
characterizations correlate fairly well with those inferred from the asymmetry
The asymmetry can
now be related to predominant growth of one or the other of the two halves of
the brain during stages of rapid brain growth. Humans start out with a slightly
greater left side brain size. During the 10-12 year period, there is somewhat
more right side brain growth. The greater right side growth of female brains in
that 10-12 year period means that their brains become more symmetric. During
the 14-16 year brain growth stage, there is a slight predominance of left side
brain growth. Perforce, the females who had more symmetrizing brain growth
around age 11 years now have less asymmetrizing brain growth around age 15
years. And, males who had less symmetrizing brain growth around age 11 years
now have greater brain growth around age 15 years when it adds to the
(It is interesting to note that persons who
tend to be more artistic use more right side brain . Thus, both female and
male artists tend to have a greater chance of a left forehead protrusion than
most other persons. In a broad statistical way this permits some level of
classification of humans just from observation of their foreheads.)
These anatomical gender differences parallel
the kinds of functional differences  which revealed the greater functional
asymmetry of male brains and the lack of as much asymmetry of female
It is important to stress that neuroanatomical
differences that derive from genetic programs are not the only source of such
Such programmed functions are built in to the genetic makeup of the organism
and will emerge without instruction provided there are no noxious inputs to the
fetus during gestation or to the organism during postnatal growth. If an input
always evokes a particular response, this is a simple reactive system; it only
As humans mature, they acquire additional
behaviors whose manifestations can appear in an almost reactive manner so that
some culture-based behaviors can seem to be programmed.
If the organism is capable of learning
non-programmed functions, there must be unused parts of the brain and/or (more
likely) additional connections which allow the organism to put together already
existing functions into more complex functions. Since brains initially contain
a huge oversupply of neurons, unused ones of which are later discarded by
dying, there are indeed extra neurons and networks that can serve as sources
for additional mental functions.
But, unless a deity programs these extra
neurons and networks they can be presumed not to have built-in functions. It is
well-known that connections among brain areas are modified and, thereby,
sharpened during maturation [11,12,13,14,15] ;and many other earlier
publications. Such a sharpening bespeaks the neural networks' dependence on
This dependence of actual neural network
structures on learning has been called experience-expectancy [11, 12, and 13].
Because such effects can also result from instruction, it could be more
generally called learning-dependence. It depends on having inputs that are
adequate in their nature, timing, and relevance for the particular individual
to learn. Inasmuch as such learning by humans takes place throughout life, it
follow that the actual structures of our brains, not just our minds, are
dependent on adequate and timely inputs from other individuals and from
experience. Thus we arrive at the inference that human brain structure and
development depend strongly on such inputs from parents, siblings, teachers,
and all other individuals with whom humans are in contact. Based on such a
scenario, it is more readily imagined how disadvantaged children can be
deprived and underdeveloped if raised by parents without the background,
resources, and time to do their parts in the instruction of their children. It
is also imaginable that, for lack of optimal early childhood inputs, later
inputs might not be able to achieve an optimal organization of the brain
because the efficiency of instruction may have evolved based on optimal inputs
at a series of turning points in maturation of children.
South African studies [16, 17] found that
rehabilitation of very young malnourished children restored both height and
weight to the range of normal values but the deficit in head circumference
The ability to learn new functions makes it
almost impossible for evolution to select for those functional programs any
more than dogs can be selected for fetching newspapers. Thus, we humans are
destined to remain learning-dependent for the neuroanatomical changes that
underlie the higher cognitive functions that are the basis of most of our
intelligent behaviors. Of course, selection could act to enhance the
instructability of the brain.
It will be very useful if the two
first-mentioned research groups [1,2] extend their studies to the developmental
aspects of the differences they found, for that would tell us if there are
contributions of experience and/or maturation to these functional properties.
If such developmental aspects are found, it will then present the challenge of
finding whether they stem from learned behaviors or from changes associated
with genetically programmed brain development processes. Such processes could
appear during the respective brain growth stages when there are significant
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