Male Brains slightly larger than Female Brains [but not proportionately]

R. McPherson rjmcpher at rigel.oac.uci.edu
Sun Aug 17 17:36:13 EST 1997

On Sun, 17 Aug 1997, James Howard wrote:

:I started this thread as an example of the effects of testosterone on 
:human brain evolution.  

:J Comp Neurol 1997 Jul 28;384(2):312-320, "Neocortical neuron number in 
:humans: effect of sex and age." Pakkenberg B, Gundersen HJ
: estimate the total number of neocortical neurons in the 
:normal human brain
: The average numbers of neocortical neurons were 19 
:billion in female brains and 23 billion in male brains, a 16%
:equation predicting the total neocortical neuron number in any 
:individual in which sex and age are known is provided."

I would like to respond to this if I may.
The body size to brain size correlation is an inter-specific
comparison and is not useful for comparing within a species so it is
not at all surprising that the study cited above did not find a
relationship between human body size and brain size.  A body size to
brain size correlation can be drawn only when comparing between
species of animals. 

Previous to the study above, neuroscientists have explored potential
differences between female vs. male brain mass and have concluded that
while males have larger brains, females have more invagination of the
cortex.  Thus female brains have essentially the same cortical surface
area as do male brains.  The cortex in female brains is more folded
(invaginated) enabling it to fit into the smaller cranium
(smaller body = smaller skull).

If the estimation technique used by the above study is legitimate, and
if the number of samples is sufficient, then perhaps they have
identified a characteristic difference in total neocortical neuron
numbers.  Before this characteristic is meaningful, we would need to
be able to establish the 1. distribution of those additional neurons,
2. demonstrated activity in those additional neurons, 3. overall
differences in neuronal activity during cognitive function in those
neurons, and 4 demonstration that more neurons = more function; as
opposed to the notion that trimming of neuronal connectivity
constitutes refinement of function.  The answer to all of these
questions are bound to be sometimes yes and sometimes no and it
depends upon which sensory modality is involved and whether you are
examining developmental or adult functioning.  So more neurons doesn't
mean anything right now.  

I would ask how body size was determined.  If males have a larger
somatic body surface, then perhaps more neurons will be required to
simply represent more body skin surface (or muscles etc...).  If this
were true, then more neurons would certainly be expected and would not
betray any particular propensity for increased cognitive capacity.

rjmcpher at uci.edu      Ron McPherson

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