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

James Howard jmhoward at sprynet.com
Sun Aug 17 12:01:11 EST 1997

I started this thread as an example of the effects of testosterone on 
human brain evolution.  In one of the responses, the body 
- brain proportionality issue was used to refute the importance of the 
difference between male and female brains.  In the publication that 
reported the differences between male and female brains, that is, that 
male brains, on average, exhibit 16% more neurons than female brains, 
the proportionality issue is not considered a determinant in causing the 
difference.  It contained this phrase: "...body size, per se, had no 
influence on neuron number..."  (The report follows below.)  Now, 
someone has attached "but not porportionately" to the title of this 
thread, which I am retaining for this post.  This debate has left the 
content of my original post, and that is alright, since different people 
have different ideas about this finding.  I am responding to this part 
of the thread, because I want those people to read that these findings 
are not affected by body size.

J Comp Neurol 1997 Jul 28;384(2):312-320, "Neocortical neuron number in 
humans: effect of sex and age." Pakkenberg B, Gundersen HJ

"Modern stereological methods provide precise and reliable estimates of 
the number of neurons in specific regions of the brain. We
decided to estimate the total number of neocortical neurons in the 
normal human brain and to analyze it with respect to the major macro-
and microscopical structural components, to study the internal 
relationships of these components, and to quantitate the influence of
important physiological variables on brain structure. The 94 brains 
reported represent a consecutive collection of brains from the general
Danish population. The average numbers of neocortical neurons were 19 
billion in female brains and 23 billion in male brains, a 16%
difference. In our study, which covered the age range from 20 years to 
90 years, approximately 10% of all neocortical neurons are lost over
the life span in both sexes. Sex and age were the main determinants of 
the total number of neurons in the human neocortex, whereas body
size, per se, had no influence on neuron number. Some of the data 
presented have been analyzed by using new mathematical designs. An
equation predicting the total neocortical neuron number in any 
individual in which sex and age are known is provided."

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