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

Howard Olson holson at california.com
Sun Aug 3 20:17:44 EST 1997

James Howard <jmhoward at sprynet.com> wrote:

>"Male Brains slightly larger than Female Brains"
>James Howard

>The recent finding that, on average, male brains are slightly larger 
>than female brains, adds support to my theory of human evolution.  Below 
>this is my post, "The Flynn Effect  …yes, testosterone," which explains 
>how testosterone increases brain size (posted November 11, 1996).  In 
>it, you will find my explanatin of the Flynn Effect.   The Flynn Effect 
>should produce male brains that are larger, on average, than female 

>The Flynn Effect   ...yes, testosterone

>In response to my "Current Signals of Increases in Testosterone," which
>explained how I think rising testosterone causes the secular trend and 
>affecting a number of phenomena in the U.S., I received this response:
>"But IQ scores are rising with the secular trend!"  The author of this
>comment intended to show that this, real connection between IQ scores
>and the secular trend, refutes my theory.  This increase in IQ is called
>the "Flynn Effect."  The Flynn Effect refers to an increase in IQ that
>occurs along with the secular trend.  However, the Flynn Effect is 
>an increase in "non-verbal tests."  The overall effect is primarily due 
>increases in the lower half of the IQ distribution, but it also occurs 
>in the
>"gifted," or those who score 1400 or more on the SAT.  The Flynn Effect
>is a worldwide phenomon.  (It occurs in all socioeconomic groups.)

>I suggest the Flynn Effect is due to rising testosterone.  The effect is
>small; perhaps 15 points in two generations.  In my last post, "Hominid
>Brain Size: Testosterone, DHEA, and Melatonin," I suggested that two
>kinds of increases in hominid brain size have occurred.  One, that
>causes only slight increases in brain size, I attribute to increasing
>testosterone in hominids.  This is the small increase noticeable in the
>early hominids.  This is the increase I think is due mainly to the
>increases in testosterone that started separateing us from the other
>primates.  The main increase in hominid brain size, I suggest, is due to
>the effects of the hormones, DHEA and melatonin.  The effects of these
>two hormones, according to my theory, have to occur away from the
>equator, and this is where the fossil evidence, north and south, shows
>the main increases in brain size.  The increase in testosterone will 
>through hominids, i.e., testosterone will periodically rise in humans.  
>will produce some dramatic effects, but one, I suggest, is a slight
>increase in nonverbal abilities of the brain that will cause slight 
>in overall IQ scores.  To explain this, I am including part of my post,
>"Math and English," that was posted June 13.  This was generated in
>response to findings that "reading proficiency is down in virtually 40
>states  ...and significantly down in 10."  The math scores were up
>slightly; this is the Flynn Effect in the U.S., as of April, 1996.  The
>increase in testosteorne causes the slight increases in nonverbal
>abilities, while it causes decreases in verbal abilities.

> "It is generally accepted that the two sides of the brain
>(cerebral hemispheres) interact but basically perform different
>functions.  The things we call reading and writing are usually 
>by the left half, mathematical things and spatial abilities are usually
>located in the right half.  Because of this divergence in function, boys
>are, generally, but not always, better in math than girls.  My work
>suggests a reason for this that is directly connected to the changin
>reading and math scores.

>The left hemisphrere finishes growth a little after the right.
>My work suggests brain growth is particularly dependent on the
>hormone, DHEA.  (DHEA in extremely small quantities stimulates
>formation and growth of the brain cells primarily used in thinking,
>neurons.)  Therefore, the left hemisphere depends on a continued
>supply of sufficient DHEA for final growth.  All Tissues, especially the
>brain, compete for DHEA.  The hormone, testosterone, increases use of
>DHEA by testosterone target tissues, which also includes parts of the
>brain.  Boys produce more testosterone than females so there is less
>DHEA, on average, for left hemisphere growth.  In animals studies, it
>has been demonstrated that testosterone actually reduces development
>of the left hemisphere (Behavioral and Neural Biology 1988; 49: 344).
>Therefore, boys, on average, have an increased ratio of right
>hemisphere to growth to left.  The right side is used more for
>mathematical and spatial thinking, therefore, on average, boys out-
>perform girls in these areas.

>If you want to be a good mathematician, you might be tempted to
>want more testosterone.  In tests of spatial and mathematical reasoning,
>males with high testosterone score much worse than those with low
>testosterone.  High testosterone increases lower brain growth and
>development at the expense of even the right hemisphere.  That is, in
>high testosterone, even the right hemisphere looses in the competition
>for DHEA.

>I have suggested in past letters to this paper that testosterone
>is rising in this society.  Most people see it in the 'secular trend,'
>that is, boys and girls are getting bigger and reaching puberty earlier.
>If testosterone is rising, it not only will affect the size of our
>children, but it will also affect their brains.  That is, as 
>increases it will decrease the ratio of left hemisphere to right.  This
>will be seen, on aveage, as a decline in reading ability and an increase
>in math abilities.  In areas where testosterone is very high, reading 
>math scores should both decline.  The thing that worries me most is that
>one of my references points out that 'the left hemisphere also seems to
>be the seat of analytical thinking...'  According to the National
>Assessment of Educational Progress, we may be seeing a real, and in
>some areas already significant, decline in functions of the left
>James Howard

	First of all, the brains of men and women are comparable when you
consider the  body size to brain size ratio. This is a well known fact
of paleoneurology. Secondly, testosterone is Not the only anabolic
steroid in the human body and there may well be an analogous effect
from another steroid or even a peptide (such as GH ) in the female.

	BTW, are you aware that testosterone is metabolized to estrogen BEFORE
it reaches the nuclei of target cells? Surely that throws a monkey
wrench into the simple reductionist model you propose for male


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