Male Brains slightly larger than Female Brains

James Howard jmhoward at sprynet.com
Thu Jul 31 16:21:10 EST 1997


"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 
brains.

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 
is
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 
mainly
an increase in "non-verbal tests."  The overall effect is primarily due 
to
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 
cycle
through hominids, i.e., testosterone will periodically rise in humans.  
It
will produce some dramatic effects, but one, I suggest, is a slight
increase in nonverbal abilities of the brain that will cause slight 
increases
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 
controlled
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 
testosterone
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 
and
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
hemisphere."
James Howard



More information about the Mol-evol mailing list