brain size and intelligence

Richard Norman rsnorman at
Wed Feb 7 08:58:34 EST 2001

"MS" <marshmallow5 at> wrote in message
news:5y4g6.456$WV5.59588 at
> Since a relationship exists between brain:body ratios and intelligence
> across species, any thoughts on why it doesn't pertain within a species
> (i.e. humans). Or are there studies demonstrating this which I'm not aware
> of?

I believe that historically there were attempts to correlate
brain size with "race" and thereby establish "objective
criteria" for racial superiority theories.

One major problem is defining what you mean by "intelligence"
and demonstrating that a single indicator can measure whatever
you mean by it.

Another major problem is that even a modest correlation allows
enormous numbers of individual cases where the relationship
is false.  So any result based on population data is totally
worthless when applied to individuals.  Therefore there is
no real utility in even attempting the study on humans.
 It would not be a useful indicator for anything.

A third major problem is that to establish significant results
for even a modest correlation, you need data over a very wide
range of brain sizes and "intelligence" (whatever that is).
Normal humans don't have enough variation in brain size.
You can get a wide variation in brain size by looking across
species.  A similar problem is seen in looking at the relation
between body size and metabolic rate.  Within one species,
there is not really enough variation to firmly detect whether
the power law relationship has an exponent of 2/3 or something
closer to 0.7.  But across a wide enough range of body sizes
(which requires a very diverse range of organisms) you can
clearly see that the exponent is distinctly greater than 2/3.

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