Re: Brain size, IQ

Len Piotrowski (
Wed, 28 Aug 1996 15:28:59 GMT

In article <501g5s$> (Bryant) writes:


>>The controversy is over the simple notion that brain size was heritable and
>>not something else of which head size (and thus brain size) was an effect.

>Um, so you believe that Australopithicine brains and Homo sapiens brains
>would be the same if subjected to the same set of environemntal effects?

How ever did you derive from what I said that conclusion???

>Why stop there?

I didn't even start it!

>Why not posit that plopping a canary brain in a mouse
>skull will result in a perfectly mouse-ly brain in the developed adult?

Quite imaginative, but on point? Perhaps you are implying by this that head
size and brain size aren't correlated?

>>By this definition, the human brain size can be viewed as relatively "fixed"
>>since Neanderthal times, and even relatively unvarying before that with Homo
>>erectus, which by your definition could indicate that brain size was not
>>"heritable." I'm sure this is not what you are driving at.

>I don't recall the cubic centimeters, but do seem to recall some
>substantial growth in cranial capacity since H. erectus.

Not since H. erectus, as far as I am aware, unless you are including H.
erectus in the statistic. Do you have some data that shows this trend?

> My argument was
>that brain size was clearly heritable in the evolutionary past, as
>demonstrated by the fossil record.

Well, if head size is correlated with brain size (which you may not be ready
to concede) then the only significant differences in head size resolved by the
fossil record are at speciation events. In between those episodes there
appears to be no significant "heritability" of head size in the record.
Perhaps you can clarify this anomalous situation?

>>At any rate, if developmental genes are heritable, their affect on phenotypic
>>variability of other characteristics like the brain could be considerable,
>>even if those other genetic characteristics were "fixed."

>In which case, via pleiotropy, those are also genes "for" whatever brain
>characteristics they affect, and brain development is therefore, by
>definition, not fixed and species typical.

... which by this "definition" would make "heritability" a signifier without
distinction. Every trait that was effectively "developmental" would by
this definition be "heritable" via pleiotropy, and thus incapable of
becoming "fixed," unless, of course, all of development became fixed. Perhaps
this is what you originally meant by "brain size was heritable," that is, that
"development was heritable?"