holstege@polya.stanford.edu (Mary Holstege) (11/09/88)
It took me a while to chase down this reference, sorry. This is about the
report in the Chronicle that the corpus collosum is larger in female brains
than in males. As with most things that get printed in the Chron, the truth is
more complicated. Let me just quote what Sandra F. Witelson has to say about
it in Vol 11:2 (June 1988) of Behavioral and Brain Sciences. Her discussion
is in reference to an article on differences in mean SAT-M scores, which I have
discussed from other angles elsewhere. Anyway, this is what Witelson has to
say about the corpus callosum (square brackets are my comments):
"...In one report (de Lacoste-Utamsing & Holloway 1982) the posterior one-fifth
region of the callosum (...) was observed to be larger in females than in
males, both in absolute area (...) and in maximal width. In a later study
(Holloway & de Lacoste 1986) only the sex difference of maximal splenial width
was observed. In the larger samples of the few studies done at the turn of the
century, in my own work (Witelson 1985), and in the few more recent reports,
sex differences were not found in total callosal size or in the posterior
one-fifth region, either in adults (see review, Witelson & Kigar 1987) or in
children (see review, Witelson & Kigar 1988). My interpretation of the data is
that females do not have a larger (in absolute or proportional) splenial region
than do males.
"There may be sex differences in other parts of the corpus callosum, however.
Benbow very accurately reported the results of my 1985 paper, which found a
nearly significant sex-by-hand interaction effect [e.g. left-handed females
might show a different pattern than right-handed males, etc.] for the size of
the overall posterior one-half region of the callosum.
"In a subsequent study (Witelson 1986) the posterior part of the body or trunk
of the callosum (...) showed a particularly marked difference in size between
consistent-right-handers and mixed-handers, regardless of right- or left-hand
writing (...). In a further study of subregions of the callosum with an
expanded sample of 50 cases (15 men, 35 women), statistically significant sex
differences in the size of the callosum were found in some regions (Witelson
1987b). Two sex differences are relevant here: (1) Hand preference was found
to be a factor in the absolute size of the isthmus in males only. This area was
56% larger in mixed-handers. (2) The consistently right-handed females had a
larger absolute isthmus region than the consistently right-handed males;
females did not have a larger area in any other callosal region.
"The isthmus region connects the parieto-temporal cortical regions of the two
hemispheres. These regions are involved in the representation of linguistic,
spatial, and musical skills -- skills that are represented asymmetrically in
the cortex."
OK, now that you've waded through all the gobbledegook, what have we got?
First is that there have been a number of studies (I will email complete
versions of the above references to anyone who asks) and they seem to
contradict one another a great deal. Second: None of the studies involves a
great number of brains. That isn't too surprising as people are a bit squeamish
at the prospect of having their brain chopped up, even if they have no further
use for it. This casts some doubt on the validity of some of the results. For
example, I have a hard time believing that enough mixed-handed males could be
found in a sample of 15 to say anything meaningful, when 15 is no a number that
should inspire confidence to begin with. I leave it as an exercise for those
who remember statistics better than I to explain why Witelson must be using a
definition of the phrase "statistically significant" that is questionable, at
best. If you look at enough factors with samples this size, you are *bound* to
find correlations. I'm not saying that there is no difference in the
neuro-anatomy of adult female and male brains, and indeed I would be rather
surprised if there were *not*, but it seems clear to me that it is not a simple
difference and that other factors are involved (handedness, perhaps). Also
bear in mind that since the brain forms connections as a result of both pre-
and post-natal development, it does not follow that a difference in adult
brains (dead adult brains, at that; dead adult brains of people willing to have
their brains sliced, yet) tells us anything about "natural" or "innate" ways of
being for men and women.
-- Mary
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