...within the girl’s smaller brain is a larger corpus callosum than in the boy’s brain... Boys and girls come from the womb with brains wired for different characteristics and behavior just as God intended.
--Mary Ann Bradbury, conservative Christian author.
Others have already addressed what a corpus callosum is; this node addresses a controversy surrounding this portion of the brain, sex, and size.
In 1982, a study suggested that the splenium of the corpus callosum may be larger in females than males. Although that study did not address gender differences in thinking, it did not take long for some people to conclude that clearly, this instance of sexual dimorphism must cause men and women to think differently. The argument goes something like this: the corpus callosum is a conduit for communication between the right and left hemispheres of the brain. Since it is larger in females, a woman's left brain and right brain must chat more often, doubtless sharing the kind of detail for which the manly brain has no time. This results in men being more focussed and compartmentalized, and women more.... Well, that varies, but some have held this process to be responsible for "female intuition."
Invariably, this leads to the conclusion that one gender is better-suited to a particular kind of thinking than another. "Doctor" John Gray, a favourite of Oprah Winfrey, has gone to excessive lengths to promote the notion, and relate it to gender differences. Some conservatives likewise have cited this difference as an indication of why men and women do not think alike: most famously, Dr. James Dobson, who is fond of finding "scientific" support for a fundamentalist Christian view of human nature. Certain feminists-- those with an essentialist bend-- also have embraced the corpus callosum as the seat of innate gender differences, with the implication that female thinking is more holistic and perhaps better, though difficult for men to grasp. At the same time Joe Manthey and other "men's rights" activists have made the difference in corpus callosum size a key part of their argument regarding the ways in which contemporary education fails to serve the true needs of male students.
Two problems exist here. Firstly, these people are citing a gender difference which may not exist. Secondly, if it does exist, its implications remain uncertain.
Christine De Lacoste-Utamsing and Ralph L. Holloway published the original 1982 report, "Sexual Dimorphism in the Human Corpus Callosum" after examining a rather small sample of brains. The female brains they examined had, on average, corpus callosa with larger splenia. Lacoste-Utamsing and Holloway approached their data cautiously. They noted several potential problems with the study, not the least of which was the relatively small size of their original sample. Another problem is the well-documented change to the corpus callosum with age. They never claimed that this difference-- if it truly existed-- indicated anything about male and female thinking. They naturally encouraged further research. Good scientists know that a single result taken from a small sample may become the impetus for further research, but by itself means very little.
Studies since that time have have not consistently replicated the results. Some have contradicted it.
A 1990 examination of 49 studies of the corpus callosa since shows no significant difference in shape or size. The study's authors conclude that the "widespread belief that women have a larger splenium than men and consequently think differently is untenable" (Bishop and Wahlsten). Another study has concluded that average differences in shape exist between the genders (Allen, Richey, Chai, Gorski). Holloway has noted that some of the studies which contradict the original findings do not correct for brain size, thus problematizing their results. Many other studies support the original claim that, on average, female corpus collosa contain larger splenia, relative to total brain size (Holloway 1990). Another study published in 2000 found no size difference, except that the genu of the corpus callosum actually appeared to be on average larger in males. The researchers concluded, of course, that the "clinical and physiological significance of this observation" was "uncertain". (Bermudez, Zatorres).
So do women, on average, have larger corpus collosa? Good question. But even if research does indicate sexual dimorphism in the size of the corpus callosum, there would be no strong grounds to assume that difference necessarily leads to different kinds of thinking. The usual Mars/Venus view of our corpus callosum also requires a grossly simplified and exaggerated view of right/left brain dominance-- brain hemisphere specialties being much exaggerated in pop psychology.
Research on sexual dimorphism in the human corpus callosum, then, has yielded uncertain results. Even less certain are the conclusions we can draw from brain differences. Men and women may well have inherently different ways of thinking, but any statement on this matter should be derived from good science, not ideological presuppositions.
LS Allen, MF Richey, YM Chai and RA Gorski. “Sex Differences in the Corpus Callosum of Living Human Beings” Journal of Neuroscience. 1991. http://www.jneurosci.org/cgi/content/abstract/11/4/933
Banka S; Jit I "Sexual dimorphism in the size of the corpus callosum." Journal of The Anatomical Society of India. December 1996 45(2): 77-85.
K.M. Bishop and D. Wahlsten. “Sex and the Corpus Callosum.” University of Indiana, Bloomington, Indiana. http://www.indiana.edu/~pietsch/cc-sex.html
Patrick Bermudez and Robert J. Zatorre. “Sexual Dimorphism in the Human Corpus Callosum: Methodogical Considerations in MRI Morphology” NeuroImage 13, 1121-1130 October 25, 2000.
Mary Ann Bradbury. "Boys and Girls." Lifeway. (reprinted from ParentLife). http://www.lifeway.com/lwc/article_main_page/0,1703,A%253D155239%2526M%253D50018, 00.html
Christine De Lacoste-Utamsing and Ralph L. Holloway. “Sexual Dimorphism in the Human Corpus Callosum.” Science, New Series, Vol. 216, No. 4553 June 25, 1982, 1431-1432
Ralph L. Holloway. “Sexual Dimorphism in the Human Corpus Callosam: Its Evolutionary and Clinical Implications.” From Apes to Angels: Essays in Anthropology in Honor of Phillip V. Tobias Geoffrey H. Sperber, Ed. 221-8. New York: Wiley-Liss, 1990. http://www.columbia.edu/~rlh2/CorpCallevol1990.pdf