Corpus callosum

The corpus callosum is the largest white matter structure in the mammalian brain. It consists mostly of contralateral axon projections. It appears as a wide, flat region just ventral (below) the cortex. It is missing in monotremes and marsupials. It is made up of 200-250 million nerve fibers.

The corpus callosum connects the left and right cerebral hemispheres. Most (but certainly not all) communication between regions in different halves of the brain are carried over the corpus callosum. The posterior portion of the corpus callosum is called the splenium; the anterior is called the genu (or "knee"); between the two is the body. The most anterior part is the rostrum. Agenesis of the corpus callosum is a complete or partial absence of the corpus callosum in humans.

Sexual dimorphism
In humans, disputed claims have been made about the importance for gender difference of a difference in size between the corpus callosum in males and females, and analogous racial claims. RB Bean, a Philadelphia anatomist, suggested in 1906 that the ‘‘exceptional size of the corpus callosum may mean exceptional intellectual activity” and claimed gender differences which were refuted by Franklin Mall, the director of his own laboratory (Bishop and Wahlsten, 1997).

Of much more substantial popular impact was a 1982 Science article (de Lacoste-Utamsing and Holloway) claiming to be the first report of a reliable sex difference in human brain morphology and arguing for relevance to cognitive gender differences. This paper appears to be the source of a large number of lay explanations of perceived male-female difference in behaviour: for example Newsweek stated in 1992 that the corpus callosum was ‘‘Often wider in the brains of women than in those of men, it may allow for greater cross talk between the hemispheres&mdash;possibly the basis for woman’s intuition”. It has also been used, for example, as the explanation of an increased single-task orientation of male, relative to female, learners; a smaller male organ is said to make it harder for the left and right sides of the brain to work together and to explain a feminine ability to multitask.

The relationship between known gender-specific biology (such as males having, in general, higher testosterone levels than females) and claims about behaviour (such as human males being more competitive) remains a highly contested one. Unusually, the scientific dispute in the case of the corpus callosum is not about the implications of biological difference, but whether such a difference actually exists. A substantial review paper (Bishop and Wahlsten, 1997) performed a meta-analysis of 49 studies and found, contrary to de Lacoste-Utamsing and Holloway, that males have a larger corpus callosum, a relationship that is true whether or not account is taken of larger male brain size. Bishop and Wahlstein found that "(t)he widespread belief that women have a larger splenium than men and consequently think differently is untenable."

There is no current evidence that difference in male and female cognitive behaviour can be explained by differences in the size of the corpus callosum.