Corpus Callosum
It has been claimed that the human corpus callosum shows sex differences, and in particular that the splenium (the posterior portion) is larger in women than in men. This was based on old data from the study of cadavers. More recent data from MRIs does NOT support this.
The widespread belief that women have a larger splenium than men and consequently think differently is untenable.

However, there have been more recent studies showing differences in the the number of axons by axon diameter and locaation at different ages in men and women.
See: Sex And The Corpus Callosum