Why do most of us have an interthalamic adhesion?

Neil asks a very good question. What is the difference between those with, and those without, an interthalamic adhesion. I found a 1991 article in Science by Ann Gibbons entitled “The brain as ‘sexual organ’” (Science 30 August 1991: 957-959) which states that the “massa intermedia tends to be absent altogether in men more frequently than it is in women. While the function of the massa intermedia isn't known, some early NIH studies have found a correlation between the presence of it and I.Q. scores (with different patterns in men and women). Says [Sandra J.] Witelson, [a McMaster University behavioral neuroscientist]: ‘Obviously, intelligence isn't situated in the massa intermedia, but it could be correlated with other anatomical features that are relevant to some aspects of intelligence.’”

However, I posed Neil’s question to Dr. George R. Leichnetz, neuroanatomist at Virginia Commonwealth University and author of Digital Neuroanatomy: An Interactive CD Atlas with Text (Wiley-Liss, 2006). Here is Dr Leichnetz’s emailed response to my query:
“The "interthalamic adhesion" or "massa intermedia" is (as its name implies) an adherence of the ependymal lining of the midline third ventricle. Importantly, it is not a commissure, ie. there are no inter-thalamic fibers exchanged between the two thalami. In subhuman primates and other mammals there is a true midline commissure through which fibers are exchanged. But in man the functions of the two cerebral hemispheres are specialized and separate, hence, their principal source of afferents, the thalamus, are separate. It would be functionally disadvantagious if there were a commissure at the level of the thalamus. So, the absence of this adhesion has no negative functional consequence as far as I know.”

The next time I do a brain autopsy, I’ll take a microscopic section of the interthalamic adhesion and post a photomicrograph to prove Dr. Leichnetz’ point. Given Dr. Leichnetz’ comments, it would be highly unlikely that the presence of an interthalamic adhesion would have anything directly to do with intelligence -- or anything else, for that matter!