There have been many studies done, and some artistic
endeavors (the dramatic play
The Curious Savage, a few television series, the movie
Phenomenon, etc.) that put forth similar theories. There certainly seems to be a link between heightened
mental capacity and what could be referred to as mental
instability.
Perhaps one reason for this could be tied to the theory of paradigms. Sanity is not entirely quantifiable, but it can be generalized in definition; such that it is apparent to the majority of the "sane" populace that a person is not thinking under the same set of paradigms that define "sanity". However, this same ability to think "outside the box" that would make society dub someone insane, can also give clues as to the nature of this person's mental flexibility.
There have been many brilliant people that have shown no sign of insanity, and there are plenty of legitimately insane people who do not or cannot demonstrate any high level of intelligence. However, there is enough of a relationship that it is unwise to discount the postulate that intelligence does, at times, work hand in hand with mental instability.
Besides, some of the most brilliant people I have ever met were definitely borderline. One guy I knew talked to machinery. Not just mumbling or making odd comments, I mean having deep conversations with machinery.
The mind is a vast and tumbled plane...