The mad scientist is a popular character in modern fiction. The idea of the genius who can't tie his own shoelaces. Part of this comes because as a literary device characters must have flaws that in some way match their strengths, but that is not the only reason. There is a documented connection between mental illness and genius. In fact, truly genius people are by definition think differently than others.
The most common mental illness that attacks the genius is manic depression, or bipolar disorder striking them at considerably higher rates than the general public. The question of why, is then of course important to many of the smartest people in our world and so studies of the links between the two have began.
The first thing discovered by giving personality and temperament tests to artists was that even the healthy artist had more in common with a manic depressive than with a member of the general public. The question then becomes one of the chicken and the egg. Are these people creative because it helps them deal with mental illness or does the creativity create a mental instability.
No matter the reason the connection is still clear and it is not only these two diseases. Other mental illnesses such as sociopathic tendencies, obsessive-compulsive disorder, and schizophrenia are all more common in those with high IQ's.
Among the numbers of those who have been stuck by mental illness are many of our greatest thinkers, writers and leaders. These include Earnest Hemmingway who suffered from depression, Winston Churchill who suffered from Bipolar disorder, and John Forbes a noble laurite in economics who has paranoid schizophrenia. (The movie a beautiful mind is the story of John Forbes)
Outside of true mental illness lies the socially maladjusted and awkward. Humans have always isolated and discriminated against those that are different than them and though discrimination in many ways is less acceptable now than it has been it is still often difficult for the true genius to make friends. From the kid who graduates college at 14 to the bookworm who would rather study for his SATs than go out on a Friday night this can sometimes make those of us not used to them uncomfortable, but it is not insanity.
The clich of the mad genius alone in his libratory creating wild inventions is still primarily a story telling device. However, the connections between insanity and creative genius are far more connected than was once believed and that connection is well worth exploring because in the modern age many of these sicknesses can be treated and their genius released to help us improve this world.