On of the reasons that the idea that highly intelligent people are more likely to go insane has to do with how we define intelligence. Generally, intelligence implies some sort of ability to come up with new ways of seeing the world. If the new way of seeing the world falls too far out of our idea of what is reasonable we call the person crazy. So, it is more likely that people who go against the grain might do this. I think that a better definition of insanity would relate to a person's desire to harm themselves or others, not the wackiness of their ideas. If more people thought of insanity in these terms I think we might find that more of the so-called-normal people who stand by while their state murders people or cheerfully join the army and go off to kill (I know that's not the only reason people join the army, so don't jump on me all you ROTC's) we would find that many more of us are insane than we ever expected and it's not the guy shouting about aliens on the street corner who is the problem.