First, beyond about a 145 IQ (I'll side-step, for now, the difficulties of measuring beyond 140; note that I'm talking about the underlying "g" rather than test scores themselves) typical work boredom becomes a crippling, anxiety-inducing disability but, because few people can or want to sympathize with that problem, it often gets misconstrued as an attitude issue. After a few years of professional failure, a lot of those people go into blue-collar work where their bodies are worked but their minds are free.

Second, while IQ and mental illness actually don't correlate, creativity (which is very helpful for getting a PhD in many fields) is correlated with mental health issues.