Although this is often said it simply is not true. That may be a symptom but it is not the definition.
insanity
noun
1 insanity runs in her family - mental illness, madness, dementia; lunacy, instability; mania, psychosis; informal craziness.
2 it would be insanity to take this loan - folly, foolishness, madness, idiocy, stupidity, lunacy, silliness; informal craziness.
By your definition, I guess I'm a little insane too--we both agree with that....