Donald J. Trump Tweets that he is "a very stable genius."
Needless to say, The Donald is too dumb to understand this, but a basic rule of human psychology can be summarized for laymen as: "the smarter someone claims to be, the stupider they are." To oversimplify a complex topic, that's because extremely brilliant people are so smart that they recognize all of their own limitations, while extremely dumb people are too dumb to realize what they are missing.
In other words, the President of the United States basically just said, "I am an idiot."
That phenomenon is called the Dunning-Kruger effect. While that principle does not extend to sanity and mental stability, common sense suggests that a similar principle probably applies in that arena. You don't often hear anyone volunteer "I am sane" or "I am stable" unless they do not appear to be.