James Fallows on the traits very intelligent people:

“They all know it. A lifetime of quietly comparing their ease in handling intellectual challenges—at the chess board, in the classroom, in the debating or writing arena—with the efforts of other people gave them the message.”

“Virtually none of them (need to) say it. There are a few prominent exceptions, of talented people who annoyingly go out of their way to announce that fact. Muhammad Ali is the charming extreme exception illustrating the rule: He said he was The Greatest, and was. Most greats don’t need to say so. It would be like Roger Federer introducing himself with, “You know, I’m quite graceful and gifted.” Or Meryl Streep asking, “Have you seen my awards?”

“They know what they don’t know. This to me is the most consistent marker of real intelligence. The more acute someone’s ability to perceive and assess, the more likely that person is to recognize his or her limits. These include the unevenness of any one person’s talents; the specific areas of weakness—social awkwardness, musical tin ear, being stronger with numbers than with words, or vice versa; and the incomparable vastness of what any individual person can never know. To read books seriously is to be staggered by the knowledge of how many more books will remain beyond your ken. It’s like looking up at the star-filled sky.”