I'm not sure, but it's a very tricky subject. The line of cautious confidence in your own skill is the perfect medium; but below that is both the un-challenged confidence of the ignorant and the over-zealous confidence of the insecure. But then above that line is the arrogance of the smug, and the idiocy and insanity of the truly brilliant.

Then, combine all of that with all the types of people who lack confidence.

There are so many people who are wrongly confident at all levels of intelligence, I would bet that there's basically no IQ correlation to "confidence". But I suspect what they're calling confidence is really just "verbal and physical displays of dominance".