People who complete more formal schooling tend to be more intelligent than those who don't.
IQ tests are designed so that the mean, median and mode are essentially the same, so there's no way that two-thirds of people can be above average.
Most people really do think they are pretty smart.
Does this mean they are irrational about their own intelligence?
But expressing confidence also can bestow benefits, even if that confidence is sometimes undeserved. Confident people are seen as being more competent and higher in social status than humble people.
The most reliable way to convince other people you are brilliant probably starts with sincerely believing it yourself.
Source: The Wall Street Journal, 7/21/18, by Patrick Heck and Christopher Chabris of Geisinger Health System in Pennsylvania