It comes from the reinforcement of belief by empirical results.

I think this is an oversimplified view. For example, I have plenty of evidence of my own beliefs, but am not particularly confident. My confidence improves, it's true, as the evidence grows, but not as much as it should. That, I think, is due to a lack of confidence instilled in childhood - in the same way that if you miss the language spurt in your eearly years you'll never quite be able to make up for it, or if you never throw a ball as a child you'll never be a great tennis player.

It's also possible it's partly genetic, of course, as a surprising number of personality traits are.