Generally I've found that the more someone expounds on their own credentials, the less credible (and likable) they sound.

This is true, particularly because people are different, so if you tell us a lot about yourself, you're telling us comparatively little that applies to, well, us. That's why we often want some kind of organized, even if informal, study on a group of people, particularly a group in it with enough variation that personal uniqueness washes out.