Specialists only trust themselvs in their own area. In unfamiliar territory they have to rely on other specialists in other areas. So they get used to not trusting themselves. When Story Musgrave became an astronaut (he went on to log more hours aloft than any other US astronaut) he found that his background as a surgeon and an engineer helped in space where you have to rely on yourself. But he noted that it was his farm upbringing, where he learned 'to fix everything' that gave him the right mindset to be a self-reliant person. Being polymathic, having multiple expertise (even the mildest variants help) is therefore very beneficial in improving self-trust, essential even.