Not just anyone can run around calling themselves a doctor. It’s a licensed profession with standardized training, and most of us accept this. Therefore, when someone is introduced as a doctor, we’re more inclined to hear what that person has to say on topics related to their field.
That’s how social validation works, and that’s why most people are more comfortable choosing Doctor Joe over average Joe. All other information being equal, we look for secondary indicators to decide whether someone or something is credible. Usually we’re not aware we’re doing it, but it’s a necessary mental trick we all use in the process of forming opinions and making decisions.
