I find that people, as a general rule, try to live up to high expectations.
Most people want to be well thought of. If, by my words and body language, I demonstrate that I assume them to be mature, decent and well-meaning human beings, more often than not they will go out of their way to not tarnish that image.
I apply this philosophy constantly in teaching my culinary school classes. I’ve known other instructors in the same school who begin their classes with all kinds of stern warnings about bad behavior and attempt to establish a strict disciplinary framework from the beginning. Then I hear them constantly complaining about the hooligans in their classes and the stolen tools, etc. I have never once had the slightest discipline problem from my students, and I’m convinced that it’s because they want to show that my high opinion of them is justified.