The Pygmalian effect - expectations influence results

Our expectations can subtly influence results. For example, a manager who has high expectations of his employees can have excellent results from them as they try to live up to his expectations.

if you expect the worst, you get it. If you expect the besr, you get it too.
It is named after the Greek myth of Pygmalion, the sculptor who fell so much in love with the perfectly beautiful statue he created that the statue came to life.
There are questions about the original study which posited this effect since it has not been reproduced consistently. So take with a pinch of salt

An interesting use of the Pygmalion effect might be that suggested by George Bernard Shaw’s play Pygmalion. In it, Professor Henry Higgins takes a poor flower seller from the streets, Eliza Doolittle, and by giving her elocution lessons helps her sound like a duchess. Being able to speak like a member of the upper classes is meant to open doors and give her opportunities that she would otherwise never have.
The play is, among other things, an exploration of how others’ expectations limit us. Eliza has far more potential than can be realized solely because of her accent. A critical part of the plot is that Eliza herself is all too aware of how her speech holds her back and diminishes her value in the eyes of others. She is the one who follows Higgins and cajoles him into taking her on as a student. She sees the opportunities that will follow from changing her accent.
The improvements in Eliza’s speech alone do not confer the opportunities. But being able to speak like a duchess puts her in the company of people from whom she can learn the sentiments and sensibilities of the upper class. When she begins to speak like them, they treat her differently, giving her an opening to expand her capabilities.