What is the observer-expectancy effect?
The observer-expectancy effect is a cognitive bias referring to the tendency of researchers inadvertently influencing their study participants. It can contain elements of the Pygmalion effect as well as the halo or horn effects. It is also related to the idea of self-fulfilling prophecies, which can have either a positive or negative impact.
This is one of the reasons that experimental design is so important in the crafting of a research proposal, to reduce the risk that this effect will color the eventual results.