The "Rosenthal effect" is another name for the observer-expectancy effect, or the idea that the expectations or biases of a researcher can have an effect on the results of an experiment. The effect is named for the research of Robert Rosenthal in this area.
Rosenthal conducted famous studies in which students were given rats and told that some of the rats were inherently better than others at learning to navigate mazes. In reality, there was no difference between the rats, but the rats labeled as "bright" consistently performed better. This study led to another famous study in which elementary school students were given IQ tests, and randomly selected students identified as having more potential for growth actually showed more growth due to the influence of the teacher.