Question

In a classic experiment, psychology students were assigned to work either with “bright” rats or with “dull” rats, described as such by the experimenter. after a series of tests, the students' results showed that the “bright” rats had outperformed the “dull” rats. however, in fact, all the rats were of the same strain and breed; there were no preexisting differences between the groups. what caused the difference in their performance?

Answer 1

the answer to this question is The students' expectations about the rats' performance influenced their observations and behaviors
When the students heard a term/label imposed to a subject/object, we will initially expect that subject/object act according to their labels.
Another example would be, when someone tell you that a bread is poisonous, you will automatically expect whoever eat that bread to start choking or vomiting blood.