Observer-Expectancy Effect

The observer-expectancy effect occurs when a researcher's reactivity (cognitive bias) causes them to subconsciously influence the participants of an experiment. When this occurs the researcher might first set up an experiment that is biased towards their own point of view and the interprets the data derived in accordance to their own biases. Imagine an experiment where a researcher wanted to prove that while rats are smarter than grey rats. They set up identical mazes, but offer the white rat peanut butter as a reward while rewarding the grey rats with grapefruit slices for the same actions. Which group would appear smarter?