Answer:
Professor Brandt is most likely conducting a study on: perceptual set.
Explanation:
Perceptual set refers to a biased perception of things, a tendency to see certain details and, at the same time, ignore others. In Professor Brandt's study, the group that has been told the adult-child pair is related becomes biased. They begin to expect to see similarities between the two people in the picture. On the other hand, the other group does not look for similarities because they do not think the two people are related.