Dr Fletcher Is Interested in Understanding Whether Joining a Fraternity/sorority

Essay

Dr. Fletcher is interested in understanding whether joining a fraternity/sorority causes people to become more concerned about their attractiveness and appearance. He recruits a group of 55 freshmen (25 males, 30 females) who are planning to go through fraternity/sorority recruitment on his campus. After they join, he gives them a measure of attractiveness concern/appearance concern (the Body Concern Scale, where higher scores indicate higher body concerns).
As the study is described, explain a selection threat that may be present in Dr. Fletcher's study. What could Dr. Fletcher do to address this threat?