Previous research on instruction in higher education suggests that students' personal biases strongly influence their perceptions of professor/instructor effectiveness. However, there is disagreement about the degree and direction of this influence. The present article seeks to contribute to this often conflicting body of work. The researcher developed and distributed a student rating scale (N = 202) to determine how biases toward professor/instructor traits, such as age, gender, teaching style, faculty ranking, personality, and grades awarded, mediate judgments of effectiveness. The results, which statistically indicate several pronounced student-held biases, are interpreted using a multi-theoretical framework. Recommendations to combat biased evaluations are presented. (Contains 3 graphs and 2 tables.)