The goal of this experimental study was to evaluate the influence of course type, instructor and student gender, and student individual differences (domain-specific vocational interests and confidence, personality, and gender role attitudes) on student evaluation of teaching (SET) scores. A sample of 610 college students (372 female) rated hypothetical instructors described in a vignette on eight common dimensions of teaching effectiveness. Mean SET ratings were not significantly different across instructor gender and course type. A series of multiple regressions revealed, however, that student individual differences explained a significant proportion of the variance in SET ratings. The most salient traits that were significantly related to SET ratings were agreeableness, conscientiousness, conventional and investigative confidence, and gender role attitudes. In addition, female students gave significantly higher mean ratings than male students independent of course type or instructor gender. This effect was eliminated when statistically controlling for students' individual differences. Overall, the findings of this study suggest that student individual differences can bias SET scores, which poses a threat to the validity of the ratings.