Nonverbal behaviors appear to matter much more than anything else in student ratings. Enthusiastic gestures and vocal tones can mask gobbledygook, smiles count more than sample exam questions, and impressions formed in thirty seconds accurately foretell end-of-semester evaluations. The strong connection between mere nonverbal behaviors and student evaluations creates a very narrow definition of good teaching.
…show a few seconds of a teacher on tape with the sound off. A group of students is then asked to evaluate the teacher on a number of measures. These evaluations — again based on sound off seconds — turn out to be remarkable close to the evaluations the same teachers receive at the end of a semester from their regular classes. In short, looks, movement, expressions, etc, may trump everything else.
(quoting from Bias, The Brain, and Student Evaluations of Teaching, 82 St. John's L. Rev. 235 (2008))