Sometimes, we aren't looking to divine someone's overall personality or intelligence based on a first impression; we simply want to know how good they will be at a particular skill or set of skills, like teaching. Tufts psychologist Nalini Ambady has found that students, for example, are surprisingly good at predicting a teacher's effectiveness based on first impressions. She creates these first impressions with silent video clips of teachers--clips she calls "thin slices."
In a 1993 study published in the Journal of Personality and Social Psychology (Vol. 64, No. 3), Ambady and a colleague videotaped 13 graduate teaching fellows as they taught their classes. She then took three random 10-second clips from each tape, combined them into one 30-second clip for each teacher and showed the silent clips to students who did not know the teachers. The student judges rated the teachers on 13 variables, such as "accepting," "active," "competent" and "confident." Ambady combined these individual scores into one global rating for each teacher and then correlated that rating with the teachers' end-of-semester evaluations from actual students.
"We were shocked at how high the correlation was," she says. It was 0.76. In social psychology anything above 0.6 is considered very strong.
Curious to see how thin she could make her slices before affecting the student judges' accuracy, Ambady cut the length of the silent clips to 15 seconds, and then to six. Each time, the students accurately predicted the most successful teachers.
"There was no significant difference between the results with 30-second clips and six-second clips," Ambady says.
In a later experiment in the same study, she cut out the middleman--the global variable--and simply asked students to rate, based on thin-slice video clips, the quality and performance of the teachers. Again, the ratings correlated highly with the teachers' end-of-semester evaluations. Ambady also replicated her results with high school teachers.
Of course, one could argue that the true measure of a teachers' effectiveness is not what their students say about them, but how much those students learn. Ambady, acknowledging this, has tried to measure whether students actually learn more from teachers who give a first impression of effectiveness.
In an as-yet-unpublished study, she videotaped groups of five participants, one of whom was randomly assigned to be the "teacher." The teacher spent time preparing a lesson, and then taught students a mathematical language in which combinations of letters represent different numbers, as in 10=djz or 3=vfg. The students took a test at the end of the lesson to measure their knowledge of the new language. Then, as before, strangers watched 10-second video clips of the teachers and rated them on the same variables as in the first study. The thin-slice ratings of teacher effectiveness, Ambady says, significantly predicted students' performance on the test.
"Students learned more from teachers who were seen in the thin slices as having the qualities of a better teacher," Ambady says.
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