Within each of 18 classrooms, an average of 20% of the children were reported to classroom teachers as showing unusual potential for intellectual gains. Eight months later these “unusual” children (who had actually been selected at random) showed significantly greater gains in IQ than did the remaining children in the control group. These effects of teachers’ expectancies operated primarily among the younger children.
Rosenthal, R., & Jacobson, L. (1966). Teachers’ expectancies: Determinants of pupils’ IQ gains. Psychological reports, 19(1), 115-118.