Interpersonal expectancy effects--the unintentional expectations that experimenters, teachers, and authority figures bring to experiments, classrooms, and other situations--can wield significant influence on individuals. Some of the issues surrounding expectancy effects are detailed in this paper. The effect itself has been recreated in experiments with subjects ranging from humans to rats. In one classroom study, children were administered a nonverbal test of intelligence, which was disguised as a test that would predict intellectual "blooming." Their teachers were then told that certain children would show surprising gains in intellectual competence during the next eight months of school. At the end of the school year, the children from whom the teacher had been led to expect the greater intellectual gain showed significantly greater gains than did other children, although the only difference was in the mind of the teacher. Numerous other studies were conducted to measure this Pygmalion effect, and some of these--the 10 Arrow Model and the Four Factor Theory--are examined here. Research has expanded to examine Pygmalion effects in management, in courtrooms, and in nursing homes. In all cases, it appears that much of the mediation in these studies of expectation is occurring by means of unintended nonverbal behavior. (Contains 17 endnotes.) (RJM)