Great Expectations: Studying Expectancy’s Effects

A look at the fascinating research into the power of the expectancy effect.

| Spring 2016


Knowing which group would fail when he first met with them, he had somehow contaminated them with his expectations without knowing how.

Illustration by NatBasil

Robert Rosenthal bumped into the complex and far reaching power of expectations in 1956 when he thought he ruined his doctoral research. For his Ph.D. thesis in clinical psychology at UCLA, he aimed to find out whether people project disappointment with themselves onto others. However, the subjects randomly selected for a disappointing experience on a test showed a significant difference from other subjects even before the test. Rosenthal realized he must have made a mistake. “I made them different before the experiment,” he said. “That was spooky.”    

Knowing which group would fail when he first met with them, he had somehow contaminated them with his expectations without knowing how. This finding elevated his research to another level. “My thesis committee thought I was onto something new: unconscious experimenter bias.”

He could find only one similar finding in published studies. In 1934, A.S. Barr found that different researchers had different results doing the same experiment.

Rosenthal, now distinguished professor of psychology at the University of California at Riverside, later did a study to examine interactions between researchers and subjects. Reviewing their behavior on film, he found obvious differences in tone and body language that could affect experimental outcomes. Although experimenters were then thought to be as neutral as thermometers taking a temperature, he said, they were not. But his colleagues were annoyed. “They thought I was impugning human studies in scientific research,” said Rosenthal. “So I talked to scientists who ran rats. They know that problem goes on with humans. That’s why they run rats. But how do they know it doesn’t happen with rats?”

He experimented with rats learning to run through mazes. He told one group of experimenters that their rats were bred to be “dull,” while the other group was told they had exceptionally bright rats, although the rats were randomly chosen for each group. Nevertheless, rats arbitrarily designated as smarter tended to find their way through mazes faster and more precisely, and other scientists replicated those results. Rosenthal attributed differences in maze proficiency to the ways experimenters handled the rats. “‘Bright rats’ were handled more warmly, like pets,” he said.

Rosenthal did the research while he was director of the clinical psychology program at the University of North Dakota, but was unable to publish it until he became a lecturer on clinical psychology at Harvard. He was soon asked by Lenore Jacobson, a San Francisco elementary school principal who saw the study, if he would be interested in testing expectation effects on her students. Within two weeks he was on a plane with a plan.