Expectation Effect
Experimenter's Bias · Expectation Bias · Observer Effect · Subject-Expectancy Effect
The bias for pre-existing expectations in observing a phenomenon to invariably influence the outcome of that observation.
Origin
An early illustration came in 1907 with Clever Hans, a horse whose apparent mathematical abilities were revealed to stem from subtle, unconscious cues given by expectant observers. The experimental literature was formally established by Robert Rosenthal and Kermit Fode in 1963 at Stanford, where psychology students handling rats labeled "maze-bright" or "maze-dull" obtained measurably different results — despite the rats being genetically identical. Rosenthal extended the finding to human subjects in the 1968 study Pygmalion in the Classroom, showing that teacher expectations alone could shift student academic performance.