Expectancy effect
Also known as: Expectation effect, Observer-expectancy effect
What we expect to happen quietly helps make it happen.
What it means
An expectancy effect is any case in which holding an expectation about an outcome helps produce that outcome, because the expectation changes behavior, perception, or physiology before the result is settled. It runs by two routes. The interpersonal route works through the expecter: believing something about another person, we adjust our warmth, patience, attention, and feedback, and they respond in kind — behavioral confirmation, of which the Pygmalion effect is the best-known case. The intrapersonal route works through one's own body and attention, as in the placebo response. It shows up in classrooms, management, medicine, and the lab, where a researcher's hopes leak into how data are gathered and coded. The limits matter: expectancy moves ambiguous, malleable outcomes rather than fixed ones, and estimates vary widely by setting and shrink under stronger designs. It matters because good intentions are no protection — only blinding and structure are.
Examples
A manager told a new hire is 'a rising star' hands them the harder brief, more airtime, and quicker feedback — and six months on, the strong rating looks like proof.
A runner given what they are told is a performance drink, actually water, reports the last mile as easier and pushes harder — expectation doing work the drink never could.
Rosenthal's assistants, told their rats were bred 'maze-bright' or 'maze-dull,' handled the bright ones more gently and recorded faster runs. The rats had been assigned at random.
First described in Robert Rosenthal (1963, 1966).