Observer-expectancy effect
When a researcher or observer expects a certain result, they can unknowingly give off cues, like tone, pauses, or facial expressions, that push the people or animals being studied toward that result. The study then seems to show what the observer hoped for.
In everyday life
A horse known as Clever Hans seemed to solve math problems by tapping his hoof, but he was actually reacting to tiny, unintended cues from the people asking the questions.
Interviewing customers about a new snack, you nod and smile whenever they mention the flavor you invented, and they end up praising it more and more.
Why your mind does this
Holding steady beliefs lets us act without rethinking everything each time. Favoring information that fits what we already know is fast, protects our sense of being right, and keeps our picture of the world stable and coherent.
Its family, in 30 seconds: Confirms beliefs
How to spot it
Ask yourself whether the people you are testing or interviewing could tell what answer you are hoping for.
How to counter it
Use a fixed script and, where possible, have someone who doesn't know the expected result run the test.
Same family: Confirms beliefs
SourcesRosenthal & Fode (1963)·Wikipedia