The unconscious tendency for researchers to influence their study results in ways that support their hypotheses or expectations. This can occur through subtle changes in how they interact with participants, collect data, or interpret findings.
The concept gained prominence in the 1960s through Robert Rosenthal's work, building on earlier recognition of 'observer bias' in scientific research. The term reflects growing awareness that experimenters, despite good intentions, are not neutral data collectors but active participants who can shape their findings.
Rosenthal's famous studies with 'maze-bright' and 'maze-dull' rats showed that even rodents performed differently based on experimenter expectations - the rats were actually identical! Researchers unconsciously handled the 'smart' rats more gently and gave them more encouragement, proving that bias can influence results even in animal studies.
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