People shade their survey answers to look good, even anonymously

Allen Edwards showed that how socially desirable a personality trait sounds predicts how likely people are to claim it describes them, almost regardless of whether it's true. Ask directly about smoking, drinking or prejudice and the answers drift toward whatever sounds most admirable. It's why researchers build in indirect questions rather than trusting self-reports at face value.

— Allen L. Edwards, The Social Desirability Variable in Personality Assessment and Research — The Dryden Press, 1957
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