Choice Blindness
Researchers showed picture-pairs of female faces to the participants and asked them to choose which face in each pair they found most attractive. Immediately after their choice, they were asked to verbally describe the reasons for choosing the way they did. Unknown to the participants, on certain trials, a card magic trick was used to secretly exchange one face for the other.
Less than 10% of all manipulations were detected immediately by the participants, and counting all forms of detection no more than 1/5 of all manipulated trials were exposed.
When asked to motivate their choices, the participants delivered their verbal reports with the same confidence, and with the same level of detail and emotionality for the faces that that were not chosen, as for the ones that were actually chosen.