Choice blindness: You don't know what you want

Rather than playing tricks with alternatives presented to participants, we surreptitiously altered the outcomes of their choices, and recorded how they react. For example, in an early study we showed our volunteers pairs of pictures of faces and asked them to choose the most attractive. In some trials, immediately after they made their choice, we asked people to explain the reasons behind their choices.

Unknown to them, we sometimes used a double-card magic trick to covertly exchange one face for the other so they ended up with the face they did not choose. Common sense dictates that all of us would notice such a big change in the outcome of a choice. But the result showed that in 75 per cent of the trials our participants were blind to the mismatch, even offering "reasons" for their "choice".