Ars Technica have an interesting look of the effect of cognitive dissonance . For those not familiar cognitive dissonance is the state of mind where you hold two conflicting views on a subject. This can be anything from buyers remorse after an expensive purchase, you wanted it but are then guilty. The study had people in an MRI scan while rating items then having them compare items with the same ratings. All goes to show the brain is a strange and complex beast.