Are you a moral customer?
We usually complain when we receive LESS money than we are supposed to as a change. But what happens if we receive MORE? Will you pocket the money or remind the seller that he/she has made a mistake? Let's look at this interesting hidden-camera experiment done by the Brain Games crew at National Geographic Channel.
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Morality is quite relative. Many of us tend to "take revenge" if the service is not appropriate. It is closely related to our decision-making behaviour which can be affected strongly by our emotions (similarly to all functions requiring thinking). The higher is the emotional "heat" the less able we are to make a rational, objective decision.