The tendency to demand much more to give up an object than you would be willing to pay to acquire it.
In one experiment, people demanded a higher price for a coffee mug that had been given to them, but put a lower price on one they did not yet own. People value a good or service more once their property right to it has been established. In other words, people place a higher value on objects they own than objects that they don't. The endowment effect was described as inconsistent with standard economic theory which states that a person's willingness to pay for a good should be equal to their willingness to accept compensation to be deprived of the good.
This is from Jonah Lehrer 's blog, The Frontal Cortex: