The Endowment Effect

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:

JUNE 22, 2009
I went jean shopping this weekend. Actually, I went to the mall to return a t-shirt but ended buying a pair of expensive denim pants. What happened? I made the mistake of entering the fitting room. And then the endowment effect hijacked my brain. Let me explain.
The endowment effect is a well studied by-product of loss aversion, which is the fact that losing something hurts a disproportionate amount. (In other words, a loss hurts more than a gain feels good.) First diagnosed by Richard Thaler and Daniel Kahneman, the endowment effect stipulates that once people own something - they have an established or imagined "property right" to the object - that something dramatically increases in subjective value.Wikipedia has an excellent summary of an experiment documenting the endowment effect by Dan Ariely and Ziv Carmon:
Duke University has a very small basketball stadium and the number of available tickets is much smaller than the number of people who want them, so the university has developed a complicated selection process for these tickets that is now a tradition. Roughly one week before a game, fans begin pitching tents in the grass in front of the stadium. At random intervals a university official sounds an air-horn which requires that the fans check in with the basketball authority. Anyone who doesn't check in within five minutes is cut from the waiting list. At certain more important games, even those who remain on the list until the bitter end aren't guaranteed a ticket, only an entry in a raffle in which they may or may not receive a ticket. After a final four game, Carmon and Ariely called all the students on the list who had been in the raffle. Posing as ticket scalpers, they probed those who had not won a ticket for the highest amount they would pay to buy one and received an average answer of $170. When they probed the students who had won a ticket for the lowest amount they would sell, they received an average of about $2,400. This showed that students who had won the tickets placed a value on the same tickets roughly fourteen times as high as those who had not won the tickets.
What does this have to do with fitting rooms and jeans? Once I tried on the pants, I became an implicit owner of them. I stared at myself in the mirror and admired the fit, the wash, etc. I thought about how good they would look with my shoes. I contemplated wearing them to various upcoming events and all the strangers who would look at my pants and think "Those are nice pants!" In other words, I spent a few minutes imagining my life with these new jeans and, once that happened, the pants suddenly became much more valuable. I mentally endowed myself with the object and didn't want to lose something that I didn't even own. As a result, the ridiculous price tag ($170 for Levis!) no longer seemed so ridiculous. The lesson? Don't try something on that you don't want to buy.
Update: Via a reader (thanks Alon!) comes this study, which demonstrates that merely touching an item can trigger the endowment effect.