It’s a famous study. Give a mug to a random subset of a group of people. Then ask those who got the mug (the sellers) to tell you the lowest price they’d sell the mug for, and ask those who didn’t get the mug (the buyers) to tell you the highest price they’d pay for the mug. You’ll find that sellers’ minimum selling prices exceed buyers’ maximum buying prices by a factor of 2 or 3 (.pdf).
This famous finding, known as the endowment effect, is presumed to have a famous cause: loss aversion. Just as loss aversion maintains that people dislike losses more than they like gains, the endowment effect seems to show that people put a higher price on losing a good than on gaining it. The endowment effect seems to perfectly follow from loss aversion.
But a 2012 paper by Ray Weaver and Shane Frederick convincingly shows that loss aversion is not the cause of the endowment effect (.pdf). Instead, “the endowment effect is often better understood as the reluctance to trade on unfavorable terms,” in other words “as an aversion to bad deals.” 
This paper changed how I think about the endowment effect, and so I wanted to write about it.
A Reference Price Theory Of The Endowment Effect
Weaver and Frederick’s theory is simple: Selling and buying prices reflect two concerns. First, people don’t want to sell the mug for less, or buy the mug for more, than their own value of it. Second, they don’t want to sell the mug for less, or buy the mug for more, than the market price. This is because people dislike feeling like a sucker. 
To see how this produces the endowment effect, imagine you are willing to pay $1 for the mug and you believe it usually sells for $3. As a buyer, you won’t pay more than $1, because you don’t want to pay more than it’s worth to you. But as a seller, you don’t want to sell for as little as $1, because you’ll feel like a chump selling it for much less than it is worth. . Thus, because there’s a gap between people’s perception of the market price and their valuation of the mug, there’ll be a large gap between selling ($3) and buying ($1) prices:
Weaver and Frederick predict that the endowment effect will arise whenever market prices differ from valuations.
However, when market prices are not different from valuations, you shouldn’t see the endowment effect. For example, if people value a mug at $2 and also think that its market price is $2, then both buyers and sellers will price it at $2:
And this is what Weaver and Frederick find. Repeatedly. There is no endowment effect when valuations are equal to perceived market prices. Wow.