1990, Cornell University, United States. A professor distributes pretty mugs to some of his students. A general tour of coffee? Not really… The professor is Daniel Kahneman, a psychologist who works on behavioral economics: he studies how individuals make their decisions to consume, buy, sell, invest. With his mugs, he does a little experiment. Half of the students receives one, the other half does not receive anything. Then, the professor asks each group at what price they think the mug could be sold: those who have the mug said they are ready to sell for 7 dollars. And those who do not have it are ready to buy it for 3 dollars. According to the economic theory, mugs should trade around 5 dollars: it would be a middle ground, the price which mug sellers (supply) and buyers (demand) would agree, after negotiation. This is called the equilibrium price, which sets the value of the mug in the mug market. But Kahneman tries something else: he proposes to the students without a mug to receive either the mug, or 5 dollars. He offers to the students with a mug to buy them, also for 5 dollars. The choice is the same for everyone: a mug or 5 dollars. The result is that the students who do not have a mug accept the money but the students who already have the mug refuse to give it away. Why? The reason is the so-called endowment effect: we give more value to something that we already have and we are not ready to give it away to its real value on the market. Because of this psychological bias, people have a hard time getting rid of a piece of furniture, their old car, or a house, because they overestimate the value. But this bias that can also be useful for trading: when a seller offers us for example a subscription, a mattress or ... a mug to test for free for a while, it is because they hope that in the end we will be willing to pay to keep it! And you, have you ever kept anything offered by a seller?