"Thank you for coming to this urgent, last-minute meeting today," I say to you and seven of our mutual colleagues. Pointing to a plate of 10 cookies on the table between us, I explain, "We're in luck, though. As a reward for your willingness to squeeze this meeting in, I have managed to sneak some cookies out of the executive meeting next door!"
We all smile, and that look on your face tells me that you already feel better about this meeting than you did just minutes ago. As I drone on about the urgent topic of the meeting, your mind does the math. Counting you and me, there are nine of us. It doesn't take much to figure out that there is at least one cookie for everyone in the room. Your brain signals a salivary response and depending on your current blood sugar levels, possibly a preemptive insulin release from the pancreas.
In other words, you begin to act like that cookie is yours. If I were to survey you at that moment about how appetizing the cookies seem and how much you expect to enjoy yours, you and the others might collectively estimate the cookies at 6.5 on a scale from 1 to 10. Good cookies, to be sure.
That's when a knock is heard at the door and someone enters — a confederate of mine, though you don't know this — who explains that they made a mistake in letting me take the cookies. In fact, the executives ran out of cookies, and they need eight of them back. I apologize, take two cookies from the plate and put them on a napkin for us to keep. My confederate leaves the room with the eight other cookies.
If I survey you now and ask you how appetizing the two cookies left on the table appear, what do you think happens to your estimate? If you guessed that your desire for the cookies goes up, you are in tune with human nature. Indeed, the average score for the cookies will be higher, coming in at more like 7.5, even though the cookies did not change.