This best illustrates unit bias. Unit bias is the idea that people want to have a complete unit of something, no matter what the size of it is. For example, people will consider a 12 oz soda one unit, and a 24 oz soda one unit, even though these are two different sizes of the same object. The guests at Judy's party saw the half cupcakes and weren't satisfied with the idea of not having a whole unit.
