These entities can arise when the price of a good or service outstrips the consumer's tendency to purchase similar goods or services; that is, when the income effect is greater than the substitution effect.  
Harvard's Robert Jensen and Nolan Miller attempted to prove their existence in China, and Battalio, Kagal, and Kogut's experiments with quinine water and lab rats proved that they can exist on the individual level.  These entities can arise when the price of a good or service outstrips the consumer's tendency to purchase similar goods or services; that is, when the income effect is greater than the substitution effect.  They were first postulated to exist in Alfred Marshall's Principles of Economics, which used the example of bread with meat as a substitute.  FTP, name this kind of inferior good that people buy more of when the price goes up.
Harvard's Robert Jensen and Nolan Miller attempted to prove their existence in China, and Battalio, Kagal, and Kogut's experiments with quinine water and lab rats proved that they can exist on the individual level.  
Giffen goods