When bread got more expensive, some of the poorest Victorians bought even more of it

Ordinary demand theory says people buy less of something as its price rises, but economist Alfred Marshall credited Robert Giffen with a reversal among very poor households: when bread prices rise, people too poor to afford meat may cut meat entirely and buy still more bread just to stay fed, even at the higher price. Confirmed real-world examples are rare, but the theoretical case, now called a Giffen good, remains a staple exception economists use to probe the limits of basic demand theory.

— Alfred Marshall, Principles of Economics — 1895 edition, crediting Robert Giffen