The classic example of a Giffen good is a potato during the Irish potato famine. The Harvard economists say that that the example is probably apocryphal. There is certainly no data to support it. But when they subsidized the prices of rice and wheat flour for five months in China, they found that the subsidy led to reduced consumption of rice or wheat, and its removal to more consumption. Mr. Rodrik says that while this is an exciting find, its scope is limited: Environmentalists, he says, shouldn’t aim to depress consumption of fossil fuels by reducing the price of oil. — Robin Moroney