William Stanley Jevons

This statement marked a significant departure from the classical theory of value, which stated that value derived from the labor used to produce a product or from the cost of production more generally. Thus began the neoclassical school, which is still the dominant one in economics today.

Jevons went on to define the "equation of exchange." This equation shows that for a consumer to be maximizing his or her utility, the ratio of the marginal utility of each item consumed to its price must be equal. If it is not, then he or she can, with a given income, reallocate consumption and get more utility.

Take, for example, a consumer whose marginal utility from oranges is 10 "utils," and from cookies 4 utils, when oranges and cookies are both priced at 50 cents each. The consumer's ratio of marginal utility to price for oranges is 10/$.50, or 20, and for cookies is 4/$.50, or 8. Jevons would have said (and modern economists would agree) that this does not satisfy the equation of exchange and, therefore, the consumer will change purchases. Specifically, the consumer could increase utility by spending 50 cents less on cookies and using the money to buy oranges. He would lose 4 utils on the cookies, but gain 10 on the oranges, for a net gain of 6 utils. He will have this incentive to reallocate purchases until the equation of exchange holds (that is, until the marginal utility of oranges falls and the marginal utility of cookies rises to a point where, as a ratio to their prices, they are equal).

Of course, as is true with most new developments in economic theory, one can always find earlier writers who said some of the same things. Jevons' role in the marginal revolution is no exception. Much of what he said was said earlier by Hermann Gossen in Germany, by Jules Dupuit and Antoine Cournot in France, and by Samuel Longfield in Britain. Yet historians of economic thought are sure that Jevons had never read them.

Jevons put much less thought into the production side of economics. It is ironic, therefore, that he became famous in Britain for his book The Coal Question. In it he wrote that Britain's industrial vitality depended on coal and, therefore, would decline as that resource was exhausted. As coal reserves ran out, he wrote, the price of coal would rise. This would make it feasible for producers to extract coal from poorer or deeper seams. He also argued that America would rise to become an industrial superpower. Although his forecast was right for both Britain and America, and he was right about the incentive to mine more costly seams, he was almost surely wrong that the main factor was the cost of coal. Jevons failed to appreciate the fact that as the price of an energy source rises, entrepreneurs have a strong incentive to invent, develop, and produce alternate sources. In particular, he did not anticipate oil or natural gas. Also, he did not take account of the incentive, as the price of coal rose, to use it more efficiently (see Natural Resources).

Born in Liverpool, England, Jevons studied chemistry and botany at University College, London. Because of the bankruptcy of his father's business in 1847, Jevons left school to take up the position of assayer at the Mint in Sydney, Australia. He remained there five years, resuming his studies at University College upon his return to England. He was later appointed to the post of chair in political economy at his alma mater and retired from there in 1880. Two years later, with a number of unfinished books in process, Jevons drowned while swimming. He was forty-six.