The only solution is testing for purity at every point in the supply chain, says Mr Harl.

ECONOMIST: Genetically modified crops

Award-winning Tulane University professor Kenneth W. Harl, who delivers passionate lectures on the Roman Empire, is one of the lucky ones.

FORBES: Magazine Article

Mr Harl thinks the Chinese will also use freer trade to acquire more advanced technology, so that they can harvest bigger and better crops.

ECONOMIST: Farming

Owning a piece of the homeland makes sense if you're investing for the long term, looking for an inflation hedge or betting that world demand for food won't subside, says Neil Harl, an Iowa State agriculture economist.

FORBES: For down-to-earth investors, farmland can yield rewards.

Recruited in 1999, Harl has now published eight courses with the Teaching Company, including The World of Byzantium and Rome and the Barbarians. (Harl recently whipped up a curriculum on Vikings because Hidalgo's research suggested that it would be a popular course.) Harl admits he earns more from his courses sold by Hidalgo (and related speaking engagements) than from Tulane.

FORBES: Magazine Article