Edward L. Glaeser is an economist at Harvard.

Rarely, if ever, has an economics Nobel laureate been as widely known before receiving the prize as Paul Krugman. His New York Times columns have been read by millions; he has argued economic policy eloquently in a large number of popular books. Yet these pursuits had little to do with the decision of the Nobel committee. They gave this prize to honor a truly seminal figure in economic trade and geography. Mr. Krugman’s fame as a public intellectual should not lead anyone to think that they understand his contributions to economic research just because they regularly read his columns.

The Nobel Prize citation highlights two distinct but connected contributions: Mr. Krugman’s development of the “new trade theory” and his work on the “new economic geography.” International trade has a long history in economics, and for the bulk of the field’s history, patterns of trade have been explained by factor endowments and comparative advantage. Why does England export wool and Portugal export wine? The cold winters of Yorkshire produce really fluffy sheep and the banks of the Douro produce splendid grapes. Yet comparative advantage does little to explain much of modern international trade, especially not trade within industries.

Mr. Krugman published two seminal papers in 1979 and 1980 that made sense of the fact that Toyota sells cars in Germany and Mercedes-Benz sells cars in Japan. Mr. Krugman started with a variant of Edward Chamberlain’s model of monopolistic competition. In this model, every firm sells a slightly different good — an Infiniti is not exactly the same thing as a BMW. There are fixed costs of production, which means that producers get more efficient as they sell more. Finally, consumers like variety, so that even if they live in the Land of the Rising Sun, with its abundant well-made cars, they still occasionally want something a little more Teutonic.

These ingredients came together and provided a framework than can match the world’s trade patterns better than the 19th-century framework of David Ricardo, or the mid-20th-century models of Eli Heckscher, Bertil Ohlin and Paul Samuelson. The fact that two out of three of those 20th-century giants are themselves Swedes should remind us of how seriously the Swedes take their trade theory, and what a big deal it is for them to admit Mr. Krugman to the pantheon.

Mr. Krugman’s trade models became the standard in the economics profession both because they fit the world a bit better and because they were masterpieces of mathematical modeling. His models’ combination of realism, elegance and tractability meant that they could provide the underpinnings for thousands of subsequent papers on trade, economic growth, political economy and especially economic geography.

Mr. Krugman’s 1991 Journal of Political Economy paper, “Increasing Returns and Economic Geography,” is the first article that provides a clear, internally consistent (注意啊! 前後一致的重要性.這是任何一個模型都必須具備的基楚.單這點,台灣那些評論與報導就很難找到及格的了!)mathematically rigorous framework for thinking simultaneously about trade and the location of people and firms across space. It is one of only two models that I insist that Harvard’s Ph.D. students in urban economics be able to regurgitate, equation by equation.

The model begins with the same basic elements as the new trade theory: monopolistic competition, scale economics, love of variety. To these elements Mr. Krugman adds free migration of workers across space and industries. Because workers are able to move, real wages equalize across space. People in New York City may be paid more, but they give some of that back in the form of higher housing prices. The paper provides economists with a clear framework that can make sense of where we all live. Firms and workers are pulled toward the same location to reduce transportation costs of shipping goods. For example, the garment industry located in New York City, in part because of the vast trade in textiles that was already moving through the city and because of the large number of customers already living in America’s largest city.

Of course, we don’t all live in the same city. A good model of geography needs both a centripetal and a centrifugal force. In Mr. Krugman’s model, populations are pulled apart by the desire to be close to natural inputs, like land or coal mines. Cyrus McCormick moved his reaper business from Virginia to Chicago to be closer to his rural customers in the Midwest. Later models incorporated traffic congestion and other forces that limit the growth of a single large urban area. Mr. Krugman’s model proved to quite adaptable; it has received thousands of citations.

In his public role, Paul Krugman is often a polarizing figure, loved by millions but also intensely disliked by his political opponents. I still chuckle over an old New Yorker cartoon with one plutocrat saying to another that he gets some satisfaction from the fact that his vote will cancel out the vote of Paul Krugman. Within the less divided world of the academy, Mr. Krugman’s economic research has generated plenty of light, but far less heat. His papers are universally acknowledged to be immense contributions that helped to create two distinct fields. His Nobel Prize is extremely well deserved and not unexpected. I, for one, had bet on him in Harvard’s Nobel Prize winner pool.

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[註] 1) 開放與自由貿易,這不是左派鼓吹的主張啊!!  2)我不是很清楚該文的作者所言的政治上觀點是否指克魯曼是支持共和黨還是什麼意思. 政治上的觀點,如果就社會價值觀來講,民主黨相對於共和黨是右派的.也就是說民主黨通常比較開放,共和黨趨近於保守,例如反同志婚姻之類.不過弔詭的是, 經濟上,這兩黨的主張卻是左右相反的. 共和黨基本上是主張無為政府(或小政府,不是無政府),少稅少干預,開放管制與自由貿易的.  所以不要搞混了.  我也是被我美國同學講了好多次才終於不再搞混的啊!

延伸閱讀: 克鲁格曼独自得奖 (我很少看簡中的文章,因為看不懂,半猜半讀實在太累.不過這篇推薦喔!)