From the point of view of a modern economist, the most striking feature of the works of high is their adherence to a discursive, nonmathematical styl… - Paul Krugman

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From the point of view of a modern economist, the most striking feature of the works of high is their adherence to a discursive, nonmathematical style. Economics has, of course, become vastly more mathematical over time. Nonetheless, development economics was archaic in style even for its own time. Of the four most famous high development works, Rosenstein-Rodan's was approximately contemporary with Samuelson's formulation of the Heckscher–Ohlin model, while Lewis, Myrdal, and Hirschman were all roughly contemporary with Solow's initial statement of growth theory.
This lack of formality was not because development economists were peculiarly mathematically incapable. Hirschman made a significant contribution to the formal theory of in the 1940s, while Fleming helped create the still influential of s. Moreover, the development field itself was at the same time generating mathematical planning models—first Harrod–Domar type growth models, then linear programming approaches that were actually quite technically advanced for their time.
So why didn't high development theory get expressed in formal models? Almost certainly for one basic reason: the difficulty of reconciling economies of scale with a competitive market structure.