A good number of commentators thought that the slave economy was profitable only because of the expansion of cotton to new territories, that required an infusion of new slaves. Since slaves could not longer be imported, the plantations acted as [indirect and sometimes direct] breeders. The profits from selling "surplus" slaves allowed the old plantations to be profitable.
Also, Karl Marx used Fredrick Law Olmstead's articles on slavery [the 2 corresponded with each other] to show how that slavery would necessarily be unprofitable in the long run.
Here is an echo of Olmstead in modern economics:
Kauffmann, Kyle. D. 1993. "Why Was the Mule Used in Southern Agriculture? Empirical Evidence of Principal-Agent Solutions." Explorations in Economic History, 30: 3 (July): pp. 336-51. He shows that, even in the twentieth century, mules were more frequently used where sharecropping was most common, since croppers tended to use the landlord's work stock.
340: Olmstead, Frederick Law. 1904. A Journey in the Seaboard Slave States in the Years 1853-1854 (NY: Putnam): p. 51 "When I ask why mules are so universally substituted for horses on the farm, the first reason given ... is, that horses cannot bear the treatment that they get from negroes." He gives other sources. 336: Mules are consistently more expensive than horses. 337: Mules were used more extensively in the South since sharecroppers did not own the farm animals. They had little incentive to conserve the stock. 339: Mules resist injury more than horses. They resist overwork and require less grooming than horses.
348: In the North, mules were used more by the lumberman, again suggesting a principal-agent situation.
349-50: He uses data from Georgia to show that mules were used more frequently in counties where sharecropping was more common.
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Michael Perelman Economics Department California State University michael at ecst.csuchico.edu Chico, CA 95929 530-898-5321 fax 530-898-5901