If the planters of the Old South engaged in deliberate breeding for export, it was a minor 'crop.' The total value of all the slaves sold from east to west in 1860 was about $3,000,000. (pg. 48)This is an important point in the context of antebellum Southern historiography, because the extent of slave trading (and breeding for export) signifies the extent to which plantation owners had exhausted the soil of the Old South. The story goes that the soil of the Old South was depleted of its nutrients through harsh, aggressive harvesting, leaving the plantation owners to resort to slave breeding via a rising interregional market for slaves. (The significance of their claim comes up again in Chapter 3 on the "Myth of Slave-Breeding".)
The story fits nicely within what Fogel and Engerman call "traditional" accounts of the slave South, in which irrational and inhumane masters stupidly destroy the basis of their livelihood and then resort to raising slaves as they would any other crop as a means of maintaining a high profit margin. This leads to larger issues of broken families and the disintegration of black family structure. The fact that Fogel and Engerman do not find evidence of significant trading suggests that family structure was more cohesive, and plantation owners more scientific in their agricultural methods, than the traditional story suggests.
In the latest (September 2013) Journal of Economic History, Richard Steckel and Nicolas Ziebarth push back on the issue of interregional slave trading. They argue that the interregional slave trade was more significant than what Fogel and Engerman suggest. The main contribution of the research comes from its new dataset. They use the fact that, after the ban on the Atlantic slave trade in 1808, U.S. law required ships with slaves to report each slave's origin and destination - these were recorded in the "slave manifests", which they collect to assemble the largest dataset yet on the slave trade. Using this dataset, they find that over half and possibly up to 60% of slaves travelling West (specifically, to the lower South) were traded.
In contrast, Fogel and Engerman draw only from sales data in Ann Arundel County, Maryland, as well as some related findings for New Orleans - a major trade destination, but not the only one - and conclude that 16% of slaves travelling from Old to New South were traded.
Does Steckel and Ziebarth's research give more weight to the traditional view of the plantation owner of the Old South - irrational agriculturalist and slave-breeding? They don't mention how their work might place in the more general historiography of the South. Unlike the older, more ambitious cliometricians, Steckel and Ziebarth are much more humble about what their conclusions might imply. But their results do suggest that a reexamination of the behavioral foundations of the southern plantation owner might bear more fruit.
A copy of the paper can be found here: https://www.dropbox.com/s/46mwc7lknqvokcn/Steckel%20and%20Ziebarth%20galley.pdf