A new study by a University of Iowa economist puts new light on how slavery pulled apart black families before the Civil War, which some researchers say damaged African-American social cohesion so severely it continues to reverberate to this day.

The study, co-authored by Nicolas Ziebarth, professor of economics in the Tippie College of Business, refines an estimate of the number of slaves who were shipped by commercial traders from the South to the West. He says this is significant because slaves shipped by traders were likely young men who were purchased away from their families in the South and moved to newly developing areas in the West that needed labor. But those who were not shipped by a trader likely moved with their owner, and were more likely to be moved with an intact family unit.

As slave immigration become more commercial, the more damaging it was to the cohesiveness of black families, creating an instability that made it even more difficult for newly-freed slaves to begin new lives after the Civil War.

Ziebarth and his co-author, Richard Steckel of Ohio State University, say that most previous estimates put the number of commercially transported slaves in the antebellum South at between 40 and 60, with some outliers as high as 70 percent. But using new historical data, Ziebarth estimates that 55 percent of slaves in the Deep South were taken from their families and moved west in the commercial trade.

“Under any reasonable threshold, a large share, if not a majority, of slaves were delivered by traders,” Ziebarth and Steckel write.

That number only increased in the 1840s and 1850s, as the need for labor grew in the rapidly developing West. In the 1830s, 46 percent of Southern slaves sent west were sent by traders, a number that jumped to 60 percent in the 1840s and 57 percent in the 1850s.

Ziebarth and Steckel were able to refine the number by reviewing 24,400 shipping manifests kept by the National Archives that had been ignored by prior researchers. The manifests record the movements of about 135,000 slaves who were shipped from East Coast ports to western ports on the Gulf of Mexico and Mississippi River—mainly New Orleans—during the years 1810 to 1861.

The researchers used the records in those manifests—along with newspaper advertisements, census reports, and other records—to determine whether immigrating slaves were shipped as cargo, or as passengers. While it still doesn’t include the number of slaves who were sent overland, they say the new records help to show the extent of the damage done to black families as a result of the western slave trade.

Ziebarth’s and Steckel’s paper, “A Troublesome Statistic: Traders and Coastal Shipments in the Westward Movement of Slaves,” is published in the current issue of The Journal of Economic History.