"To understand if slavery is an “efficient” method of economic organization, we have to ask the standard economist question: Compared to what?
There is no doubt that a healthy adult slave in a region with adequate natural resources can produce more than a subsistence amount of output, allowing for the owner to keep the slave alive and keep the surplus for himself, living up to the Marxist vision of how labor markets work in general. So if the question is, “Was US output higher with millions of productive slaves working, than it would be if those slaves suddenly disappeared?” then the answer is, “Yes, of course, slavery was ‘productive’ in this sense.”
But that’s not really the question. The question is, if all of the plantation owners in (say) the year 1850 had suddenly freed all of their slaves and turned them into free laborers, what would that have done to the course of US economic development? Is it really true that this change would have made the country as a whole poorer?
The issue isn’t whether cotton was an important export, or whether the expected future flow of labor of the slaves was a valuable financial asset (codified in the market price received in auctions). The issue is whether had the slaves all been freed, would that change have made cotton exports grow more slowly over the 19th century, and would it have made the productivity of the (former) slaves’ labor grow more slowly? Those are the types of questions we need to answer, if we want to know whether slavery was a boon for American capitalism rather than a blight that was not only immoral, but also inefficient."