One of the advantages of having commenters who are smarter than you is that they can help you through things that you're too dense to get. On this subject, I'll take the advice of anyone with a decent knowledge of American history and some rudimentary understanding of economics.

I'm re-listening to David Blight's lectures on slavery and the Civil War. In his lecture on slavery and expansionism, Blight notes that southern slaveholders believed that slavery had to expand or eventually die. He says they made the economic calculation that if the country kept growing and slavery was isolated in the South, the price of slaves in border states would go down, and those slaves would be then sold into the Deep South. He argues that this would represent "an economy turning in on itself."

I've been listening to this trying to understand why this would be true. My basic read is that it's a supply and demand problem. If slavery can't expand, and slaves keep reproducing (as they were in the South) you'll end up with a glut of slaves in a small area, thus causing the price of slaves to fall. I have two questions. 1.) Does my read sound right? 2.) Why would the falling price of slaves be bad for the South? Wouldn't that be good, in that it would require even less investment in labor?

UPDATE: I don't know if this is important, but during the period Blight is discussing, the Trans-Atlantic slave trade has been outlawed. So the only real supply of slaves is internal.

UPDATE#2: Forgive the lack of links. Blight's lectures are, as mentioned in comments, free. Here's the quote I'm referencing:

One of the things that southern statesmen feared the most--and I cannot stress enough, because it's going to be right there at the heart of their secession debates in 1860--is they feared what they kept calling now this--it's more than a theory to them--this theory of a shrinking south. If they couldn't expand this system beyond its limits and beyond its borders--get into Arkansas, get out into Oklahoma, Texas, West Texas, further west, Caribbean--that the south would begin to shrink as an economic entity, as a political culture, as a force in the national government. And if you cordoned off slavery, what's going to happen to the price of slaves around its borders? Well, they might begin to go down. What happens if the price of slaves starts going down in Kentucky, Virginia, Maryland, around the edges? Well, people start selling them off. Where are they going to sell them? Alabama, Louisiana, Mississippi. And you would have the beginnings now of an economy turning in on itself. Southerners, many southerners, came to believe slavery had to expand or it could die.

Below is an embed of the lecture. It's incredible.

Watch it on Academic Earth

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