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Economics, 10th Edition, Paul Samuelson, 1976, page 783:
"The newer farmlands of the Mississippi afforded a high marginal-product to the slaveowners. . . . . Hence, the Invisible Hand of competition caused the tidewater regions to specialize in the production and reproduction of slaves, for sale to the fertile lands westward."
And therefore, it seems to me, that the people who opposed the expansion of slavery were kind of in the right and may have had a pretty good understanding of what was going on.