[ubernostrum]

The primary cash crop -- cotton -- required so much manual labor, prior to the invention of the cotton gin, that cotton producers, even with access to slave labor, could only really operate on a small scale.

The cotton gin changed that, making large-scale cotton production profitable and drastically increasing the demand for slaves (and thus creating an incentive to find justifications for and defenses of the practice of slavery).

[taejo]

This is an instance of Jevon's paradox (http://en.wikipedia.org/wiki/Jevons_paradox): making a process more efficient in terms of resource requirements (in this case labour), causes the process to become more profitable and grow, therefore using more rather than less of the resource.

[taejo]

This is assuming sufficiently elastic demand (i.e. there is unfulfilled demand at the old price).