Economists say that transactions in the inter-regional slave market were driven primarily by differences in the marginal productivity of labor, which were based in the relative advantage between climates for the production of staple goods. The trade was strongly influenced by the invention of the cotton gin, which made short-staple cotton profitable for cultivation across large swathes of the upland Deep South (the Black Belt). Previously the commodity was based on long-staple cotton cultivated in coastal areas and the Sea Islands.