Why did the institution of slavery expand from 1775 to 1830?
The single most important reason for the barbaric growth of slavery in the United States was the invention and subsequent patent of Eli Whitney's Cotton Gin. By the 1830's there was a popular saying among the souths largest plantation owners; 'the cotton gin made slavery pay'. The United States Constitution prohibits any further importation of slaves in 1808, however with the invention of the cotton gin, the patent, and those interested in benefiting from its financial potential, the value of slavery rose rapidly. In addition, although the importation of slavery was prohibited after 1808, the number of slaves in America grew every year. Any child born of a slave was the property of the slave owner, therefore it was not uncommon for slave women to basically be kept pregnant as often as possible. (forced) The cotton gin was responsible for doubling the amount of cotton produced every ten years post 1800. Combine this profit incentive in conjunction with all the other new industrial inventions of the time, such as the new and improved textile machinery of northern businesses along with the surplus of cheap industrial immigrant labor available in the northern cities and the growth of slavery was liken to collateral damage.
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The institution of slavery expanded during this time period largely due to the opening up of new land where slavery could exist and due to the invention of the cotton gin.
During this time period, America expanded inland to some extent. Slaves moved into new areas like Alabama and Lousiana and Mississippi as these areas became part of the United States.
Perhaps more important, though, was the development of the cotton gin. Before this machine was invented, slavery was dying out as the tobacco industry became less viable. After the cotton gin was invented, cotton became a much more profitable crop. As cotton production increased, the need for slaves increased as well.