Most of these states had a higher proportion of free labor than in the South and economies based on different industries.They abolished slavery by the end of the 18th century, some with gradual systems that kept adults as slaves for two decades.New communities of African-American culture were developed in the Deep South, and the total slave population in the South eventually reached 4 million before liberation.As the West was developed for settlement, the Southern state governments wanted to keep a balance between the number of slave and free states to maintain a political balance of power in Congress.The United States became polarized over the issue of slavery, represented by the slave and free states, in effect divided by the Mason–Dixon line which delineated (free) Pennsylvania from (slave) Maryland and Delaware.Congress during the Jefferson administration prohibited the importation of slaves, effective 1808, although smuggling (illegal importing) was not unusual.The historian Ira Berlin noted that what he called the "charter generation" in the colonies was sometimes made up of mixed-race men (Atlantic Creoles) who were indentured servants, and whose ancestry was African and Iberian.
The largest denominations, the Baptist, Methodist, and Presbyterian churches, split over the slavery issue into regional organizations of the North and South.
In some cases, convicted criminals were transported to the colonies as indentured laborers, rather than being imprisoned.
The indentured laborers were not slaves, but were required to work for four to seven years in Virginia to pay the cost of their passage and maintenance.
Domestic slave trading, however, continued at a rapid pace, driven by labor demands from the development of cotton plantations in the Deep South.
More than one million slaves were sold from the Upper South, which had a surplus of labor, and taken to the Deep South in a forced migration, splitting up many families.