Final answer:
The domestic slave trade was the main source of the increase in the slave population in the U.S. as enslaved African Americans were sold and transported from the Upper South to the Lower South to meet the demand for labor in the cotton and sugar economies.
Step-by-step explanation:
The source of most of the increase in the slave population in the U.S. was the domestic slave trade, specifically the sale and transportation of enslaved African Americans from the Upper South to the Lower South. Between 1790 and 1860, over 1 million African American slaves, approximately two-thirds of whom were sold, were transported from states like Virginia, Maryland, and others to meet the increasing demand for labor in the cotton and sugar economies of the Lower South.