Slavery in North American colonies often contrasted with other colonial areas in the Atlantic World. Comparing trans-Atlantic slave trade numbers and enslaved populations in North America to different Atlantic World regions provides insight into why slavery proved to be unique in this context. Even with these variations, however, enslaved Africans in North America still struggled with similar terms of racial oppression, coerced labor, and violence found throughout New World chattel slavery systems.

Of the over twelve million Africans forced into the trans-Atlantic slave trade from the sixteenth to the nineteenth centuries, only four percent – roughly 470,000 men, women, and children – were sent to North America. The overwhelming majority of enslaved Africans in the trans-Atlantic slave  trade went to sugar plantations in the Caribbean and Brazil. This significant difference in trade numbers stems from various factors, particularly contrasting mortality and reproduction rates for enslaved populations in different regions.

In the Caribbean and Brazil, enslaved Africans often experienced high mortality rates and unbalanced gender ratios, which limited natural population increase through reproduction. This kept market demand active for new shipments of enslaved Africans in these areas. In contrast, while the conditions of climate, disease, and labor in North America were often extreme, they generally proved to be less lethal for enslaved Africans in this region in comparison to their enslaved counterparts on Caribbean and South American plantations. This allowed for greater rates of natural increase overall for North American slave populations, and less reliance on the trans-Atlantic trade.

Family of African Americans on Smith's Plantation, Beaufort, South Carolina, ca. 1862, courtesy of the Library of Congress and learnnc.org.

Population growth factors for enslaved populations also varied within specific North American regions and at different points in time. For example, when the plantation economy was rapidly growing in the Carolina Lowcountry during the late seventeenth and eighteenth centuries, shipments of enslaved Africans to the port of Charleston increased significantly to meet planter demands. Mortality rates for these new arrivals could be as high as Caribbean and South American sugar plantations, particularly as enslaved Africans struggled with the recent physical and psychological trauma of the Middle Passage.

Over time, however, survival rates improved in Carolina, and throughout North American colonies. Growing numbers of enslaved African Americans were born in North America. Even with this general pattern of population growth, South Carolina planters continued to purchase significant numbers of new arrivals through the trans-Atlantic slave trade to provide skills and labor for the Lowcountry's numerous plantations. As described in Barbadians in Carolina, Carolina settlers' trade and migration connections to the English West Indies during the colonial period distinctly shaped how slavery developed in this region. In contrast to other North American colonies, the Carolina Lowcountry initially functioned as an extension of the West Indian slavery and plantation system, where the trans-Atlantic slave trade was generally in great demand.

Frontispiece for Incidents in the Life of a Slave Girl, Written by Herself, by Harriet Ann Jacobs, 1861, courtesy of Documenting the American South. Harriet Ann Jacobs (February 11, 1813 – March 7, 1897) was an African American writer who escaped from slavery in North Carolina. Her autobiography was one of the first published accounts of the struggles of female slaves and the sexual abuse they frequently endured under slavery.

Despite exceptional mortality rates and trans-Atlantic slave trade numbers in the Carolina Lowcountry, North American colonies overall experienced higher rates of natural increase for enslaved populations in comparison to other Atlantic World colonies. In addition to a more temperate climate, scholars argue that North American slaveholders strategically sought to improve survival rates through a more balanced gender ratio. With access to greater numbers of enslaved women, slaveholders could increase their enslaved population through childbirth as well as purchase, because the offspring of enslaved women legally inherited their mother's status.

A number of scholars argue that sexual exploitation played a role in increasing childbirth rates among enslaved women. A more balanced gender ratio in North America colonies also meant that enslaved men and women could develop partnerships and family ties. Under chattel slavery, such ties were often constrained by legal restrictions against slave marriages, and all relationships between enslaved people were threatened by separation through sale.

Ultimately, a large number of African Americans descended from the relatively small number of Africans sent to North America. This led to a large domestic slave population in North America, particularly in southeastern colonies with significant plantation economies. Access to enslaved Africans and African Americans in North America, through both the trans-Atlantic and domestic trades, meant that slavery became a major labor system in many of the early English, Dutch, French, and Spanish colonies that later formed the United States. Even after the trans-Atlantic slave trade legally ended in the United States in 1808, slaveholders could acquire enslaved African Americans through inheritance, the domestic slave trade, natural increase, and even the illegal trans-Atlantic trade. By 1860, on the eve of the U.S. Civil War, four million African Americans lived in chattel bondage in the United States. They were valued at roughly three billion dollars, which equaled three times the value of U.S. manufacturing or railroads, seven times the net worth of all banks, and forty-eight times the expenditures of the federal government. As historian Ira Berlin asserts, the economic, social, and political history of North America, particularly the United States, cannot be understood without addressing the central role of slavery in this nation's foundation.