American slaves was much greater than for the population of any nation in Europe, and was nearly twice as rapid as that of Britain. This was sometimes attributed to very high birth rates: "U.S. slaves, then, reached similar rates of natural increase to whites not because of any special privileges but through a process of great suffering and material deprivation". The Southern Colonies (Maryland, Virginia, and Carolina) imported more slaves, initially from long-established European colonies in the West Indies. Like them, the mainland colonies rapidly increased restrictions that defined slavery as a racial caste associated with African ethnicity. In 1663 Virginia adopted the principle in slave law of partus sequitur ventrem, according to which children were born into the status of their mother, rather than taking the status of their father, as was then customary for English subjects under common law. Other colonies followed suit. This meant that children of slave mothers in colonial America were also slaves, regardless of their fathers' ethnicity. In some cases, this could result in a person being legally White under Virginia law of the time, although born into slavery. According to Paul Heinegg, most of the free black families established in the Thirteen Colonies before the American Revolution of the late 18th century descended from unions between White women (whether indentured servants or free) and African men (whether indentured servant, free, or enslaved). These relationships took place mostly among the working class, reflecting the fluid societies of the time. Because such mixed-race children were born to free women, they were free. Through use of court documents, deeds, wills, and other records, Heinegg traced such families as the ancestors of nearly 80 percent of the free Negroes or free blacks recorded in the censuses of the Upper South from 1790 to 1810. In addition, slave owners manumitted slaves for various reasons: to reward long years of service, because heirs did not want to take on slaves, or to free slave concubines and/or their children. Slaves were sometimes allowed to buy their freedom; they might be permitted to save money from fees paid when they were "hired out" to work for other parties. In the mid-to-late 18th century, Methodist and Baptist evangelists during the period of the First Great Awakening ( 1730–1755) encouraged slave owners to free their slaves, in their belief that all men were equal before God. They converted many slaves to Christianity and