Laws of manumission varied widely from society to society and within societies across time. They are often viewed as the litmus test of a particular society’s views of the slave, that is, of the capacities the slave was likely to exhibit as a free human being. Many Islamic societies, broadly interpreting the Hebrew prescription, generally prescribed that slave owners had to free their slaves after the passage of a number of years, essentially the length of time they considered it took for an “outsider” to become an “insider.” Most other societies allowed masters to free their slaves whenever they wished, although there were exceptions. Some legal systems prescribed manumission when the slave adopted the religion of his owner. It is hardly surprising that manumission was more frequent in systems of household slavery, for intimate relations between master and slave soon converted the outsider into an insider. With notable exceptions, such as Athens, Rome, Muscovy, and some circum-Caribbean societies, many societies required manumission after three generations.

Birth was occasionally a route to manumission. In thriving slave systems such as those of the New World, in harsh systems such as those among the Northwest Coast Indians and the medieval Germanic peoples, or even in milder systems such as those of the Chinese and the Muscovites, a slave’s offspring simply added to the slave population. But that was not universally the case; African slave societies, such as the Dahomeans of West Africa, the Ashanti of Ghana, or the Azande living between the Congo and the Nile, prescribed that the offspring of slaves should be free, as part of the process of incorporation into a new lineage. Although Islamic law did not require manumission upon birth, the Qurʾān recommended it, and slave owners were often inclined to follow the religious tenet. The Aztecs freed all children born in slavery except the offspring of traitors. In Thailand emancipation was considered a pious act, and at their death many owners freed their slaves.

The rate of manumission did not necessarily correspond to the legal ease of manumission. It should be noted, however, that in Rome manumission was relatively easy and was widely practiced, even though there was a 5 percent tax on manumission in the Republic, and the Lex Fufia Caninia of 2 bce forbade manumission by testament of more than a fifth to a half of one’s slaves, depending on the number owned. In much of sub-Saharan Africa, manumission was common in most periods, and the freed person typically became a kind of relative in a process of assimilation. In Neo-Babylonia, in Late and Middle Assyria, and in Muscovy manumission was easy but rare; in the American South manumission was comparatively difficult and almost never happened after the prohibition on importing new slaves. The factors of institutional dynamism, expansionism, and profitability, as well as race (see below), may have been the most crucial variants for the South, where manumission was even forbidden in South Carolina in 1820, Mississippi in 1822, Arkansas in 1858, and Maryland and Alabama in 1860; other factors were at work in the ancient Middle East and Muscovy.

There was considerable variation among societies as to whether a slave was allowed to accumulate property that he might keep after manumission. One form of such accumulation was the Roman peculium, which legally belonged to the master. One of its heirs was called coartación, the self-purchase system, widely used 1,500 years later in Latin America.

After manumission, most societies prescribed a period of legal transition to freedom. In the Roman Empire, China, and elsewhere, this period took three generations and might mean that the grandchild of a slave owner (the “patron”) was legally responsible for the grandchild of a slave (his “client”). Thereafter the descendants of the freedman became full members of society, although perhaps still despised. The reason for the legally mandated period of transition to freedom was clear: the slave initially was not a member of the society but an outsider (see below), and it took time to become integrated into the new society. Equally important, the slave was dependent on his owner, and it took time for the freedman and his heirs to become fully self-reliant members of society. If the slave owner and his heirs were not responsible for the freedmen, the fear was, as expressed in the Louisiana Slave Code of 1824, that the latter might otherwise become public wards.