Paul Heinegg has shown that most free African American families listed in the censuses of 1790–1810 were, in fact, descended from unions between White women and African men in colonial Virginia, from the years when working classes lived and worked closely together, and before slavery had hardened as a racial caste. In the United States, social and legal conventions developed over time by Whites that classified individuals of mixed ancestry into simplified racial categories, but these were always porous. The decennial censuses conducted since 1790, after slavery was well established in the United States, included classification of persons by race: White, Black, mulatto, and Indian. But, the inclusion of mulatto was an explicit acknowledgement of mixed race. In addition, before the Civil War, Virginia and some other states had a legal definition of "whiteness" that provided for people being classified as White if no more than 1/8 Black. For example, if not born into slavery, Thomas Jefferson's children by his slave Sally Hemings would have been classified as legally White, as they were 7/8 White by ancestry. Three of the four surviving children entered White society as adults, and their descendants have identified as White. In the late 18th and 19th centuries, people of mixed race often migrated to frontiers where societies were more open, and they might be accepted as White if satisfying obligations of citizenship. The more familiar "one-drop rule" was not adopted by Virginia and other states until the 20th century, but it classified persons with any known African ancestry as Black (Davis 2001). Passage of such laws was often urged by White supremacists and people promoting "racial purity" through eugenics, having forgotten the long history of multi-racial unions in the South that comprised the ancestry of many families. In other countries in the Americas, where mixing among groups was overtly more extensive, social categories have tended to be more numerous and fluid. In some cases, people may move into or out of categories on the basis of a combination of socioeconomic status, social class, ancestry, and appearance (Mörner 1967). The term Hispanic as an ethnonym emerged in the 20th century, with the rise of migration of laborers from Spanish-speaking countries of the western hemisphere to the United States. It includes people who may have been considered racially distinct (Black, White, Amerindian or other mixed groups) in their home countries. Today, the word "Latino" is often used