Kin Groups and Descent. The Amish tend to maintain social relations mainly but not exclusively with members of their group. In-group marriages and kinship solidarity reinforce the family-based social structure. Amish marriages occur in what is essentially a large kin group. The extent of Intermarriage that has resulted in the intermingling of Genealogies for more than two centuries is evident in various Amish localities by the relatively few surnames. In naming their Children, Amish parents may recognize both maternal and paternal sides of the family. Children have their fathers' surnames and middle names that are often their mothers' maiden names.
Several hereditary diseases have been studied among Amish populations. Although they are not a single, genetically closed population, the Amish have separate inbreeding communities within the larger group. The inbred character is indicated by the history of their migration patterns, by the unique family names in each community, and by the distribution of blood types. Of at least twelve "new" recessive diseases ascertained, several are especially pronounced: dwarfism, a rare blood cell disease, hemophilia, muscular dystrophy, and diseases associated with metabolism. The low rate of some hereditary diseases that are common in the general population has also been noted.