Racial segregation is generally outlawed, but may exist through social norms, even when there is no strong individual preference for it, as suggested by Thomas Schelling's models of segregation and subsequent work.[4] Segregation may be maintained by means ranging from discrimination in hiring and in the rental and sale of housing to certain races to vigilante violence (such as lynchings) Generally, a situation that arises when members of different races mutually prefer to associate and do business with members of their own race would usually be described as separation or de facto separation of the races rather than segregation. In the United States, legal segregation was required in some states and came with "anti-miscegenation laws" (prohibitions against interracial marriage).[5] Segregation, however, often allowed close contact in hierarchical situations, such as allowing a person of one race to work as a servant for a member of another race. Segregation can involve spatial separation of the races, and mandatory use of different institutions, such as schools and hospitals by people of different races.