 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.Centuries of European colonialism in the Americas, Africa and Asia were often justified by white supremacist attitudes.During the early 20th century, the phrase "The White Man's Burden" was widely used to justify an imperialist policy as a noble enterprise.