US law defined whiteness for much of the country's history. The Naturalization Act of 1790 limited citizenship to "free white persons." Courts later had to decide who counted. In Ozawa v. United States in 1922, the Supreme Court ruled that a Japanese immigrant was not white. In United States v. Bhagat Singh Thind in 1923, it ruled that an Indian immigrant was not white either, despite arguments based on anthropology.
The boundaries moved. Irish, Italian and Jewish immigrants faced discrimination in the 19th and early 20th centuries and were later absorbed into whiteness, partly through access to suburbs, unions and federally backed mortgages. Legal scholar Cheryl Harris argued in a 1993 Harvard Law Review article, "Whiteness as Property," that whiteness worked like a valuable asset protected by law.
Studying whiteness shifts the focus from people of color as the "problem" to the system that ranks groups. It helps explain why white people may not think of themselves as having a race at all, which is one way the norm stays invisible.