In full public view, on the floor of the House of Representatives during the debate on what became the Civil Rights Act of 1964, sex was added to the list of characteristics to be protected from employment discrimination. But why this addition was made remains an intriguing mystery.

Representative Howard W. Smith of Virginia, chairman of the House Rules Committee and a diehard segregationist who had done his best to kill the bill, introduced a floor amendment that added sex to a list that prominently included race, religion, and national origin. What was he trying to accomplish? Recent scholarship displays a surprising lack of consensus about this question. Clay Risen’s The Bill of the Century: The Epic Battle for the Civil Rights Act (2014) claims that Smith added sex to the mix as a poison pill, “a move that he hoped would turn off congressional male chauvinists.” Indeed, the amendment’s introduction was greeted on the House floor with laughter and macho banter.

A more complex account in Todd S. Purdum’s An Idea Whose Time Has Come: Two Presidents, Two Parties, and the Battle for the Civil Rights Act of 1964 (2014) presents Smith as a bit of a feminist. Whatever his “deepest motives,” Purdum notes, Smith was actually a longtime supporter of women’s rights. His door was open to lobbyists from the National Women’s Party, which was a vocal supporter of bringing sex discrimination under the bill’s protective umbrella. Yet according to still another account in Fred Strebeigh’s Equal: Women Reshape American Law (2009), Smith later told Representative Martha Griffiths of Michigan that he introduced the amendment as “a joke.”

In Because of Sex: One Law, Ten Cases, and Fifty Years That Changed American Women’s Lives at Work, Gillian Thomas focuses on the afterlife of the Smith amendment as part of the Civil Rights Act’s Title VII, which deals with employment. (Other titles concern voting, education, and access to public facilities and public “accommodations,” meaning hotels, restaurants, and other private businesses serving the public. Title IX, a law we hear much about these days, is not part of the Civil Rights Act but of a different law, the Education Amendments of 1972.) Thomas, a lawyer with the American Civil Liberties Union’s Women’s Rights Project who has litigated sex discrimination cases for many years, is interested not in how Title VII’s “because of sex” clause came to be, but in what has become…