Some people have pointed to this strange case as an illustration that race is malleable. I submit that Ms. Dolezal is a reminder that it is not. Racial identity cannot be fluid as long as the definition of whiteness is fixed. And historically, the path to whiteness has been extremely narrow.

The “one-drop rule,” which, for much of American history, legally defined as black anyone with a black ancestor, was used to keep black people from adopting whiteness. Ironically, it has made it easier for Ms. Dolezal to claim blackness without others questioning the assertion. If they are not themselves of a similar hue to Ms. Dolezal, many black people watching her story unfold can recognize in her features a cousin, parent or grandparent. African-Americans vary in appearance from light-skinned to coal black, straight- to curly-haired, keen- to broad-featured, and every possible combination in between.

This diversity is partly a result of this one-drop rule. The original intent of it was to protect racial privilege. Sometimes, if their appearance borrowed enough from white ancestors, black Americans could “pass” in white society. But that social sleight of hand came with many dangers, such as the chance that black lineage would be outed in the skin or hair of one’s progeny. Segregation simply would not work if society was fuzzy on who got the nice drinking fountain, the front seat on the bus and the right to vote.

In 1924, Virginia passed the Racial Integrity Act, a series of anti-miscegenation laws that would be overturned by the landmark case Loving v. Virginia in 1967. The act defined a white person as someone with blood that was “entirely white, having no known, demonstrable or ascertainable admixture of the blood of another race.” Yet when faced with the fact that many rich Virginia planters claimed to be descendants of Pocahontas, the state allowed that a white person could be up to one-sixteenth Native American. A person with one-sixteenth black blood, however, was still black.

The definition of blackness under the one-drop rule included those with the barest African ancestry. For example, in 1977, Susie Guillory Phipps, a 43-year-old Louisiana native who had been raised as a white woman, was “sickened” to receive a birth certificate that listed her race as “colored.” Under a 1970 Louisiana state law, anyone with one thirty-second “Negro blood” was black. Challenged by Ms. Phipps, the state spent thousands of dollars tracing her heritage back to an enslaved black great-great-great-great-grandmother. Louisiana overturned its version of the one-drop rule in 1983.