THE terrorist attack in Charleston, S.C., an atrocity like so many other shameful episodes in American history, has overshadowed the drama of Rachel A. Dolezal’s yearslong passing for black. And for good reason: Hateful mass murder is, of course, more consequential than one woman’s fiction. But the two are connected in a way that is relevant to many Americans.

An essential problem here is the inadequacy of white identity. Everyone loves to talk about blackness, a fascinating thing. But bring up whiteness and fewer people want to talk about it. Whiteness is on a toggle switch between “bland nothingness” and “racist hatred.”...

We don’t know the history of whiteness, and therefore are ignorant of the many ways it has changed over the years. If you investigate that history, you’ll see that white identity has been no more stable than black identity. While we recognize the evolution of “negro” to “colored” to “Negro” to “Afro-American” to “African-American,” we draw a blank when it comes to whiteness. To the contrary, whiteness has a history of multiplicity.

Constructions of whiteness have changed over time, shifting to accommodate the demands of social change. Before the mid-19th century, the existence of more than one white race was commonly accepted, in popular culture and scholarship. Indeed, there were several. Many people in the United States were seen as white — and could vote (if they were adult white men) — but were nonetheless classified as inferior (or superior) white races. Irish-Americans present one example.

In the mid- to late-19th century, the existence of several white races was widely assumed: notably, the superior Saxons and the inferior Celts. Each race — and they were called races — had its characteristic racial temperament. “Temperament” has been and still is a crucial facet of racial classification since its 18th-century Linnaean origins. Color has always been only one part of it (as the case of Ms. Dolezal shows)....