She knew, but she didn’t participate — not fully. She participated, but she didn’t know — not everything. She was a bystander. She was an anomaly.

The full role of white women in slavery has long been one of the “slave trade’s best-kept secrets.” “They Were Her Property,” a taut and cogent corrective, by Stephanie E. Jones-Rogers, who teaches at the University of California, Berkeley, examines how historians have misunderstood and misrepresented white women as reluctant actors. The scholarship of the 1970s and ’80s, in particular, did much to minimize their involvement, depicting them as masters in name only and even, grotesquely, as natural allies to enslaved people — both suffered beneath the boot of Southern patriarchy, the argument goes.

Jones-Rogers puts the matter plainly. White slave-owning women were ubiquitous. Not only did they profit from, and passionately defend, slavery, but the institution “was their freedom.” White women were more likely to inherit enslaved people than land. Their wealth brought them suitors and gave them bargaining power in their marriages. If their husbands proved unsatisfactory slave owners in their eyes, the women might petition for the right to manage their “property” themselves, which they did, with imaginative sadism.

How have so many historians gotten it so wrong?