Late one afternoon in the ­Ecuadoran Amazon, a short but imposing Achuar tribeswoman walked up to me with a knife in her hand. The Achuar are the tribe next door to the Shuar, who are known for their historical tradition of shrinking the heads of slain enemies. (Both tribes were formerly, and politically incorrectly, known as the Jívaro, which comes from the Spanish jíbaro, meaning “savage.”) The Achuar had, at the time I visited in 1998, the world’s second-highest murder rate. I was there with an anthropologist named John Patton, who studies intratribal murder and revenge, and the Conambo River Valley was a fruitful place for him to be. Achuar men do not so much as go out for a piss without bringing a rifle.

The woman spoke loudly in words I couldn’t understand. With her free hand, she grabbed my hair. “She wants to make paintbrushes,” Patton said. My hair is finer than Achuar hair, and the woman saw its potential for achieving precise lines and decorative ­embellishments on the clay bowls she crafted. I went back to the States minus a crudely lopped hank of hair and with a new story that grew with each telling. The knife, which might have been a pair of scissors—I honestly don’t recall—became a machete. The machete ­acquired bloodstains. The potter took on a stony glower that I claimed to have interpreted as: This scrawny woman in the bulbous shoes ­annoys me, and I will take her head.

It was a preposterous story. The Achuar were not head shrinkers—as adversaries of the Shuar, they were the shrinkees—and I knew this. I was the latest in a long line of white folk who’ve visited Jívaro country and come home with embroidered tales of scary encounters.

America's fascination with ­“savages” and shrunken heads began in the early 1900s, with the publication of the first English-­language Jívaro ethnographies and the arrival of the first tsantsas, as ceremonial heads are known, in U.S. museums. The fascination flourished ...