Tap, tap tap. Scott Standfast knocked on the door again. He’d been at it for several minutes, standing in front of a one-story brick house in Niceville, Florida, on a warm, dry day in November 2015. The then-59-year-old former Marine and his wife had driven more than 11 hours to get here, hoping to answer a question that’s haunted him for 40 years. He knocked again. This time, harder. Boom, boom, boom! Still no answer.

About four decades ago, Standfast fought in the last battle of the Vietnam War, and his memory of it is sharp—from the location of enemy positions to the smothering jungle foliage. But it’s not what he remembers that troubles him; it’s what he can’t recall about that traumatic day. He’s tried everything. In 2015, he even joined a group of veterans for a trip back to the battlefield where they met their former enemies. Some shook hands, trying to forgive and move on. The experience helped but not enough. “It's blocked out,” he tells me on the phone, choking up. “I'm sorry.”

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In 1975, researchers at Stanford invited a group of undergraduates to take part in a study about suicide. They were presented with pairs of suicide notes. In each pair, one note had been composed by a random individual, the other by a person who had subsequently taken his own life. The students were then asked to distinguish between the genuine notes and the fake ones.

Some students discovered that they had a genius for the task. Out of twenty-five pairs of notes, they correctly identified the real one twenty-four times. Others discovered that they were hopeless. They identified the real note in only ten instances.

As is often the case with psychological studies, the whole setup was a put-on. Though half the notes were indeed genuine—they’d been obtained from the Los Angeles County coroner’s office—the scores were fictitious. The students who’d been told they were almost always right were, on average, no more discerning than those who had been told they were mostly wrong.

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