In 1906, Hugo Münsterberg, the chair of the psychology laboratory at Harvard University and the president of the American Psychological Association, wrote in the Times Magazine about a case of false confession. A woman had been found dead in Chicago, garroted with a copper wire and left in a barnyard, and the simpleminded farmer’s son who had discovered her body stood accused. The young man had an alibi, but after questioning by police he admitted to the murder. He did not simply confess, Münsterberg wrote; “he was quite willing to repeat his confession again and again. Each time it became richer in detail.” The young man’s account, he continued, was “absurd and contradictory,” a clear instance of “the involuntary elaboration of a suggestion” from his interrogators. Münsterberg cited the Salem witch trials, in which similarly vulnerable people were coerced into self-incrimination. He shared his opinion in a letter to a Chicago nerve specialist, which made the local press. A week later, the farmer’s son was hanged.

Münsterberg was ahead of his time. It would be decades before the legal and psychological communities began to understand how powerfully suggestion can shape memory and, in turn, the course of justice. In the early nineteen-nineties, American society was recuperating from another panic over occult influence; Satanists had replaced witches. One case, the McMartin Preschool trial, hinged on nine young victims’ memories of molestation and ritual abuse—memories that they had supposedly forgotten and then, after being interviewed, recovered. The case fell apart, in 1990, because the prosecution could produce no persuasive evidence of the victims’ claims. A cognitive psychologist named Elizabeth Loftus, who had consulted on the case, wondered whether the children’s memories might have been fabricated—in Münsterberg’s formulation, involuntarily elaborated—rather than actually recovered.

To test her theory, Loftus gave a group of volunteers the rudimentary outlines of a childhood experience: getting lost in a mall and being rescued by a kindly adult. She told the subjects, falsely, that the scenario was real and had taken place when they were young. (For verisimilitude, Loftus asked their parents for biographical details that she could plant in each story.) Then she debriefed the subjects twice, with the interviews separated by one or two weeks. By the second interview, six of the twenty-four test subjects had internalized the story, weaving in sensory and emotional details of their own. Loftus and other researchers have since used similar techniques to create false memories of near-drownings, animal attacks, and encounters with Bugs Bunny at Disneyland (impossible, since Bugs is a Warner Bros. character).

Earlier this year, two forensic psychologists—Julia Shaw, of the University of Bedfordshire, and Stephen Porter, of the University of British Columbia—upped the ante. Writing in the January issue of the journal Psychological Science, they described a method for implanting false memories, not of getting lost in childhood but of committing a crime in adolescence. They modelled their work on Loftus’s, sending questionnaires to each of their participant’s parents to gather background information. (Any past run-ins with the law would eliminate a student from the study.) Then they divided the students into two groups and told each a different kind of false story. One group was prompted to remember an emotional event, such as getting attacked by a dog. The other was prompted to remember a crime—an assault, for example—that led to an encounter with the police. At no time during the experiments were the participants allowed to communicate with their parents.

What Shaw and Porter found astonished them. “We thought we’d have something like a thirty-per-cent success rate, and we ended up having over seventy,” Shaw told me. “We only had a handful of people who didn’t believe us.” After three debriefing sessions, seventy-six per cent of the students claimed to remember the false emotional event; nearly the same amount—seventy per cent—remembered the fictional crime. Shaw and Porter hadn’t put undue stress on the students; in fact, they had treated them in a friendly way. All it took was a suggestion from an authoritative source, and the subjects’ imaginations did the rest. As Münsterberg observed of the farmer’s son, the students seemed almost eager to self-incriminate.

One young woman spun a story about a kind of love triangle. In the first debriefing, she remembered the incident as a fistfight between her and another girl. In the second, she remembered having thrown a small rock at her adversary after the girl uttered a slur. By the third debriefing, the rock had grown to the size of her fist and she had hurled it at the girl’s face. “It was very emotional,” Shaw said. “Each time she’d reënact the event, the rock would fill her hand a little bit more.” Nothing in the woman’s affect suggested that the memory was false. She earnestly believed in the truth of her confession, as most of her fellow-participants did theirs. The memory was vivid, loaded with details about the crime that the interviewer had not furnished. Moreover, Shaw and Porter could find no personality traits that distinguished the false confessors from the few holdouts, and no way of identifying who was most susceptible.

These are troubling findings. They mimic, in the gentlest way, what can happen during police questioning: a small lie, told to shake loose the truth, rattles around in a suspect’s imagination and takes root. The psychologist Saul Kassin has studied interrogation and false confession for decades. He told me that Shaw and Porter’s experiment illustrates perfectly how social pressure can make innocent people admit to wrongdoing. “Think about the dilemma the suspect now faces: ‘I don’t have a memory for this, but the person who took care of me does. Therefore it must be true and I have to find a way to remember it.’ ”

Kassin cited the example of Martin Tankleff, a high-school senior from Long Island who, in 1988, awoke to find his parents bleeding on the floor. Both had been repeatedly stabbed; his mother was dead and his father was dying. He called the police. Later, at the station, he was harshly interrogated. For five hours, Tankleff resisted. Finally, an officer told him that his father had regained consciousness at the hospital and named him as the killer. (In truth, the father died without ever waking.) Overwhelmed by the news, Tankleff took responsibility, saying that he must have blacked out and killed his parents unwittingly. A jury convicted him of murder. He spent seventeen years in prison before the real murderers were found. Kassin condemns the practice of lying to suspects, which is illegal in many countries but not here. The American court system, he said, should address it. “Lying puts innocent people at risk, and there’s a hundred years of psychology to show it,” he said.

Shaw and Porter’s study also provides further evidence of the inaccuracy and malleability of human memory, evidence that is already compelling enough to have persuaded the state supreme courts of New Jersey and Massachusetts to mandate that judges instruct juries that eyewitness testimony is inherently unreliable. “Evolutionary theorists say memory is good enough—just good enough for us to survive and to reproduce,” Shaw told me. “But, at the very least, this research calls into question whether we should be putting so much weight on any memory in court”—especially in the absence of corroborating proof. “It’s sort of a reality check.”