Frightening new evidence of the brain's susceptibility to suggestion was presented on Sunday to the American Association for the Advancement of Science.
Psychologists described recent experiments in which they implanted false memories, altered perceptions through subliminal messages, and demonstrated the intense emotional distress of people who believed they had been abducted by aliens.
Psychologists warn on power of suggestion
By Clive Cookson in Denver
Financial Times February 17 2003
The researchers said their work showed that courts, police and other agencies needed to be extremely wary of relying solely on the memory of witnesses - for example in sex abuse cases - because even the most intense memories could be false.
Elizabeth Loftus of the University of California, Irvine, described several studies in which false memories were implanted through suggestive questioning. For example, working with Russian colleagues, she interviewed 80 volunteers in Moscow about television coverage of Chechen bomb attacks in the city three years ago.
During a preliminary interview Prof Loftus planted the false idea that a wounded animal had featured in the coverage. When she asked the subjects in a second interview six months later: "Can you remember the wounded animal that you mentioned last time?", 10 of them had such memories, such as seeing a cat lying bleeding in the dust after an explosion.
In another study, Prof Loftus recruited American men and women to talk about childhood visits to Disneyland, with a newspaper advertisement showing a fake scene with Bugs Bunny in the Californian theme park. When interviewed, 33 per cent of the volunteers described meeting Bugs Bunny at Disneyland as children - an impossible event since Bugs Bunny is a Warner Bros character and has never appeared at Disneyland.
Joel Weinberger of Adelphi University is investigating subliminal suggestion, a subject with a long history in popular culture. In the early years of the cinema, frames saying "Drink Coke" or "Eat Popcorn" were said to have been inserted into films. And during the last US presidential election the Bush campaign was accused of inserting the word "Rats" into one of its advertisements in an effort to associate Democrats with rats - a charge vehemently denied by the Republicans.
Prof Weinberger's research shows that subliminal messages can affect memory and perception. In one study he asked volunteers to rate hypothetical candidates for political office on the basis of a short video. When "Rats" was flashed for less than a hundredth of a second - too quickly for conscious perception - participants reported a significantly less favourable impression than when neutral words were inserted.
In a second experiment, people were asked to describe childhood memories after watching a video clip. Subliminal insertion of the words "Mommy and I are One" made them described their relations with their mother in more positive terms.
Richard McNally of Harvard University studied 10 people who had reported being abducted by aliens and subjected to traumatic experiences. Participants recorded these experiences on audiotapes, which were played back to them later in the laboratory while researchers measured their heart rate, skin sweating and muscle tension.
The physiological symptoms of emotional distress shown by the abductees during playback were similar to those of people suffering from real post-traumatic stress.
Prof McNally said his experiment should sound a warning to some psychotherapists who believed "recovered memories" of past traumas such as childhood sex abuse are genuine just because they induce intense emotion.
"The intensity of emotional reaction associated with a memory cannot confirm the authenticity of the memory," he said.