Mind: The Certainty of Memory Has Its Day in Court

For scientists, memory has been on trial for decades, and courts and public opinion are only now catching up with the verdict. It has come as little surprise to researchers that about 75% of DNA-based exonerations have come in cases where witnesses got it wrong.

While most of us tend to think memory works like a video recorder, it is actually more like a grainy slide show. Lost details, including imaginary ones, often are added later. One of the earliest and more famous experiments to demonstrate that memories are malleable was conducted by Elizabeth Loftus, a psychology professor at the University of California, Irvine, and an early pioneer of witness memory research.

In a 1974 study published in The Journal of Verbal Learning and Verbal Behavior, she asked participants to view films of fender-benders in which no car windows or headlights were broken. Later, the subjects who were asked how fast the cars were going when they smashed into each other — as opposed to hit — were more likely to report speeding and describe shattered glass they never actually saw.

In another experiment, conducted in Scotland, participants were four times as likely to report a memory of a nonexistent event — in this case, a nurse removing a skin sample from their little finger — if they had been asked to imagine it just one week before. Others in the experiment read a description, but were not asked to picture it happening.

Even the process of police questioning and prepping for trial can crystallize a person's own faulty reconstruction. In 2000, Dr. Tversky published a series of experiments conducted at Stanford University in the journal Cognitive Psychology. In one, volunteers read profiles of fictitious roommates with both charming and annoying habits; they were then asked to write either a letter of recommendation or letter making a case for a replacement.

When later asked to repeat the original description, the volunteers' recollections were skewed by the type of letter they had written. Their minds had shed qualities that didn't match the first draft of their own recall and had embellished those that did.

When we don't remember, we make inferences, Dr. Tversky said.

Sometimes we miss details because we weren't paying attention, but sometimes we are concentrating too hard on something else. Nothing is as obvious as it seems.

All this makes sense, he said, when you consider the purpose of memory. He and his colleagues believe that memory is designed not just to keep track of what has happened, but to offer a script for something that might.

Evidence for this also comes from brain scans. Just as the recall of a bogus event lights up the brain's memory centers, so does thinking about something that might occur.

Because the brain uses memories for mental dress rehearsal, we are not wired to retain every facet of an event, scientists say. We don't have to. A general framework is all that's necessary to keep from getting lost, or find food, or know what to do when a storm is coming.

For more, see The Certainty of Memory Has Its Day in Court by Laura Beil, November 28, 2011 at NYTimes.com.