Kluge: The Haphazard Construction of the Human Mind (Houghton Mifflin; 2008)

For most species, most of the time, remembering gist rather than detail is enough. If you are a beaver, you need to know how to build dams, but you don't need to remember where each individual branch is. For most of evolution, the costs and benefits of context-dependent memory worked out fine: fast for gist, poor for detail; so be it.

If you are human, though, things are often different; societies and circumstances sometimes require of us a precision that wasn't demanded of our ancestors. In the courtroom, for example, it's not enough to know that some guy committed a crime; we need to know which guy did — which is often more than the average human can remember. Yet, until recently, with the rise of DNA evidence, eyewitness testimony has often been treated as the final arbiter; when an honest-looking witness appears confident, juries usually assume that this person speaks the truth.

Such trust is almost certainly misplaced — not because honest people lie, but because even the most honorable witness is just human— saddled with contextually driven memory. Oodles of evidence for this comes from the lab of the psychologist Elizabeth Loftus. In a typical study, Loftus shows her subjects a film of a car accident and asks them afterward what happened. Distortion and interference rule the day. For example, in one experiment, Loftus showed people slides of a car running a stop sign. Subjects who later heard mention of a yield sign would often blend what they saw with what they heard and misremember the car as driving past a yield sign rather than a stop sign.

In another experiment, Loftus asked several different groups of subjects (all of whom had seen a film of another car accident) slightly different questions, such as How fast were the cars going when they hit each other? or How fast were the cars going when they smashed into each other? All that varied from one version to the next was the final verb (hit, smashed, contacted, and so forth). Yet this slight difference in wording was enough to affect people's memory: subjects who heard verbs like smashed estimated the crash as occurring at 40.8

miles per hour, a significantly greater speed than that reported by those who heard verbs with milder connotations, like hit (34.0) and contacted (31.8). The word smashed cues different memories than hit, subtly influencing people's estimates.

Both studies confirm what most lawyers already know: questions can "lead witnesses." This research also makes clear just how unreliable memory can be. As far as we can tell, this pattern holds just as strongly outside the lab. One recent real-world study, admittedly small, concerned people who had been wrongly imprisoned (and were subsequently cleared on the basis of DNA tests). Over 90 percent of their convictions had hinged on faulty eyewitness testimony.

When we consider the evolutionary origins of memory, we can start to understand this problem. Eyewitness testimony is unreliable because our memories are stored in bits and pieces; without a proper system for locating or keeping them together, context affects how well we retrieve them. Expecting human memory to have the fidelity of a video recorder (as juries often do) is patently unrealistic. Memories related to accidents and crimes are, like all memories, vulnerable to distortion.

A memorable line from George Orwell's novel 1984 states that "Oceania had always been at war with Eurasia" — the irony being, of course, that until recently (in the time frame of the book) Oceania had not in fact been at war with Eurasia. ("As Winston well knew, it was only four years since Oceania had been at war with Eastasia and in alliance with Eurasia.") The dictators of 1984 manipulate the masses by revising history. This idea is, of course, essential to the book, but when I read it as a smug teenager, I found the whole thing implausible: wouldn't people remember that the battle lines only recently had been redrawn? Who was fooling whom?

Now I realize that Orwell's conceit wasn't so far-fetched. All memories — even those concerning our own history — are constantly being revised. Every time we access a memory, it becomes "labile," subject to change, and this seems to be true even for memories that seem especially important and firmly established, such as those of political events or our own experiences.