Unreliable Eyewitness Testimony

Factoid of the day:

Bad eyewitness identifications contributed to 75 percent of wrongful convictions in cases that were overturned by DNA evidence.

Given these dismal statistics, some states have tried to fix their procedures for eyewitnesses. New Jersey, for example, used to do police lineups the standard way: witnesses identified suspects from an in-person lineup as detectives stood next to them. The police officers would sometimes offer words of encouragement. New Jersey has now done away with the lineup, and instead presents people one after the other. This is supposed to prevent witnesses from making "relative judgments about which individual most looks like the perpetrator."

From the perspective of neuroscience, eyewitness testimony is an extremely unreliable type of evidence. The first reason is that our vision is vulnerable to all sorts of top-down influences, which alter (corrupt, some might say) the actual inputs witnessed by our eye. To paraphrase Paul Simon, "A man sees what he wants to see and disregards the rest." This doesn't mean that our senses aren't rooted in reality, but it does suggest that we shouldn't place too much trust in the details of perception, especially when the event in question happened fast, from a distance, and in bad lighting.

The second reason eyewitness testimony is unreliable has to do with the nature of memory. Neuroscience now knows that every time we remember a memory, that memory is "reconsolidated," slyly remade and reconfigured. The idea of reconsolidation should make us distrustful of our memories. They do not directly represent reality. Instead, they are an imperfect copy of what actually happened, a Xerox of a Xerox of a mimeograph of the original photograph. This directly contradicts an underlying assumption of eyewitness testimony, which assumes that our memories are immutable impressions, locked away in the brain's vast file cabinet. Recalling the memory shouldn't change the original memory.

But it does change the memory. The very act of remembering what you saw changes the neural substrate of the original memory trace. For more on this research, check out the work of Karim Nader.

Of course, our legal system will always need eyewitness testimony. The important thing is to make eyewitness testimony more reliable, to design legal situations that discourage eyewitness biases and mistakes. More states should follow the lead of New Jersey.

But I think this research also speaks to another controversy perpetually roiling the legal system: the death penalty. The awareness of our own epistemic limitations - and neuroscience is great at illuminating those limits - should make us skeptical of our verdicts. People make sensory and memory mistakes all the time. That's just the nature of the brain. But nobody should die because of a mistake.


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Memory will change as the result of excessive interrogation. You the witness -- or suspect -- give me your story and then we pick at it, going over and over and over, jumping around in time, getting you to make adjustments here and there, caving in again and again, taking direction from us, until finally we get the story we want. Then you get to sign on the line to a cooked up 'statement' that you never made, but you will sign it if you want to get out of here. Then, if need be, we'll bring you into court and make you agree that you authored the story we created.

No changes in the rule book will stop cops from doing what's worked for them in the past.

By BitterCynic (not verified) on 03 May 2007 #permalink

The US overpunishes by cultural design; response to manipulation through moral panic.

I am a jury junkie. We rely too much on the eyewitness report, and too much on prosecutorial devices such as plea bargaining (which some enlightened nations have refused to engage in). When we as a nation, refuse to engage in plea bargains and overprosecution, there will be hope for us, until then -- doubtful.

From PD

The theoretical conclusion of PD is one reason why, in many countries, plea bargaining is forbidden. Often, precisely the PD scenario applies: it is in the interest of both suspects to confess and testify against the other prisoner/suspect, even if each is innocent of the alleged crime. Arguably, the worst case is when only one party is guilty -- here, the innocent one is unlikely to confess, while the guilty one is likely to confess and testify against the innocent.

But I think this research also speaks to another controversy perpetually roiling the legal system: the death penalty.

All of the arguments I've heard against the death penalty, this is the only one that I found convincing in the slightest. It was, however, enough to convince me of the necessity of not enforcing capital punishment.

By valhar2000 (not verified) on 04 May 2007 #permalink

I can agree with the 'eyewitness bias' from personal experience...

I was involved in a fracas as a young man of 17.... and selected a person from the lineup whom I swore was present. That person was, in fact, quite convingly elsewhere at the time in question (he was actually held at another police station, but that's beside the point).

I have since always questioned my own eyewitness 'evidence' and challenge my personal memory of events.

This helps me (as a consultant). I always take notes. And I always 'replay' my notes and get agreement before moving on. I never take unsubstantiated memory as proof - even if it costs me time or money. If I don't have physical evidence, then I don't have evidence - simple as that.

As regards crimes & appropriate punishment.... I disagree with capital punishment in most cases - especially in 'eyewitness' cases. But - and this is a very important but - if an individual is indisputably identified as the perpetrator of a particular capital crime -- through physical and forensic evidence such as DNA plus video plus.... then I would not be against capital punishment in that case. If such indisputable evidence were not present, then capital punishment should NEVER be considered.

The evidence needs to be incontrovertible.