Thursday, December 13, 2012

But I saw it with my own eyes!!

Last week the Oregon supreme court ruled unanimously to change the legal policy on eyewitness identification. The new ruling places the burden of proof that an eyewitness statement should be admitted to court proceedings on the prosecutors instead of the defendants. What does this have to do with science? I'm glad you asked. 

Eyewitness testimony is often very wrong - even missing important details. Take for example this video. Can you correctly count the number of times that this basketball is passed between the players wearing white shirts?

How about in this version?


We tend to give more merit to our own memories than we should. We trust ourselves and say things like "I know what I saw". However, even seeing something with your own eyes does not mean that it happened as you remember. Not only do we miss details, but our memories are very malleable - they are easily influenced by time, what other people say, or even what we wish the memory to be. Not only do we trust our own eyes more than we should, but we trust everyone else's eyes more than we should too. The supreme court has even said that:
Despite its inherent unreliability, much eyewitness identification evidence has a powerful impact on juries. Juries seem most receptive to, and not inclined to discredit, testimony of a witness who states that he saw the defendant commit the crime. 
Eyewitness testimony is likely to be believed by jurors, especially when it is offered with a high level of confidence, even though the accuracy of an eyewitness and the confidence of that witness may not be related to one another at all. All the evidence points rather strikingly to the conclusion that there is almost nothing more convincing than a live human being who takes the stand, points a finger at the defendant, and says 'That's the one!'
In other words, juries are highly influenced by a confident eyewitness testimony - even though how confident you has nothing to do with how right you are.

One tragic story of eyewitness misidentification is that of Ronald Cotton. In 1984, Jennifer Thompson awoke to a man standing over her bed. A knife was held to her neck and she was raped. During the attack she made a conscious effort to memorize important details of her attacker. Later that day at the police station Thompson was shown six photos. After examining them for several minutes she chose the Ronald Cotton from the photo line-up. "Did I do OK?", she asked the detectives. "You did great" was the response. This positive feedback solidified her confidence that Ronald Cotton had raped her. 

Later she was asked to identify Ronald Cotton from a physical line-up. Once again she chose Ronald Cotton (who was the only person in both the photo and physical line-up) and once again her confidence in the identification increased when she was told she picked the "right guy". However, DNA evidence has since cleared Ronald Cotton. Thompson and Cotton have since written a book on their story - Picking Cotton: Our Memoir of Injustice and Redemption.

Cotton's story can help us see how our eyes can easily deceive us. Our own personal memories - no matter how vivid - are not by themselves convincing evidence of anything. That's one reason why scientists use so many measurement techniques. The ones and zeros that the instrument in my lab outputs may seem impersonal, but they don't change over time. I can always go back and reanalyze the data to be sure of what really happened.

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