March 8, 2012

The Role of the Expert Witness in Assessing Witness Reliability

In response to a legal report today that Pennsylvania may allow expert witnesses to offer opinions on circumstances where research shows that eyewitnesses may be unreliable, a CECON expert offers the following perspective:

As a human factors psychologist and one who couldn’t recognize his exact double walking down the street towards me, I would like to comment on the role of the expert in assessing eyewitness reliability. The article points out that research has shown there are many factors that affect eyewitness identification reliability and the article mentions only a very few. Could a witness distinguish between twins? Or ignore some noticeable characteristic of clothing? 




Even the procedures and manner of presentation in a lineup can cause false positives. We have long known about “demand characteristics” which in a lineup translates into “if I am shown five suspects at the same time, one of them must be the one!” This relates to one of the four conditions for unreliability mentioned in the article, but there are better solutions to such procedural biases than just informing the witness, but the better procedures have not been universally adopted. A jury would never know that these issues existed or what their effects may be on testimony.


We have long known about individual differences. Does the witness have “face blindness” that is at one end of a continuum in the population, as opposed to those people who truly never forget a face at the other end of the spectrum? Human factors experts often reference inattentional blindness (not seeing something that is obvious when you take a second look). And there has been excellent research that demonstrates the limitations on the number of features that can be processed after observing a large set of features. That research shows that which features are recallable depends on the instruction for recalling them.

It seems to me that an expert could adequately tell a jury what must be considered in eyewitness testimony and establish the case that the eyewitness’s lawyer or the prosecutor must conduct an evaluation of the witness’s ability. The expert should be allowed to rebut the evaluation if it was not done to scientific standards. But we must also remember that the evaluation cannot “prove” that the witness’s identification was correct; only that it is likely to be incorrect if a poor ability was demonstrated in the evaluation. The strength of the expert is knowing what science can tell us and the likelihood of being right about it. Then it is up to the jury.






The author holds a Ph.D. in Experimental Psychology and is an expert in human factors psychology, perception and vision, cognitive psychology, and situational awareness. To read more about this expert, click here.

To read the article that prompted this response, click here.

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