Expert Evidence

Evidence from expert witnesses also plays a significant role in US courts. One study of the use of experts in courts in three American cities suggests that this kind of evidence is introduced in 60% of all criminal trials (Shuman and Champagne, 1997). That figure is undoubtedly higher in complex, civil cases.

Expert evidence, particularly related to psychological and psychiatric issues, plays a less prominent role in English courts (Lloyd-Bostock and Thomas, 1999) where, for example, jurors are assumed to have common knowledge of the vagaries of eyewitness testimony. As a result, expert testimony on this issue is not admitted (Gudjonsson, 1996). Suggested Lord Justice Lawton: 'jurors do not need psychiatrists to tell them how ordinary folk who are not suffering from any mental illness are likely to react to the stresses and strains of life' (quoted by Lloyd-Bostock and Thomas).

Empirical research on the effects of expert testimony on eyewitness reliability is relevant here. Can jurors, in fact, be aided by the testimony of an expert on eyewitness identification? Or will the expert render them overly skeptical of eyewitnesses? Cutler, Penrod and Dexter (1989) showed mock jurors a realistic videotaped trial that focused on the accuracy of an eyewitness's identification of an armed robbery defendant. Some participants heard an expert testify about the effects of identification conditions on accuracy, others did not. In addition, some mock jurors heard evidence that the witnessing and identification conditions were good, and others heard that they were bad. Analysis of the verdicts and mock jurors' post-trial sentiments indicated that jurors who were exposed to the expert testimony more carefully evaluated the role of various factors (e.g. weapon focus, line-up procedures) on eyewitness reliability than did jurors who had no such testimony. Those who heard an expert also gave less weight to the expressed confidence of eyewitnesses (a desirable finding in light of evidence that witness confidence and witness accuracy are only weakly related). These findings suggest that the effect of expert testimony was to generally sensitize jurors to the importance of witnessing and identification conditions rather than to make jurors skeptical of the eyewitness.

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