Abstract
Expert evidence can be very difficult for jurors to understand. To overcome this
problem, experts often use visual aids to help convey their testimony. But although
visual aids can help with retention and learning, they could also have unintended
negative effects. More specifically, the increase in cognitive fluency associated with
visual aids might lead jurors to judge expert testimony more positively, to see it as more
true, and to believe they understand it better than they actually do. To test this
possibility, we exposed mock jurors (N = 834) to a simulated gunshot homicide case in
which the defence argued that the death was an accident. Jurors read a transcript of
skeletal trauma evidence provided by a forensic anthropologist, accompanied either by
probative skeletal diagrams, non-probative skeletal diagrams, or no diagrams. We also
manipulated whether the expert was called by the prosecution lawyer or the defence
lawyer. Both diagram conditions increased mock jurors’ self-reported comprehension of
the evidence—but only when the evidence was presented by the prosecution. There
was, however, no increase in objective comprehension. Furthermore, regardless of
lawyer, both diagram conditions increased mock jurors’ confidence in their verdict. No
differences were found between non-probative and probative diagram conditions.
Although all these effects were small, our results suggest that, despite their high face
value, skeletal diagrams might only create an illusion of understanding. These findings
have important implications, both for decision-making theory and for the way in which
evidence is presented to jurors.