Mental illness in the courtroom : does a psychiatric diagnosis affect perception of a defendant's speech dynamics on the witness stand?

The present study aimed to investigate prospective juror's perceptions of the dynamics of social interaction in a courtroom where the defendant is said to have a psychiatric diagnosis. The experiment adopted Communication Accommodation Theory (CAT) to examine the effect of lawyer and defendant's converging and diverging speech rates on the jurors' perception of the defendant's credibility, likeability, cooperativeness, intent and guilt. One-hundred and eighty-six participants were allocated to one of 18 conditions, in which they listened to a reenactment of part of an edited court case and then filled in questionnaires. It was hypothesised that rapid speed of speech would act as a credibility cue, resulting in an increase in ratings of defendant credibility. It was further expected that ratings of cooperativeness and likeability would increase when the lawyer and defendant's speech rates converged and that ratings of cooperativeness and likeability would vary across convergence and divergence depending on whether the defendant's change in speech rate was perceived as being internally or externally motivated (intent). In regards to the effect of the defendant's mental health label, it was hypothesised that where the defendant was said to have a psychiatric diagnosis this label would override the effect of the speech rate manipulations. Little support was found for the hypotheses outlined in this study. Possible reasons for this lack of support, as well as suggestions for further research are outlined in the discussion.