Expert testimony and the effects of a biological approach, psychopathy, and juror attitudes in cases of insanity.

Amid growing psychological controversy and legal interest surrounding the uses of PCL-R and biological evidence in the legal system, this mock jury study assessed the effects of PCL-R and biological evidence on outcomes in an insanity defense case. A sample of 428 undergraduates read a trial transcript of an insanity defense murder case. Three variables of interest were manipulated: rebuttal illness (no mental illness, personality disorder, or psychopathy), evidentiary basis (biological or psychological), and evidentiary strength (moderately strong or moderately weak). Consistent with the hypotheses, biological evidence was more persuasive than psychological evidence, and the rebuttal was slightly more successful when the prosecution labeled the defendant as a "psychopath" than when they described him simply as "not mentally ill."