Law & Psychology Review
During a trial, a witness's job is to supply the facts by telling the jury what she saw, heard, or otherwise experienced that is relevant to the legal questions the jury must answer. The jury's job is to decide how much weight and credibility to accord a witness's testimony. Jurors are expected, even instructed, to rely on their own knowledge about the world when deciding whether and how much to believe a witness. Most of the time, jurors' own experiences are sufficient to allow them to accurately assess a witness's testimony. However, jurors are sometimes called upon to assess testimony that their own experiences have not prepared them to assess accurately. In these cases, expert witnesses can provide jurors with the knowledge that they need to evaluate the evidence properly. By definition, an expert witness is someone who has knowledge that would be helpful to jurors—helpful both because jurors are unlikely to have this knowledge and because this knowledge is important to properly understanding something at issue in the case.
Dora W. Klein, Memoir as Witness to Mental Illness, 43 Law & Psychol. Rev. 133 (2018).