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Ipse dixit is a Latin phrase meaning "he said it himself." In a legal context, ipse dixit refers to trial testimony that is not substantiated by other evidence but accepted based on the authority of the person delivering it. In most instances in which the term is used, it pertains to expert witness testimony about subjects of significant complexity or detail to warrant conclusive opinions from authoritative specialists in the field. Problems have arisen, however, in the distinction between legitimate expertise and pseudoscience. This has prompted the United States Supreme Court to deliver several decisions that stress the trial judge role in determining the admissibility of evidence based on its reliability and relevance.
The 1993 case Daubert v. Merrill Dow Pharmaceuticals, Inc. led to a key Supreme Court decision regarding ipse dixit testimony. Concerns over the introduction of flawed science into courtroom testimony, including opinions based on pure feelings, unreliable methodology, and personal experience, led the litigants to seek an opinion from the higher court regarding defined standards for expert testimony. The Supreme Court held that expert witnesses needed to back up their assertions with peer-reviewed articles, tests, statistics, and descriptions of their methodology. The justices also ruled that trial judges should limit ipse dixit evidence and closely scrutinize the applicability and trustworthiness of such evidence. These rulings were broadened to apply even to highly subjective scientific areas, in which hard data may be lacking, such as psychiatry in Kumho Tire Co. Ltd. V. Carmichael.
Another form of admissible ipse dixit in courtrooms is the opening statement of an attorney, in which he presents a preview of his case. Legal studies indicate that about 80 percent of jurors decide the merit of a case immediately after hearing the opening statements of the attorneys. In light of this, expert witnesses can play a significant role in the preparation of opening statements for the attorneys that accurately represent their own expert opinions in easily understood language. They can also help to criticize the weaknesses of the scientific case of the opposing side.
Experts must maintain their roles as educators, not as advocates. If an expert is biased towards one side of the issue, his authority upon which the ipse dixit evidence is admitted becomes questionable. Furthermore, authorities must be experts in the applicable field, not just in any discipline. For example, a physician who practices gastroenterology and treats digestive diseases would be considered an inappropriate witness to present ipse dixit evidence on cardiovascular surgery in most courtrooms, even though he may truly understand a tremendous amount about cardiovascular surgery. Courts also recognize that even experts make mistakes, disagree with their colleagues on some issues, and depart from commonly accepted views at times, making all ipse dixit statements somewhat debatable.