An historian testifying as an expert witness is a bit like a dog cutting your lawn — You don’t care whether the dog mows in a straight line, or cuts too high or too low, or even whether the dog rakes up after cutting. Dogs should not be cutting the grass at all.
Historians may have a great deal to contribute to litigation by identifying primary sources and suggesting lines of argument or narratives from the evidence that they collect and synthesize. Historians’ testimony, however, should not substitute for lawyers’ proving their cases by admissible evidence, and by persuading the fact finder with argument. The principal problem with historian expert witness testimony is that it circumvents the rules of evidence, and injects final argument into the middle of the trial in the guise of testimony. Lawyers might welcome this opportunity for an additional final argument, and for the relief from the tedium of actually proving the factual predicate of their cases, but expedience is a slim justification that does not outweigh circumventing the structure of the trial and rules of evidence.
Caution! Suggesting that historians should not serve as expert witnesses may cause the dog to bite. See, e.g., D. Rosner & G. Markowitz, “The Trials and Tribulations of Two Historians: Adjudicating Responsibility for Pollution and Personal Harm, 53 Medical History 271, 280-81 (2009)(criticizing legal counsel for expressing the view that historians should not be permitted to testify and thereby circumvent the rules of evidence); D. Rosner & G. Markowitz, “L’histoire au prétoire. Deux historiens dans les procès des maladies professionnelles et environnementales,” 56 Revue D’Histoire Moderne & Contemporaine 227, 238-39 (2009) (same); D. Rosner, “Trials and Tribulations: What Happens When Historians Enter the Courtroom,” 72 Law & Contemporary Problems 137, 152 (2009) (same).
Apparently historians do not have prohibitions against duplicative (or triplicative) publications.