Anne Bowen Poulin, A Fair Subject for Expert Testimony?

59 Fla. L. Rev. 991 (2007) | | | |

INTRODUCTION :: How can two leading commentators take diametrically opposed positions concerning the use of expert testimony to address witness credibility? After the adoption of the Federal Rules of Evidence, Judge Weinstein stated categorically that the Federal Rules permit experts to testify concerning a witness’s truthful character, but Professor Michael Graham stated with equal confidence that they do not. Courts have also adopted conflicting positions on whether expert testimony on witness credibility is admissible. The Fifth Circuit has asserted that “[t]he readily apparent principle is that the jury should, within reason, be informed of all matters affecting a witness’s credibility to aid in their determination of the truth.” Other courts regard expert testimony relating to credibility as an invasion of the jury’s province, taking the view that “[c]redibility . . . is for the jury-the jury is the lie detector in the courtroom.”

The explanation for the conflicting lines of authority lies in the residual strength of the common-law maxim that witnesses-particularly expert witnesses-must not invade the jury’s province by vouching for or bolstering a witness’s credibility. It also lies in the extension of that maxim to preclude expert testimony that explains weaknesses in credibility. Judge Weinstein looks at the Federal Rules of Evidence, which allow opinion testimony and expert testimony, and concludes that the Rules permit experts to testify concerning credibility. Professor Graham examines judicial decisions and concludes that the testimony is not admissible. Graham’s assertion reflects the judiciary’s persistent adherence to the common-law maxim, even though the maxim was not codified in the Federal Rules of Evidence.


The common-law prohibition against expert testimony on credibility should not continue to restrict the admissibility of evidence bearing on credibility. Instead, courts should set aside the maxim’s broadly stated prohibition and should eliminate the overprotection of the jury’s “special province.” The courts should forthrightly engage the evidentiary issues raised by specific types of expert testimony that bolsters or impeaches. The Federal Rules of Evidence do not support the continued prohibition on expert testimony addressing credibility. Moreover, the courts cannot expect juries to function as accurate lie detectors; modern research has documented human weakness at divining credibility. The legal system should respond to this insight by providing juries with as much assistance as possible as they evaluate which witnesses to believe and which facts to credit. Expert witnesses are sometimes able to supply this assistance.

An expert witness may provide insight concerning a specific witness based on expert evaluation of that witness or may educate the jury on matters that the jury can bring to bear when assessing credibility. A particular expert’s testimony may address witness credibility in one of several ways. Courts should base admissibility decisions on the way in which the testimony addresses witness credibility.

First, an expert may be in a position to testify concerning a witness’s general tendency to be truthful or untruthful-the witness’s character for truthfulness. The notorious case, United States v. Hiss, involved this type of testimony. The trial court admitted the defense expert’s testimony that the government’s key witness had a pathological condition that caused him to lie persistently. Although courts have rarely followed the precedent established in Hiss, the modern Federal Rules of Evidence expressly provide that opinion concerning a witness’s character for truthfulness is admissible, and the Rules do not carve out a special rule for expert opinion. Expert testimony, like lay witness testimony, that addresses character for truthfulness should be admissible under the Rules.

Second, and far more commonly, expert testimony may address the witness’s perception and memory or the witness’s bias. Testimony that bolsters or attacks credibility in these ways is not governed by a specific evidence rule. Instead, it is governed by the general rules providing that relevant evidence is admissible, although the court may exclude relevant evidence if its negative characteristics substantially outweigh its probative value. Evidence that helps the jury understand the witness’s perception, memory, or bias has high probative value, and evidence falling into this category should generally be admitted.

Third, an expert witness may help the jury understand the way in which a witness’s conduct reflects on the witness’s credibility. When a witness’s conduct may suggest a lack of credibility to the jury, expert insight into that conduct may bolster the witness’s credibility. Like evidence of perception, memory, or bias, testimony in this category is not governed by a specific rule of evidence and can play an important role in assisting the jury.

Courts do not always recognize the different roles expert testimony can play, and, as a result, they apply evidentiary rules inappropriately or simply invoke the outdated common-law maxim to bar expert testimony without providing a thorough analysis. Courts should consider in a more nuanced manner the role proffered expert testimony plays and how evidence rules affect this testimony’s admissibility. Doing so should lead courts to embrace the promise of the modern rules of evidence and to permit experts to assist juries in assessing credibility. This Article explores the ways in which experts can help the jury and suggests the analytical approaches courts should apply to varying types of expert testimony.

In Part II, this Article briefly summarizes the relevant Federal Rules of Evidence to provide the context for the discussion. Part III of this Article describes the common-law maxim and then demonstrates the flaws in the maxim’s foundation. In Parts IV and V, this Article considers the different ways that expert testimony can help the jury assess credibility and argues that courts should admit this testimony far more readily. Parts VI and VII consider some of the barriers that have prevented courts from abandoning the common-law maxim and have limited the admissibility of expert testimony bearing on credibility. Finally, in Part VIII, this Article examines and cautions against overreaching by the parties who present expert testimony on credibility and suggests that a more restrained approach will increase the willingness of courts to allow such testimony.

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