Excluding expert evidence in connection to witness credibility and self-explanatory evidence


In the case of United States v Robert McDonnell [2015 US Dist. LEXIS 88874], the Court of Appeals held that exclusion of expert evidence was not abuse of discretion on part of the district court as expert evidence could not be used solely to discredit a witness. In the present case, the Appellants used a legal expert to explain the unusual nature of the immunity agreement of one of the witnesses. This was done with a view to highlight the possibility that the witness had incentive to exaggerate facts in his testimony. The Court held that the sole purpose of this testimony was to discredit the particular witness which the district court was justified in exercising its jurisdiction to exclude. The Court also held that the expert testimony was not needed to explain the complexity of a Statement of Economic Interest to the jurors.


This was a case filed against the former governor of Virginia, Robert F McDonnell, on charges of corruption. The defendant was accused of accepting certain gifts in consideration for assisting a Virginia company to secure state university testing of a dietary supplement developed by the said company. In an appeal from the lower court conviction, the defendant challenged the conviction on a number of errors made by the district court, including an error in the evidentiary rulings. The defendant contested the ruling of the district court in excluding expert testimony on two counts.


There were two questions before the Court with respect to the exclusion of expert evidence by the district court. To begin with, the Appellants sought to discredit the testimony offered by Williams by showing that the transactional immunity that he had obtained from the government was of an exceptional nature and thus he had an incentive to exaggerate in his testimony. To prove this, the Appellants used the expert opinion offered by Peter White, a lawyer, to help the jury understand the nuances of the immunity agreement. The Court however sided with the district court in excluding this testimony because an expert witness cannot be used solely used to undermine the credibility of another witness. The Court referred to United States and Allen [716 F.3d 98, 105-06 (4th Cir. 2013)] and upheld the ratio that credibility of witnesses is best left for cross examination and there is no requirement for expert testimony to be included.

The second point that was challenged was the district court’s decision to exclude expert testimony for explaining the Statements of Economic Interest. The Appellants had used the testimony of Normas Thomas, a private attorney to explain to the jury the complexity of the Statement of Economic Interest. The Court used the ratio of United States v Lespier [725 F.3d 437, 449 (4th Cir. 2013)], to hold that under Rule 702 of the Federal Rules of Evidence, expert testimony cannot be used to explain an issue which was self-explanatory. The operative part of Rule 702 lies in the fact that expert evidence is only admissible as long as it is helpful to the trier of the fact to understand an issue. In this case, the jurors did not require any special skill to understand the complexity of the Statement of Economic Interest. Hence, the Court upheld the lower court’s decision to exclude the expert evidence on this count.