Robert S. Litt is a former federal prosecutor and Justice Department official.

At the Sept. 27 Senate Judiciary Committee hearing, both Judge Brett M. Kavanaugh and research psychologist Christine Blasey Ford expressed “100 percent” confidence in their version of events concerning her allegations of sexual assault. Both versions cannot be true, of course, and some have characterized the matter as a “he said, she said” standoff that cannot be resolved.

Yet juries in civil and criminal cases are asked every day to resolve similar conflicts, aided by instructions from judges about how to evaluate witnesses’ credibility. Those instructions could provide helpful guidance for senators seeking to assess evidence regarding Kavanaugh’s past.

A treatise commonly consulted by judges, “Modern Federal Jury Instructions,” written by Judge Leonard B. Sand (who died in 2016) and other legal experts, tells jurors in civil cases: “Everything a witness said or did on the witness stand counts in your determination. How did the witness impress you? Did he or she appear to be frank, forthright, and candid, or evasive and edgy as if hiding something? How did the witness appear; what was his or her demeanor — that is, his or her carriage, behavior, bearing, manner, and appearance while testifying? Often it is not what a person says but how he or she says it that moves us.”

The instructions in criminal cases are similar and end by asking jurors to consider, “Did the witness strike you as someone who was trying to report his or her knowledge accurately?” Like jurors, senators assessing the credibility of Ford and Kavanaugh could begin by contemplating their demeanor when testifying.

Jurors are also instructed to “consider any bias or hostility the witness may have shown for or against any party as well as any interest the witness has in the outcome of the case.” This factor is fleshed out in additional instructions: Jurors should consider that “evidence that a witness may benefit in some way from the outcome of the case” creates “a motive to testify falsely” that “may sway a witness to testify in a way that advances his own interests.” Then again, “it does not automatically follow that testimony given by an interested witness is to be disbelieved” because “many people,” regardless of their interest in the outcome of the case, “would not testify falsely.”

Senators could likewise consider whether a witness has a motive to testify falsely or to shade the truth, and, if so, whether that motive affected the witness’s testimony.

The model instructions also caution jurors to “consider whether a witness had an opportunity to observe the facts he or she testified about.” While this principle might not seem to be relevant in judging the credibility of Kavanaugh’s and Ford’s conflicting testimony, it might help senators evaluate the weight to be given to the testimony of other witnesses.

Finally, jurors are instructed to consider “whether the witness’s recollection of the facts stands up in light of the other evidence in the case” — in other words, whether other evidence corroborates it, contradicts it, or is neutral. In particular, the model jury instructions give guidance on how to assess inconsistencies in a witness’s statements or conflicts with the statements of others.

In civil cases, for example, jurors are told that “evidence of discrepancies may be a basis to disbelieve a witness’s testimony. On the other hand, discrepancies in a witness’s testimony or between his or her testimony and that of others do not necessarily mean that the witness’s entire testimony should be disbelieved.”

The instructions go on to note that people “sometimes forget things and even a truthful witness may be nervous and contradict himself” and that “two people witnessing an event will see or hear it differently.” Jurors are therefore told to assess whether a discrepancy “pertains to a fact of importance or only to a trivial detail,” but should bear in mind that “a willful falsehood always is a matter of importance and should be considered seriously.”

The task facing senators is ultimately no more difficult than that which confronts ordinary citizens in courtrooms, although the stakes for the country are great. In the words of the model instructions, senators as well as jurors should use their common sense to try “to size up a person” just as they “would in any important matter.” In doing so, senators could profit from following the accumulated wisdom of decades of judicial experience.

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