In court, on impeaching expert testimony: Imagine that Einstein is still alive, that he hadn't done anything since he actually died, that he was never famous (so no one in a modern courtroom except for two lawyers knows who he is), and that the world's knowledge of relativity had not changed since he actually died. At someone else's trial, he testifies on relativity as an expert. Answering questions, he explains both his special and general theories in terms a jury understands. Counsel thanks him. Opposing counsel in cross-examination asks Einstein if his theory includes a "cosmological constant". Einstein testifies firmly: "No." Opposing counsel reads to the court from an earlier publication by Einstein that there is a cosmological constant. Einstein seems urgently about to say something but the judge reminds him that he may only answer questions he is asked and no questions are pending at the moment. Opposing counsel has no further questions, counsel who brought Einstein does not seek to ask anything (perhaps because not expecting this turn of events) and the witness is dismissed. Someone searching history would find that Einstein had publicly said that the cosmological constant was the "biggest blunder of his life", but a jury is not allowed to do outside research before issuing its verdict and this is not a nonjury trial. I understand that impeachment frequently works to persuade a trier of fact to disregard impeached testimony; assume such persuasiveness succeeds in this case.

I think this illustrates an inadequacy of the system of determining the state of scholarship through expert testimony, among other reasons because it discourages bringing an expert who might ever have published a statement they would later dispute, and that would exclude most research scholars with long publication histories. Am I legally wrong about this hypothetical Einstein being easily impeachable? Does this vary much by jurisdiction of place?

I'm not opening a debate on reform, just trying to see if I misunderstand the general law.

2 Answers 2


Your main misunderstanding is that opposing counsel cannot testify. He can ask a question, such as "Did you say ...?", which provides Einstein an opportunity to answer in a way that maintains the credibility of his testimony, but consel cannot just enter his own testimony into the record. In addition, counsel on "his side" has the opportunity to pose appropriate clarifying questions, e.g. could ask "Did you repudiate that idea 88 years ago?", though there is a problem that the attorney may not have the technical expertise to recognize the subject-matter question that they should ask.

Expert testimony does not attempt to determine the state of scholarship in an area, it attempts to resolve a specific factual matter. The two are not the same thing. Einstein would not testify as to what his general and special theories of relativity say, except in a bizarre universe where some person is accused of the crime of denying the theory of relativity. Instead, he might be testifying for the defense, where the defendant is accused of an act that (it turns out) is physically impossible in light of the theory of relativity. If Einstein had been in the habit of posing random mutually-contradictory conjectures on a weekly basis, opposing counsel's question could be relevant to establishing that the witness's testimony is not reliable. Theoretically, counsel could "hint" that the witness is scientifically flighty, so Einstein would have to have a credible explanation for changing his position. It is easy to demonstrate that scientists in general do change their positions as knowledge expands, so Einstein's burden would not be very substantial.

  • Counsel don't testify but, I read, in at least 1 jurisdiction opposing counsel can read into the record an impeaching document (maybe after "Is this your name on this article?"), even if 1st counsel didn't prep, didn't know the expert's history, and can't redirect. I understand an expert can say enough subject background to help a jury find on facts. Couldn't an ordinary case leave a sub-Einstein expert witness impeached without the witness free to speak up unless 1st counsel knew what to ask?
    – Nick
    Jul 16, 2019 at 23:59

Impeaching the witness means that the witness gives an answer that contradicts previous testimony or other sworn statements. The process you are outlining is called Voir Dire (Voir rhymes withe "Spa", Dire is pronounced like "Deer:). Voir Dire is used to establish a witnesses has credibility to speak on a subject matter to give expert testimony (fancy way of saying "opinion". Eye witnesses can only give factual testimony of events witnessed. Expert Witnesses can testify to opinions based on alaysis in subject matter areas. An eye witness can testify that the defendant fired a gun. An expert can testify the caliber of the gun and the casing matched the bullet found at the scene and that the description of the gun matches the eye witnesses' description of the gun).

So the first problem here is the council Voir Diring Einstein would have to have a relavant reason to the case for invoking a "Cosmological Constant" and why one point having believed it and then disavowed it would be relevent to Einstein's expert testimony. In another famous example, and one that doesn't involve physics that a lay person doesn't understand, in one book, the character Watson reveals that among the things that Sherlock Holmes does not know are matters relating to astronomy and surprisingly does not know that the Earth revolves around the Sun! Of course, when poking fun at Sherlock for not knowing basic common knowlege, Sherlock Holmes points out he doesn't want to know because it's useless to him in solving crime mysteriories and he would only care to know it if it became relevant. As he points out, he has yet to encounter a serial murder or a kidnapper who used heliocentric sciences to conceal a crime in London but knowing the soil composition of all of London's districts is vital because criminals tend to touch dirt.

Opposing council may not know what the hell this cosmological constant thing is, but he should know enough about trial law to shout "Objection: Relevancy" which would cause the Judge to ask the Voir Diring council to explain the importance and allow opposing council to formulate questions to ask Einstein about whether he feels the flip-flop is relevant. The film "My Cousin Vinny" hinges on the Voir Dire of an expert witness and even shows the Voir Diring council slip in a trick question and the witness testifying to why the question is "bull shit" in an expert opinion. I highly recommend a watch (there is a required Language Warning.). I'd say go watch the scene, but considering it's the climax of the film you need to watch the whole film to understand the situation.

  • This is hypothetical and I didn't want to write voluminous assumptions. I presumed relevance. Counsel may be smart enough to fill in on the spot but suppose the issue would need counsel to have prepared before a trial date, for example by buying a contradictory text at a bookstore (useful for either attorney).
    – Nick
    Jul 17, 2019 at 0:12
  • @Nick: Lawyers will almost never ask a question they do not know the answer too. This is actually on display in the "My Cousin Vinny" but is more subtle. Not only does Vinny know what the picture is telling him, but he knows his witness will also notice this and realize the same thing... not only that, he phrases the question in a way that will entice the witness to want to answer his question (she is also his fiance who at this point is pissed at him. So he asks if the picture shows that "the defense's case hold water?" The answer that he's wrong is exactly the answer he wants.
    – hszmv
    Nov 19, 2020 at 17:26

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