Say Alice from the defense/prosecution meets with Bob, some expert/witness, to establish some "facts", and then Alice agrees to call Bob to the stand to establish those "facts" for the court.

But when called the the stand, Bob presents the opposite statements, and completely sabotage's everything Alice was banking on.

Are there any repercussions for Bob, or anything to dissuade him from doing this? Because if there are no repercussions, what's stopping Bob from going to the opposing side to accept money/a favour to do this for them?

Also, just to be clear, consider the topic to not be a "fact", but more so something that is highly contested/controversial. E.g. some "experts" may classify someone as insane, while others would classify the opposite, meaning there's no actual undisputed "fact" per se, and any "expert" is at liberty to think one side or the other.

As one final example about a bad-acting witness/expert, consider the following scenario: Alice wants to establish her client was suffering some malediction. What's stopping her from calling Dr. SnakeOil (or Dr. Oz) to testify as such? Does the expert have to be established as legit by going through some interview as if he were applying to a company?

3 Answers 3


The first thing to keep in mind is that, before appearing on the stand, an expert witness will have given a sworn deposition and delivered a written report of his or her findings. They could expect to be questioned about anything they say under oath that contradicts something else they said under oath. It’s not illegal for an expert to admit they were wrong before, or that there are other facts that support a different conclusion—indeed, a witness on the stand might be obligated to—but it might not do their professional reputation and their credibility with the jury any good, either.

In the U.S., the opposing counsel is generally allowed to contact a witness, and take a statement, but may not ask the witness to testify falsely or offer any inducement prohibited by law. (See Supreme Court Rule (SCR) 173/Model Rule (MR) 3.4.) It would certainly be illegal for a witness to take money from both parties in the case without informing them or the court, or to swear that whatever the highest bidder pays them to say is their expert opinion.

If this happened under direct examination, the lawyer would probably cut their losses by asking no further questions and getting the witness off the stand. Putting the witness on the stand would give the other side an opportunity to cross-examine them. Grilling your own expert as a hostile witness, even if the judge allowed it, would only make your position seem tendentious. Nothing stops an expert witness from giving testimony that is more helpful to the other side. They are witnesses, not lawyers, and their duty is to tell the truth, not to zealously advocate on behalf of some client.

  • Re the 1st paragraph: Meaning we don't really expect them to do the opposite because of some deposition or affidavit prior to their testimony?
    – BCLC
    Jun 14, 2022 at 13:20
  • @BCLC This also gives the other side a chance to read over, evaluate and rebut the expert’s testimony.
    – Davislor
    Jun 14, 2022 at 17:12

The question is based on a false premise, that one side or the other can proffer random "experts" to offer "facts" that bear on a trial. Instead, a person can only testify to an actual fact, and they have to be competent to testify to that fact. To take your example of insanity, a doctor would not be allowed testify that the defendant is insane, and if the doctor asserted "The defendant is insane!", the testimony would be stricken (the bell would have been rung, the jury would then be instructed to disregard the statement, and if a pattern of such inadmissible testimony developed, the judge could hold the doctor in contempt). However, neither side can e.g. sue the doctor for not testifying the way they wanted him to testify.

The parties will know more or less what the expert will say, which then allows the other side to object that the testimony is inadmissible. The doctor could testify to admissible facts about the defendant's behavior which could support (or not) a finding regarding insanity. Under cross-examination, facts could be brought out that were not in the written report, and that could seriously damage the testimony of the expert. There is no direct penalty attached to setting one side up for an embarassment. There could be a professional penalty (self-inflicted damage to reputation) if you suppress relevant facts, because both sides get to call witnesses, and the opposing witness is likely to be aware of / be able to independently find those facts.

  • 4
    This answer seems to disregard the difference between a fact witness and an expert witness. Anyone can testify to facts that they personally witnessed. Someone qualified as an expert witness can testify as to what conclusions they, in their expert opinion, drew from the fact they are aware of. So if a doctor is established as being an expert in mental health, then yes, they can testify that in their expert opinion, the defendant is insane. Jun 13, 2022 at 1:41
  • @Acccumulation whether the defendant is insane is a question of law. The doctor can offer expert opinion concerning facts that must be proven to establish a finding of insanity, notably the defendant's state of mind at the time of the crime.
    – phoog
    Jun 13, 2022 at 11:04
  • 1
    @phoog No, whether the defendant is insane is a question of fact, and is decided on by the trier of fact. Jun 13, 2022 at 14:44
  • I have seen too many transcripts of experts offering provably false facts.
    – Joshua
    Jun 13, 2022 at 17:30

A witness is not an advocate

A witness’ role in a court case is to state the facts (or opinion for expert witnesses) and not to argue for the case for either side - that’s the lawyers job. A witness that crosses into advocacy will be pulled up by the court and reminded what their role is.

While witnesses are described as for the prosecution/plaintiff or defence, that is just an identification of which party called them - it is not a suggestion that they are or should be on anybody’s side.

Expert witnesses must be approved as such by the court. The party wishing to call the expert will submit their qualifications and experience to the court and the court will determine if they are experts and the scope of their expertise: a civil engineer is not an electrical engineer and a oncologist is not an epidemiologist for example.

An expert’s report will detail the information they were given or collected, the assumptions they made, their conclusions and the reasoning that led them to those conclusions. The experts for the other side will critique the report and provide their own reports.

Expert conclaves are commonly held; the experts meet without the lawyers or the judge/arbitrator being present in order to narrow the range of the dispute and prepare a joint report. My instructions to my experts (who are usually engineers) when the conclave goes something like: “You’re engineers. That means you’re problem solvers. Solve the problem. And remember, you don’t work for the guy who’s paying you, you work for me.”

The difference between experts is rarely very great in any event.

Real law isn’t like they pretend on TV

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