Rarely if ever will the cross-examining attorney know more about a subject than the expert witness being confronted.  Indeed, that is why discovery rules mandate pre-trial disclosure of expert reports – “They allow attorneys, not experts in the fields at issue, to prepare intelligently for trial and to solicit the views of other experts…”  Metavante Corp. v. Emigrant Sav. Bank, 619 F.3d 748, 762 (7th Cir. 2010).  So a strategy of beating the expert at her own game is not a sure path to success; it may bore or otherwise lose the jury, and it risks letting the expert explain.

Nonetheless, an expert’s claims can be checked, challenged and undercut in a number of ways.  Beyond the impeachment methods available for all witnesses – inconsistent statements, dishonest character, contrary information [with experts, often via a learned treatise] – there are expert-specific lines of inquiry.  What is essential is that opposing counsel develop the list of potential attacks, test them pre-trial to ensure their applicability, and then organize the cross-examination to maximize their utility.

Let’s begin with the challenge to credentials.  Because the standard for qualifying an expert is exceptionally lax – according to one court, it is “whether the witness has  any reasonable pretension to specialized knowledge  on the subject under investigation” – the likelihood of exclusion is low.  But the proponent of the expert has engaged in permissible bolstering, and the qualifications voir dire offers the chance to take the witness down a notch or two in the eyes of jurors.

In the qualifications arena, the following subjects are key to any cross-examiner’s checklist:

  • Limited practical experience
  • Missing credentials (e.g., a lack of Board certification)
  • Inflated credentials

This last point warrants amplification.  An expert may be a member, or even an official, of a “professional” organization – but investigation may reveal that membership comes from an application and a fee, and not based on any performance standard or testing.  For an extreme example, see The Emperor of Junk Science Forensics Has Died,, (last visited September 23, 2018).

Related to the problem of credentials, at least as to the weight of the expert’s testimony, is that of bias.  Bias comes in many forms, some going to the witness’ credentials and others to the shaping of the actual opinion(s), with admittedly some overlap.  In the bias area, the following subjects warrant exploration:

  • Fees (assuming the fee paid to the opponent’s expert is disproportionate to that paid for the party’s mown expert)
  • The relationship of this witness to counsel, as when this witness has been used repeatedly by the same counsel or lawfirm
  • What Stephen Lubet has denominated “positional bias,” the loyalty of the witness to a particular side or the witness’ willingness to only find one [the same] conclusion over a range of cases (See Lubet, MODERN TRIAL ADVOCACY, §

There are at least two other forms of subconscious biasing that must be evaluated, although developing a cross on either may be difficult.   The first arises merely by virtue of who hired the expert, as research has shown that there is a phenomenon of “adversarial bias” where experts begin to align their expectations and conclusions dependent on which party hired them.  As elaborated in one research paper, “working for one side in an adversarial case causes some experts’ opinions to drift toward the party retaining their services, even on ostensibly objective instruments and procedures…”  Adversarial Allegiance Among Expert Witnesses, (last visited September 23, 2018).

And the second?  It arises from “domain irrelevant information,” and occurs when an expert is given information extraneous to the task but impactful on the ‘lens’ through which the expert views the evidence.  Telling a fingerprint expert that the suspect confessed offers no useful information in how to compare two prints but has been shown to affect judgment in cases where the prints are ambiguous or unclear.  See, e.g. Dror et al, Cognitive Bias and Its Impact on Expert Witnesses and the Court, (last visited September 23, 2018).  This may be shown by demonstrating what the expert was asked [or “told”] to look for; and then have the expert concede that certain information provided is not part of the normal decision-making process in the particular discipline.

Making the opposing expert yours is a critical part of any cross.  The opposing expert may be willing to validate your expert’s

  • credentials
  • methodology
  • sources of data
  • conclusions at least in part

Making the expert one’s own is desirable and should precede any attack.   But attack is often the primary focus of the cross-examination.  A preliminary topic may be to show the limits of the discipline, even where it has survived a challenge under Frye, Daubert, or 702 principles.  Here, questioning may track the admissibility criteria of Daubert.  Questioning may highlight any of the following:

  • Testability (or the lack thereof)
  • Peer Review/Publication (again, the lack thereof)
  • Error Rate (here, the lack of one, or a problematic error rate)
  • Existence of Standards (again, the lack of standards from examiner to examiner)
  • General Acceptance (again, the lack thereof or substantial challenges to the theory or method)

Beyond an attack on the discipline are several potential ones on the expert’s approach and conclusions.  One such attack is premised on assumptions.  Where the adverse expert’s conclusions are premised on certain assumptions, a skillful cross can begin with an agreement that the quality of the opinion depends on the correctness of the assumptions.  Here, the list of potential cross-examination points includes:

  • the danger for experts of relying on assumptions
  • assumptions made in this case
  • the concession that if the assumed facts are wrong then the opinion might be wrong as well

Where a pre-trial deposition has occurred, counsel may already have secured the concession that under a different factual scenario – i.e.¸ where the assumptions are different – the conclusion changes and becomes one favorable to the cross-examiner.

Another area for inquiry may be dubbed the “more” category.  Here, the cross-examination focuses on the limits imposed on the expert, whether due to finances, time or the inadequacy of the materials to be reviewed or the testing options available.  Questions may focus on:

  • More data would have been better because…
  • More tests would have been better because

Finally, the expert may be pressed on what the evidence does not show.  By way of example, a fingerprint expert will have to concede, in most cases, that how, why or when a latent print was left at a crime scene is unknown.  Such concessions may dovetail with the expert having to agree that the evidence could have been left in a manner consistent with the cross-examiner’s theory of the case.

Will every one of these challenges be available in a particular case?  The answer is almost certainly “no.”  But unless each case is screened against these criteria, the cross-examiner may be missing essential grounds for undercutting the opposing expert.