By Rehana Vohra
Assistant District Attorney in Harris County
Remember that scene in My Cousin Vinny, where Marisa Tomei’s character, Mona Lisa Vito, owns the stand (and steals the show) as an expert witness? The prosecutor, Jim Trotter, challenges Ms. Vito’s qualifications as an expert in automobiles, thinking she’s just an “out-of-work hairdresser.” After the judge grants Mr. Trotter’s request to take Ms. Vito on voir dire, the fireworks begin.
Mr. Trotter asks Ms. Vito, “What would the correct ignition timing be on a 1955 Bel Air Chevrolet with a 327-cubic-inch engine and a four-barrel carburetor?” which Ms. Vito expertly explains is a trick question. “’Cause Chevy didn’t make a 327 in ’55—the 327 didn’t come out till ’62. And it wasn’t offered in the Bel Air with a four-barrel carb till ’64. However, in 1964, the correct ignition timing would be four degrees before top dead center.” Mr. Trotter has to sheepishly concede: “Well, uh, she’s acceptable, Your Honor.”
If every defense expert presented like Mona Lisa Vito, I would not feel the need to write this article. But the Mona Lisas of the courtroom are rare.
Challenging a defense expert’s qualifications to render opinions can be intimidating. However, it is absolutely necessary when their opinions mislead or wade into unreliable territory. Accredited laboratories that employ forensic analysts in Texas are beholden to regulatory entities such as the Texas Forensic Science Commission. However, private experts employed by the defense bar are often beholden to no one. Therefore, it is up to us as prosecutors to make sure junk science is not getting through the courthouse doors without testing it. To quote the Honorable Kevin Yeary in a recent concurring opinion: “Our adversarial system works better when the parties actually serve as adversaries.”[1] I could not agree more with Judge Yeary on this issue.
The basics
Texas Rule of Evidence 702 is a good place to start: “A witness who is qualified as an expert by knowledge, skill, experience, training, or education may testify in the form of an opinion or otherwise if the expert’s scientific, technical, or other specialized knowledge will help the trier of fact to understand the evidence or to determine a fact in issue.” The key takeaway in this rule is that the expert should help the judge or jury understand the evidence or determine a fact in issue. This is where a lot of the attack can occur from a rational standpoint. I’ve noticed that defense experts like to address issues that are common sense and do not require certain qualifications to explain to a judge or jury. For example, a psychologist for the defense in a child sexual assault case may want to tell the jury that children can lie. But a general statement such as this really does not require any specialized knowledge from a Ph.D. to explain or to understand.
Texas Rule of Evidence 705(c) further provides, “An expert’s opinion is inadmissible if the underlying facts or data do not provide a sufficient basis for the opinion.” In Vela v. State,[2] the Court of Criminal Appeals examined the appellate court’s holding that the trial court abused its discretion when it excluded the testimony of a defense expert, who was a “certified legal nurse consultant.” In a sexual assault case, this expert would have testified that if there is no physical evidence, then no rape occurred.[3] The Court of Criminal Appeals faulted the appellate court’s analysis related to the trial court’s determination of whether this testimony was reliable. On remand, the appellate court held that the trial court did not abuse its discretion when it held that this expert’s opinion was not reliable, and the appellate court affirmed the conviction.[4]
It is well-settled law, after Kelly v. State,[5] that scientific evidence must also meet three criteria to be reliable:
“(a) the underlying scientific theory must be valid;
“(b) the technique applying the theory must be valid; and
“(c) the technique must have been properly applied on the occasion in question.”
The Kelly Court went on to explain that a non-exhaustive list of factors that could affect a trial court’s determination of reliability included the following:
“1) the extent to which the underlying scientific theory and technique are accepted as valid by the relevant scientific community, if such a community can be ascertained;
“2) the qualifications of the expert(s) testifying;
“3) the existence of literature supporting or rejecting the underlying scientific theory and technique;
“4) the potential rate of error of the technique;
“5) the availability of other experts to test and evaluate the technique;
“6) the clarity with which the underlying scientific theory and technique can be explained to the court; and
“7) the experience and skill of the person(s) who applied the technique on the occasion in question.”[6]
A note, here, that Nenno v. State[7] recognized that the reliability inquiry may require a more flexible approach when evaluating the admissibility of a “soft science,” such as psychology, as opposed to a “hard science” (mathematics) under the Kelly standard above.
Texas Rules of Evidence 402 and 403 are the last pieces to consider in terms of your attack. The expert’s proposed testimony must still be relevant to the facts of the case. Even if the testimony is relevant, it may still not be admissible under 403 if the probative value of the evidence is substantially outweighed by a danger of one or more of the following: unfair prejudice, confusing the issues, misleading the jury, undue delay, or needlessly presenting cumulative evidence.
How to prepare
I once encountered a software engineer attempting to give his opinion about the unreliability of forensic DNA testing in a case. He had never worked in a forensic lab, nor had he analyzed DNA before. These are the types of defense experts we should not be afraid to challenge, even if that person is ultimately permitted to testify in your case. If you go in with disqualification on the brain and lose the battle, you can still win the war in cross-examination because you will have already discovered the weaknesses and limitations of their opinions.
Once you receive notice from the defense attorney that he intends to use an expert at trial, ask him for an updated copy of the expert’s curriculum vitae or résumé. He may say no, which is fine (and that could be a part of your cross-examination later). But if he produces it to you or you find it publicly available on your own, spend time reading it from top to bottom. Look up the expert’s published articles and actually read them. Look up her presentations and study them. Search for her on YouTube giving talks and watch how she presents herself. You can learn a lot about an expert by carefully studying and researching her professional works. You may realize that she’s been working on her master’s degree for more than 10 years (that’s a long time). You may discover that even though she is critiquing your medical examiner’s work, she has not conducted an autopsy in 15 years. You may find that she is not actually qualified in the area in which the defense attorney seeks to use her. You may see that she authored an unhinged piece of work for some wacky publication. You may also find out that she did little to no work on the articles she is credited on. If the expert’s name appears anywhere other than the first author listed on a publication, you should find out what she actually contributed to the publication itself. The first author is typically the one who made the most significant contribution, and the level of contribution decreases with each successive author listed.
Maybe it’s just me, but I really enjoy reading prior testimony from defense experts as a part of my preparation. You should promptly enter the expert’s name in Westlaw or LexisNexis and see whether (and when and where) she has testified previously. You can find out if her testimony has ever been excluded by a court (and that should also be a question you ask during your voir dire examination of the expert). You can ask other jurisdictions to share a record with you based on your searches. By reading prior testimony, you get to study how the expert answers questions. You might also quickly figure out if she is going to be antagonistic or not. You get to see how she comes across on a record. You also get to see how your peers frame questions and what points they make on cross-examination, which is helpful for when it’s your turn. You can learn from their mistakes, but you can also borrow from their questioning. Even if you don’t learn anything that you can use substantively in your case, you will at least learn about the expert’s courtroom persona and how she operates under questioning by both sides.
To prepare to effectively voir dire the defense expert, you should outline your questions. Your research about the expert will help structure them in a flow that is persuasive to the fact-finder. I find that getting the expert witness in a rhythm of saying “no” to questions about her qualifications is an effective way to present a case for disqualification. For example, if you know that the expert has not reviewed your case, you can structure a line of questioning that will result in the witness having to say “no” to your questions about whether she reviewed a litany of important evidence in your case.
Also during your voir dire examination, try to nail down whether any written materials have been produced by the expert in connection with your case. Once your questioning establishes that this expert has notes or a report, ask the court to order the expert to produce it to you under Texas Rule of Evidence 705(a) and (b). Beware that some seasoned defense experts know not to write reports or take notes because they realize this is discoverable information by the State. If they don’t have anything to produce or say that they “forgot the file at the office,” you can still use their lack of note-taking or report-writing to demonstrate incompetence, incredibility, or evasiveness.
Going the extra mile
Remember that you cannot communicate with a member of the defense team, including any of their experts, without permission from defense counsel.[8] But consider asking the defense attorney if you can talk to the expert before trial. On two different occasions, I wanted to talk to the defense expert, and I was glad the two different defense attorneys granted that permission. Now, this happened for me in a post-conviction context, but it can translate pretrial as well. I can tell you that it was very helpful for me to speak to these two experts beforehand. One was a forensic pathologist who seemed legitimate, and I genuinely wanted to understand the position she had taken on a case. When we spoke, her opinion was a lot more watered-down than I expected. I was satisfied that she was not going to do any harm to my case.
The other expert was a psychologist whom I knew to do credible work. I also knew that he was frequently used by the State on competency and sanity issues. In my case, the defense was using this psychologist to support the theory that the defendant had been diagnosed with Factitious Disorder Imposed on Another (FDIA), better known as Munchausen Syndrome by Proxy. After I spoke to the psychologist, he was able to level with me on some of his opinions. This discussion provided a basis for me to later argue to the court that his testimony was a double-edged sword for the defendant.
Keep in mind that you don’t want to do this on every case because you don’t want to waste your time. My instincts told me that these two experts might be open and honest with me based on my prior research of their work, and I am thankful it paid off in both cases.
Conclusion
One of the best parts of being a prosecutor is knowing that all you have to do is the right thing. There’s a sense of internal peace and freedom when this is our one and only job—we are beholden to the truth and to justice. So while it may seem daunting to challenge a defense expert in her area of expertise, take comfort in knowing that you have the facts and the evidence on your side. Let that be your guide.
[1] Ex Parte Horvath, No. WR-88,478-01, 2025 WL 1699335, n.2 (Tex. Crim. App. June 18, 2025) (Yeary, J., concurring, Parker, J., joining).
[2] 209 S.W.3d 128 (Tex. Crim. App. 2006).
[3] Id.
[4] Vela v. State, 251 S.W.3d 794 (Tex. App.—Corpus Christi–Edinburg 2008).
[5] 824 S.W.2d 568, 573 (Tex. Crim. App. 1992).
[6] Id.
[7] 970 S.W.2d 549, 560-61 (Tex. Crim. App. 1998).
[8] Tex. Disciplinary Rules Prof’l Conduct R. 4.02 (b).