Some courtroom lines never die. More than thirty years later, everybody still knows the glove, the struggle to pull it on, and the rhyme that went with it — “If it doesn’t fit, you must acquit.” Here’s the thing, though. Long before anyone was trying on gloves in Los Angeles, the Third Circuit’s Judge Becker had come up with “fit,” and the Supreme Court adopted it in Daubert. Scientific validity for one purpose is not necessarily scientific validity for another. A study of one population, one exposure, and one outcome doesn’t automatically prove anything about a different population, a different exposure, or a different outcome. If the science doesn’t fit the plaintiff, the expert doesn’t get to the jury. Or, updating the rhyme for Rule 702 — if the study doesn’t fit, you must exclude it.
Today’s case isn’t a drug or device case. It comes out of the Deepwater Horizon cleanup. But it’s a published Fifth Circuit decision affirming exclusion of plaintiff’s causation experts under Rule 702, and affirming summary judgment, by applying what the court called “widely accepted toxic tort causation principles.” DuFour v. BP Exploration & Production, Inc., ___ F.4th ___, 2026 WL 3215980, at *4 (5th Cir. Oct. 27, 2026). Toxic tort causation is our neighborhood, so we’re going to walk through it.
Plaintiff alleges that his exposure to toxic chemicals as a cleanup worker caused chronic worsening of his asthma. Id. at *1. Note the word “worsening.” Plaintiff had asthma long before the spill, going back to childhood. Id. at *7. That fact is important. To prove causation, plaintiff offered two experts, an epidemiologist and one of his treating pulmonologists. The district court denied his request for a Rule 702 evidentiary hearing, excluded both experts, and granted summary judgment. Id. The Fifth Circuit affirmed.
The parties agreed that general maritime law governed. That raised the question of what causation looks like in a maritime toxic exposure case. Plaintiff made it easy. He conceded that “[w]ithout an expert, [he] could not prove the central issue in this litigation – the level, extent, and duration of his exposure to harmful substances.” Id. at *2. He also agreed he had to prove “(1) the scientific knowledge of the harmful level of exposure to a particular chemical, and (2) that he was exposed to such quantities.” Id. That’s dose. That’s the ballgame in any toxic tort case. That, plus lots of precedent, made it easy for the court to find that toxic exposure injuries with multiple possible causes require expert testimony. Id. at *3. The court then held that the general-causation and specific-causation framework that has governed non-maritime toxic torts for decades is part of maritime law, either because enough decisions have established it or because the federal courts have created that rule. Id. at *4.
Plaintiff’s first argument was that the district court abused its discretion by not holding a live evidentiary hearing. His theory was that the case was scientifically complex and that his experts deserved a chance to explain away whatever deficiencies the court saw. Id. at *5. The Fifth Circuit disagreed. Trial courts have “considerable leeway” in deciding how to assess reliability. The district court had thorough briefing, depositions, declarations, and expert reports. It also had the benefit of its familiarity with the Deepwater Horizon litigation. That was enough. Id. at *6. A Rule 702 hearing isn’t a second chance to write the report you should have written the first time. Plaintiffs don’t get unlimited mulligans.
The court began its Rule 702 analysis with dose-response. It noted that even if Mississippi law applied instead of maritime law (remember that point when we get to the dissent), the Mississippi Supreme Court has affirmed exclusion of an expert whose studies did not provide a dose-response relationship that allowed the plaintiff’s exposure to be compared with the exposures of study subjects. Id. at *6 (discussing Watts v. Radiator Specialty Co., 990 So. 2d 143 (Miss. 2008)).
Expert No. 1: Plaintiff’s epidemiologist relied on three studies. They didn’t examine the same exposure-disease relationship. They didn’t address the same conditions. And none of them “appear[ed] to address the specific worsening of preexisting chronic asthma that is the central issue in [plaintiff’s] case.” Id. They weren’t even based on the same exposures. One focused on inhalation; the other two didn’t distinguish between exposure routes. Id.
The main study, referred to as the Lawrence Study, had two problems, either of which would have been enough. First, the expert himself acknowledged possible “misclassification and over reporting of asthma due to the utilization of self-reported wheezing in the case definition.” When self-reported wheezing was removed from the definition, the results were “largely attenuated.” Id. at *7. So, the association depended heavily on people saying they wheezed.
Second, and more importantly, the study excluded anyone who reported an asthma diagnosis before the spill. Plaintiff had asthma since childhood, so he would have been excluded. As the district court put it, and the Fifth Circuit agreed, that “demonstrates that he is not on common footing with any participant in that Study, and as such [the expert] did not reliably apply the Lawrence Study to this case.” Id. That’s fit in its purest form. The study was built for someone else’s hand.
Then there was the problem of symptoms versus disease. The dose-response the expert described concerned “the reporting of symptoms, not the diagnosis or worsening of asthma or any other chronic conditions.” Id. The expert conceded that wheezing and shortness of breath are symptoms of asthma but “they’re not specifically asthma worsening,” and that “not everybody who wheezes or has shortness of breath, et cetera, has asthma.” Id.
Next, statistical significance. Plaintiff (and the dissent) leaned hard on the study’s p-value. However, the fact that results are statistically significant “says nothing about the magnitude of any association . . . or about the biological, clinical, or public health importance of the finding.” Id. Significance “cannot make up for” the study’s other limitations. Id. at *8. The study did report a relative risk above 2.0 for some categories of workers, but the expert never tried to show that plaintiff would have fallen into any of those categories, even setting aside that the study excluded him. Id. A statistically significant result for a group you’re not in tells the jury nothing about you.
The bottom line — the expert offered no “reasonable basis for using respiratory symptoms alone as a proxy for chronic asthma.” Jumping from increased respiratory symptoms to the conclusion that the cleanup was a substantial factor in worsening plaintiff’s asthma “constitutes ipse dixit testimony and leaves too great of an analytical gap between the data and his causation opinion.” Id. Not an abuse of discretion. Too big a leap.
Expert No. 2: Plaintiff’s treating pulmonologist didn’t prepare an expert report. The district court ruled that a treater who doesn’t provide a report is limited to the facts and opinions in his disclosed medical records, meaning what he actually recorded while treating the patient. If those records don’t say plaintiff’s condition was caused by a particular substance, the treater can’t offer that causation opinion later in litigation. Id. at *8. Plaintiff didn’t rely on the records. He tried to use causation opinions the treater gave at his deposition, which the district court held “would plainly circumvent Rule 26’s written report requirement.” Id. (The dissent points to a chart note saying plaintiff “may have” work-related asthma. Id. at *10 (dissent). “May have” is something a doctor writes down to look into, not an opinion that something more likely than not caused the injury.) Whether treaters must file reports for opinions formed for litigation is not a settled issue in the Fifth Circuit, so the appellate court affirmed the exclusion on other grounds. Id. at *8 n.69.
Those other grounds were solid. Even if the opinions had been properly disclosed, nothing in the record showed the treater was qualified to opine on what the substances at issue do to the human body, in general or to plaintiff specifically. Id. at *9. He offered no explanation for ruling out other causes. He relied on timing alone. He admitted he didn’t know much about the substances involved and did no research into what might have worsened plaintiff’s asthma. Id. That’s ipse dixit again. With both experts out, summary judgment followed.
There was a dissent, so we’ll address it, briefly. The dissent’s central theory is that “[w]e cannot uphold the district court’s Rule 702 ruling without first identifying the relevant substantive law. That is because Federal Rule of Evidence 702 and Daubert depend on the substantive law applicable to the plaintiff’s claim.” Id. at *10. Then comes a hypothetical about a serial spider-bite victim, from which the dissent concludes that “[d]istrict courts must reach different Daubert rulings when applying State A’s law and State B’s law to factually identical spider-bite suits.” Id. at *12. And because the dissent says we don’t know which law governs here, it would not uphold the exclusion. Id.
We don’t buy it. Rule 702 shouldn’t mean different things in different federal courtrooms, whether the court is sitting in admiralty or in diversity. Here’s how the division of labor actually works. State substantive law decides what must be proved. Rule 702 decides admissibility. State law sets the elements, the causation standard, whether expert proof is required, any degree-of-certainty requirement, and whether the evidence is sufficient. Federal law decides whether an expert’s opinion is admissible in the first place. Which state’s law governs doesn’t change how Rule 702 is applied.
The text of the Federal Rules backs this up. When the drafters wanted state law to control an evidentiary question in civil cases, they said so expressly: Rule 302 (presumptions), Rule 501 (privilege), and Rule 601 (witness competency) all defer to state law where state law supplies the rule of decision. Rule 702 has no such clause. It’s a uniform federal standard that should be applied uniformly.
Yes, “fit” asks whether the testimony will help determine “a fact in issue,” and the substantive law identifies those facts. But the fact in issue here was whether cleanup exposure worsened plaintiff’s preexisting asthma. That’s the same question under maritime law, Mississippi law, or any other law we can think of. No jurisdiction makes causation optional, and plaintiff conceded he needed expert proof of dose. Whether an expert reliably connected his data to that fact doesn’t depend on which body of tort law supplies the rule of decision. And as the majority pointed out, Mississippi applies Daubert-based dose-response principles anyway. Id. at *6. The choice-of-law question the dissent raises makes no difference to the result.
The dissent also complains that no court has ever held “an expert cannot rely on scientific studies unless the plaintiff himself qualified as a subject of the study.” Id. at *13 (dissent). That isn’t what the court held. The holding was that the expert never bridged the gap between the population the study examined and the plaintiff in the case, and never even tried to place plaintiff in the categories that showed elevated risk. Id. at *8. That’s a Rule 702(d) reliable-application failure, plain and simple. Finally, the dissent called the new Reference Manual “biased and scandal-ridden.” Id. The majority’s answer was low-key. The quoted passage on statistical significance is nearly identical to language in the “uncontroversial third edition,” and nobody has criticized either edition’s treatment of statistics. Id. at *7 n.67. Whatever anyone thinks of the fourth edition, the principle that statistical significance isn’t the same thing as relevance is basic statistics.
Just to recap the highlights of DuFour applicable to drug and device cases: (i) a Rule 702 hearing isn’t required when the record is fully developed; (ii) an expert’s studies must address the same exposure, the same outcome, and a population the plaintiff actually belongs to; (iii) symptoms are not a stand-in for disease without a reliable scientific bridge; (iv) statistical significance can’t rescue a study that answers the wrong question; and (v) a treater who skips the report is limited to what’s in the chart, and a timeline is not a methodology. All of that applies whenever a plaintiff’s expert offers a study of somebody else’s disease in somebody else’s population.
Back to the glove. That jury was told that if it didn’t fit, they had to acquit. Here, the district court tried plaintiff’s science on and found it was made for a different hand. A different population, a different outcome, a different question. The Fifth Circuit agreed. The study didn’t fit, so it was excluded.
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