In the “Daubert trilogy,” Rule 702 spawned three children, all special in their own way. The firstborn, Daubert v. Merrell Dow Pharms., Inc., 509 U.S. 579 (1993), naturally receives most of the attention, being the pioneer. The middle child, General Elec. Co. v. Joiner, 522 U.S. 136 (1997), tends to be comparatively underappreciated in the shadow of its predecessor. Kumho Tire Co., Ltd. v. Carmichael, 526 U.S. 137 (1999), the youngest, generally receives the least attention.

Daubert’s broad pronouncements about gatekeeping principles dominate the Rule 702 landscape. No one calls a motion to exclude a “Joiner motion”; no one participates in a “Kumho hearing.” But in the broad wake of Daubert, Joiner played a particularly important and multifaceted role in shaping the ongoing development of Rule 702 jurisprudence. Its influence is worth revisiting.

Standard of Review

Joiner is celebrated primarily for its unanimous holding that the gatekeeping decisions of district courts must receive deference on appeal. As every appellate lawyer knows, the standard of review on appeal is analogous in importance to the burden of proof at trial and the standards for evaluating dispositive motions.

The usual standard for reviewing evidentiary rulings is the “deferential abuse of discretion” standard. In Joiner, the Eleventh Circuit applied “a particularly stringent standard of review” because the exclusion of expert testimony was “outcome-determinative” – it had resulted in an absence of proof of causation and a summary judgment. The U.S. Supreme Court reversed, holding that ordinary deferential review must be applied, whether or not the ruling dooms the claim or case.

And that’s important, but the Court went slightly, but significantly, further. It also held unequivocally that deferential review applies equally across the board to decisions both to admit and to exclude expert testimony. The Court rejected the argument that because the Federal Rules of Evidence generally “display a preference for admissibility” the exclusion of expert testimony is disfavored and justifies a comparatively “harder look” on appeal. Rather, the courts of appeal “may not categorically distinguish between rulings allowing expert testimony and rulings that disallow it.”

By destigmatizing exclusion rulings and limiting the district courts’ fear of reversal, Joiner facilitated and normalized the exclusion of overreaching experts. The district courts were freed, as a practical matter, to make tough decisions to exclude unsupported expert opinions. That freedom has been critical to the development of Rule 702 jurisprudence.

The importance of Joiner in this regard is highlighted in the breach. For example, the Ninth Circuit has systematically circumvented Joiner, rigorously reviewing and often reversing exclusions. See, e.g., Messick v. Novartis Pharms. Corp., 747 F.3d 1193 (9th Cir. 2014); Wendell v. GlaxoSmithKline LLC, 858 F.3d 1227 (9th Cir. 2017). The message to district courts in the Ninth Circuit, antithetical to both the letter and spirit of Joiner, is that they exclude expert testimony at their peril. See In re Roundup Products Liability Litigation, 390 F.Supp.3d 1102, 1112-13 (N.D. Cal. 2018). That’s a longer story for another day, and another post, but it demonstrates how the standard of review can operate to shape the decisions of the lower courts.

Beyond appellate procedure, Joiner has significantly influenced the way Daubert motions substantively are argued, evaluated and decided.

The Scope of Gatekeeping – Foundation and Reasoning

Joiner also strengthened Rule 702 by authorizing review of more than a qualified expert’s methodology. The Joiners claimed the only issue for the district court was whether their experts had relied on appropriate types of studies, i.e., animal studies and epidemiological studies, to reach their causation conclusions. The Supreme Court found this approach too narrow and general; rather, the question was whether the experts’ conclusions were sufficiently supported by the studies. Joiner therefore established that gatekeeping included examination of the expert’s factual support and chain of reasoning.

Joiner confirmed that under Rule 702 and Daubert an expert’s reasoning and foundation needs to hang together uninterrupted by any leaps of faith or logic, and be adequately supported at “every step.” Joiner memorably conscripted judges to hunt down impermissible “ipse dixits” and “analytical gaps” between the data and the expert’s conclusion. Daubert had suggested that the focus of gatekeeping should be the expert’s methods and not their conclusions. Joiner effectively broadened the focus, authorizing scrutiny into whether the conclusions were reasonably supported by the science.

This enlargement of the mission was later reflected in amendments to Rule 702, inquiring whether the opinion testimony was based on sufficient facts or data and whether reliable principles and methods were reliably applied to the facts.

Gatekeeping Rigor

Joiner also instructively modeled the rigor expected of district courts in addressing objections under Rule 702. As discussed below, the Supreme Court dug deeply into the details of the cited studies and how the experts employed them to reach general causation conclusions. And two years later, in Kumho, the Court elegantly described the rigor gatekeepers should require of experts – the courts should admit expert testimony only if the expert has “employ[ed] in the courtroom the same level of intellectual rigor that characterizes the practice of an expert in the relevant field.” The interrelationship is exquisitely symmetrical – it takes rigor to enforce rigor.

Toxicology 101 and the Limits of Extrapolation

As noted, in reviewing the district court’s exercise of discretion in excluding the causation opinions, the Court recognized several scientific principles important to the assessment of such testimony in toxic tort cases:

• When an expert relies on scientific studies, the court should examine the studies both individually and in combination to determine whether their causal conclusion is sufficiently supported.

• The court may reject extrapolation from animal studies to infer human causality based on dissimilarities between the study and the exposure circumstances and medical endpoint at issue. The district court could reject opinions based on animal studies injecting high doses directly into the lining of the abdominal cavity in infant mice resulting in one type of cancer where the adult plaintiff developed a different cancer after low dose dermal exposure. Joiner recognized that the courts may reject reliance on studies based on material differences in species, dose, route of exposure and endpoint, absent a scientifically supportable justification for the expert’s extrapolation, based on lack of “fit” and on excessive analytical

• Joiner also recognized that ordinarily experts may not read studies more broadly than the authors, and cannot draw conclusions regarding substances not addressed, or not isolated, in the studies. As Daubert recognized, scientists are ordinarily conservative in reaching conclusions. They fundamentally require specificity in attaching specific consequences to specific types of exposures to specific types of substances.

Over the past two or so decades, each of these propositions has figured prominently in numerous Daubert motions and decisions. Daubert called on district courts to preserve scientific conservatism in the courtroom. Joiner echoed that and showed the lower courts how it is done.

In retrospect, Joiner did more than incrementally develop the common law of Rule 702 – it was a turning point in weaponizing the Rule against junk science. It demonstrated the level of deference required of circuit courts, analytical rigor required of district courts and scientific rigor required of expert witnesses. It also established several specific scientific principles of general causation often encountered in toxic tort cases, and reinforced that scientific rigor cannot be relaxed in the courtroom. Where Daubert triggered a spirited debate about the role of the courts in the modern era and how junk science should be addressed in the courtroom, its sibling, Joiner, answered several of those questions and demonstrated how the gate is kept.