Edward K. Cheng is the Hess Professor of Law in absentia from Vanderbilt Law School, while serving this fall as a visiting professor at Harvard. Professor Cheng is one of the authors of the multi-volume treatise, Modern Scientific Evidence, and the author of many articles on scientific and statistical evidence. Cheng’s most recent article, “The Consensus Rule: A New Approach to Scientific Evidence,”[1] while thought provoking, follows in the long-standing tradition of law school professors to advocate evidence law reforms, based upon theoretical considerations devoid of practical or real-world support.

Cheng’s argument for a radical restructuring of Rule 702 is based upon his judgment that jurors and judges are epistemically incompetent to evaluate expert witness opinion testimony. The current legal approach has trial judges acting as gatekeepers of expert witness testimony, and jurors acting as judges of factual scientific claims. Cheng would abolish these roles as beyond their ken.[2] Lay persons can, however, determine which party’s position is supported by the relevant expert community, which he presumes (without evidence) possesses the needed epistemic competence. Accordingly, Cheng would rewrite the legal system’s approach to important legal disputes, such as disputes over causal claims, from:

Whether a given substance causes a given disease

to

Whether the expert community believes that a given substance causes a given disease.

Cheng channels the philosophical understanding of the ancients who realized that one must have expertise to judge whether someone else has used that expertise correctly. And he channels the contemporary understanding that knowledge is a social endeavor, not the unique perspective of an individual in isolation. From these twin premisses, Cheng derives a radical and cynical proposal to reform the law of expert witness testimony. In his vision, experts would come to court not to give their own opinions, and certainly not to try to explain how they arrive at their opinions from the available evidence. For him, the current procedure is too much like playing chess with a monkey. The expert function would consist of telling the jury what the expert witness’s community believes.[3] Jurors would not decide the “actual substantive questions,” but simply decide what they believe the relevant expert witness community accepts as a consensus. This radical restructuring is what Cheng calls the “consensus rule.”

In this proposed “consensus rule,” there is no room for gatekeeping. Parties continue to call expert witnesses, but only as conduits for the “consensus” opinions of their fields. Indeed, Cheng’s proposal would radically limit expert witness to service as pollsters; their testimony would present only their views of what the consensus is in their fields. This polling information is the only evidence that the jury hear from expert witnesses, because this is the only evidence that Cheng believes the jury is epistemically competent to assess.[4]

Under Cheng’s Consensus Rule, when there is no consensus in the realm, the expert witness regime defaults to “anything goes,” without gatekeeping.[5] Judges would continue to exercise some control over who is qualified to testify, but only as far as the proposed experts must be in a position to know what the consensus is in their fields.

Cheng does not explain why, under his proposed “consensus rule,” subject matter experts are needed at all.  The parties might call librarians, or sociologists of science, to talk about the relevant evidence of consensus. If a party cannot afford a librarian expert witness, then perhaps lawyers could present directly the results of their PubMed, and other internet searches.

Cheng may be right that his “deferential approach” would eliminate having the inexpert passing judgment on the expert. The “consensus rule” would reduce science to polling, conducted informally, often without documentation or recording, by partisan expert witnesses. This proposal hardly better reflects, as he argues, the “true” nature of science. In Cheng’s vision, science in the courtroom is just a communal opinion, without evidence and without inference. To be sure, this alternative universe is tidier and less disputatious, but it is hardly science or knowledge. We are left with opinions about opinions, without data, without internal or external validity, and without good and sufficient facts and data.

Cheng claims that his proposed Consensus Rule is epistemically superior to Rule 702 gatekeeping. For the intellectual curious and able, his proposal is a counsel of despair. Deference to the herd, he tells us “is not merely optimal—it is the only practical strategy.”[6] In perhaps the most extreme overstatement of his thesis, Cheng tells us that

“deference is arguably not due to any individual at all! Individual experts can be incompetent, biased, error prone, or fickle—their personal judgments are not and have never been the source of reliability. Rather, proper deference is to the community of experts, all of the people who have spent their careers and considerable talents accumulating knowledge in their field.”[7]

Cheng’s hypothesized community of experts, however is worthy of deference only by virtue of the soundness of its judgments. If a community has not severely tested its opinions, then its existence as a community is irrelevant. Cheng’s deference is the sort of phenomenon that helped create Lysenkoism and other intellectual fads that were beyond challenge with actual data.

There is, I fear, some partial truth to Cheng’s judgment of juries and judges as epistemically incompetent, or challenged, to judge science, but his judgment seems greatly overstated. Finding aberrant jury verdicts would be easy, but Cheng provides no meaningful examples of gatekeeping gone wrong. Professor Cheng may have over-generalized in stating that judges are epistemically incompetent to make substantive expert determinations. He surely cannot be suggesting that judges never have sufficient scientific acumen to determine the relevance and reliability of expert witness opinion. If judges can, in some cases, make a reasonable go at gatekeeping, why then is Cheng advocating a general rule that strips all judges of all gatekeeping responsibility with respect to expert witnesses?

Clearly judges lack the technical resources, time, and background training to delve deeply into the methodological issues with which they may be confronted. This situation could be ameliorated by budgeting science advisors and independent expert witnesses, and by creating specialty courts staffed with judges that have scientific training. Cheng acknowledges this response, but he suggests that conflicts with “norms about generalist judges.”[8] This retreat to norms is curious in the face of Cheng’s radical proposals, and the prevalence of using specialist judges for adjudicating commercial and patent disputes.

Although Cheng is correct that assessing validity and reliability of scientific inferences and conclusions often cannot be reduced to a cookbook or checklist approach, not all expertise is as opaque as Cheng suggests. In his view, lawyers are deluded into thinking that they can understand the relevant science, with law professors being even worse offenders.[9] Cross-examining a technical expert witness can be difficult and challenging, but lawyers on both sides of the aisle occasionally demolish the most skilled and knowledgeable expert witnesses, on substantive grounds. And these demolitions happen to expert witnesses who typically, self-servingly claim that they have robust consensuses agreeing with their opinions.

While scolding us that we must get “comfortable with relying on the expertise and authority of others,” Cheng reassures us that deferring to authority is “not laziness or an abdication of our intellectual responsibility.”[10] According to Cheng, the only reason to defer to the opinion of expert is that they are telling us what their community would say.[11] Good reasons, sound evidence, and valid inference need not worry us in Cheng’s world.

Finding Consensus

Cheng tells us that his Consensus Rule would look something like:

“Rule 702A. If the relevant scientific community believes a fact involving specialized knowledge, then that fact is established accordingly.”

Imagine the endless litigation over what the “relevant” community is. For a health effect claim about a drug and heart attacks, is it the community of cardiologists or epidemiologists? Do we accept the pronouncements of the American Heart Association or those of the American College of Cardiology. If there is a clear consensus based upon a clinical trial, which appears to be based upon suspect data, is discovery of underlying data beyond the reach of litigants because the correctness of the allegedly dispositive study is simply not in issue? Would courts have to take judicial notice of the clear consensus and shut down any attempt to get to the truth of the matter?

Cheng acknowledges that cases will involve issues that are controversial or undeveloped, without expert community consensus. Many litigations start after publication of a single study or meta-analysis, which is hardly the basis for any consensus. Cheng appears content, in this expansive area, to revert to anything goes because if the expert community has not coalesced around a unified view, or if the community is divided, then the courts cannot do better than flipping a coin! Cheng’s proposal thus has a loophole the size of the Sun.

Cheng tells us, unhelpfully, that “[d]etermining consensus is difficult in some cases, and less so in others.”[12] Determining consensus may not be straightforward, but no matter. Consensus Rule questions are not epistemically challenging and thus “far more manageable,” because they requires no special expertise. (Again, why even call a subject matter expert witness, as opposed to a science journalist or librarian?) Cheng further advises that consensus is “a bit like the reasonable person standard in negligence,” but this simply conflates normative judgments with the scientific judgments.[13]

Cheng’s Consensus Rule would allow the use of a systematic review or a meta-analysis, not for evidence of the correctness of its conclusions, but only as evidence of a consensus.[14] The thought experiment of how this suggestion plays out in the real world may cause some agita. The litigation over Avandia began within days of the publication of a meta-analysis in the New England Journal of Medicine.[15] So some evidence of consensus; right? But then the letters to the editor within a few weeks of publication showed that the meta-analysis was fatally flawed. Inadmissible! Under the Consensus Rule the correctness or the methodological appropriateness of the meta-analysis is irrelevant. A few months later, another meta-analysis is published, which fails to find the risk that the original meta-analysis claimed. Is the trial now about which meta-analysis represents the community’s consensus, or are we thrown into the game of anything goes, where expert witnesses just say things, without judicial supervision?  A few years go by, and now there is a large clinical trial that supersedes all the meta-analyses of small trials.[16] Is a single large clinical trial now admissible as evidence of a new consensus, or are only systematic reviews and meta-analyses relevant evidence?

Cheng’s Consensus Rule will be useless in most determinations of specific causation.  It will be a very rare case indeed when a scientific organization issues a consensus statement about plaintiff John Doe. Very few tort cases involve putative causal agents that are thought to cause every instance of some disease in every person exposed to the agent. Even when a scientific community has addressed general causation, it will have rarely resolved all the uncertainty about the causal efficacy of all levels of exposure or the appropriate window of latency. So Cheng’s proposal guarantees to remove specific causation from the control of Rule 702 gatekeeping.

The potential for misrepresenting consensus is even greater than the misrepresentations of actual study results. At least the data are the data, but what will jurors do when they are regaled by testimony about the informal consensus reached in the hotel lobby of the latest scientific conference. Regulatory pronouncements that are based upon precautionary principles will be misrepresented as scientific consensus.  Findings by the International Agency for Research on Cancer that a substance is a IIA “probable human carcinogen” will be hawked as a consensus, even though the classification specifically disclaims any quantitative meaning for “probable,” and it directly equates to “insufficient” evidence of carcinogencity in humans.

In some cases, as Cheng notes, organizations such as the National Research Council, or the National Academy of Science, Engineering and Medicine (NASEM), will have weighed in on a controversy that has found its way into court.[17] Any help from such organizations will likely be illusory. Consider the 2006 publication of a comprehensive review of the available studies on non-pulmonary cancers and asbestos exposure by NASEM. The writing group presented its assessment of colorectal cancer as not causally associated with occupational asbestos exposure.[18] By 2007, the following year, expert witnesses for plaintiffs argued that the NASEM publication was no longer a consensus because one or two (truly inconsequential studies) had been published after the report and thus not considered. Under Cheng’s proposal, this dodge would appear to be enough to oust the consensus rule, and default to the “anything goes” rule. The scientific record can change rapidly, and many true consensus statements quickly find their way into the dustbin of scientific history.

Cheng greatly underestimates the difficulty in ascertaining “consensus.” Sometimes, to be sure, professional societies issue consensus statements, but they are often tentative and inconclusive. In many areas of science, there will be overlapping realms of expertise, with different disciplines issuing inconsistent “consensus” statements. Even within a single expert community, there may be two schools of thoughts about a particular issue.

There are instances, perhaps more than a few, when a consensus is epistemically flawed. If, as is the case in many health effect claims, plaintiffs rely upon the so-called linear no-threshold dose-response (LNT) theory of carcinogenesis, plaintiffs will point to regulatory pronouncements that embrace LNT as “the consensus.” When scientists are being honest, they generally recognize LNT as part of a precautionary principle approach, which may make sense as the foundation of “risk assessment.” The widespread assumption of LNT in regulatory agencies, and among scientists who work in such agencies, is understandable, but LNT remains an assumption. Nonetheless, we already see LNT hawked as a consensus, which under Cheng’s Consenus Rule would become the key dispositive issue, while quashing the mountain of evidence that there are, in fact, defense mechanisms to carcinogenesis that result in practical thresholds.

Beyond, regulatory pronouncements, some areas of scientific endeavor have themselves become politicized and extremist. Tobacco smoking surely causes lung cancer, but the studies of environmental tobacco smoking and lung cancer have been oversold. In areas of non-scientific disputes, such as history of alleged corporate malfeasance, juries will be treated to “the consensus” of Marxist labor historians, without having to consider the actual underlying historical documents. Cheng tells us that his Consensus Rule is a “realistic way of treating nonscientific expertise,”[19] which would seem to cover historian expert witness. Yet here, lawyers and lay fact finders are fully capable of exploring the glib historical conclusions of historian witnesses with cross-examination on the underlying documentary facts of the proffered opinions.

The Alleged Warrant for the Consensus Rule

If Professor Cheng is correct that the current judicial system, with decisions by juries and judges, is epistemically incompetent, does his Consensus Rule necessarily follow?  Not really. If we are going to engage in radical reforms, then the institutionalization of blue-ribbon juries would make much greater sense. As for Cheng’s claim that knowledge is “social,” the law of evidence already permits the use of true consensus statements as learned treatises, both to impeach expert witnesses who disagree, and (in federal court) to urge the truth of the learned treatise.

The gatekeeping process of Rule 702, which Professor Cheng would throw overboard, has important advantages in that judges ideally will articulate reasons for finding expert witness opinion testimony admissible or not. These reasons can be evaluated, discussed, and debated, with judges, lawyers, and the public involved. This gatekeeping process is rational and socially open.

Some Other Missteps in Cheng’s Argument

Experts on Both Sides are Too Extreme

Cheng’s proposal is based, in part, upon his assessment that the adversarial system causes the parties to choose expert witnesses “at the extremes.” Here again, Cheng provides no empirical evidence for his assessment. There is a mechanical assumption often made by people who do not bother to learn the details of a scientific dispute that the truth must somehow lie in the “middle.” For instance, in MDL 926, the silicone gel breast implant litigation, presiding Judge Sam Pointer complained about the parties’ expert witnesses being too extreme. Judge Pointer  believed that MDL judges should not entertain Rule 702 challenges, which were in his view properly heard by the transferor courts. As a result, Judge Robert Jones, and then Judge Jack Weinstein, conducted thorough Rule 702 hearings and found that the plaintiffs’ expert witnesses’ opinions were unreliable and insufficiently supported by the available evidence.[20] Judge Weinstein started the process of selecting court-appointed expert witnesses for the remaining New York cases, which goaded Judge Pointer into taking the process back to the MDL court level. After appointing four, highly qualified expert witnesses, Judge Pointer continued to believe that the parties’ expert witnesses were “extremists,” and that the courts’ own experts would come down somewhere between them.  When the court-appointed experts filed their reports, Judge Pointer was shocked that all four of his experts sided with the defense in rejecting the tendentious claims of plaintiffs’ expert witnesses.

Statistical Significance

Along the way, in advocating his radical proposal, Professor Cheng made some other curious announcements. For instance, he tells us that “[w]hile historically used as a rule of thumb, statisticians have now concluded that using the 0.05 [p-value] threshold is more distortive than helpful.”[21] Cheng’s purpose here is unclear, but the source he cited does not remotely support his statement, and certainly not his gross overgeneralization about “statisticians.” If this is the way he envisions experts will report “consensus,” then his program seems broken at its inception. The American Statistical Association’s (ASA) p-value “consensus” statement articulated six principles, the third of which noted that

“[s]cientific conclusions and business or policy decisions should not be based only on whether a p-value passes a specific threshold.”

This is a few light years away from statisticians’ concluding that statistical significance thresholds are more distortive than helpful. The ASA p-value statement further explains that

“[t]he widespread use of ‘statistical significance’ (generally interpreted as ‘p < 0.05’) as a license for making a claim of a scientific finding (or implied truth) leads to considerable distortion of the scientific process.”[22]

In the science of health effects, statistical significance remains extremely important, but it has never been a license for making causal claims. As Sir Austin Bradford Hill noted in his famous after-dinner speech, ruling out chance (and bias) as an explanation for an association was merely a predicate for evaluating the association for causality.[23]

Over-endorsing Animal Studies

Under Professor Cheng’s Consensus Rule, the appropriate consensus might well be one generated solely by animal studies. Cheng tells that “perhaps” scientists do not consider toxicology when the pertinent epidemiology is “clear.” When the epidemiology, however, is unclear, scientists consider toxicology.[24] Well, of course, but the key question is whether a consensus about causation in humans will be based upon non-human animal studies. Cheng seems to answer this question in the affirmative by criticizing courts that have required epidemiologic studies “even though the entire field of toxicology uses tissue and animal studies to make inferences, often in combination with and especially in the absence of epidemiology.”[25] The vitality of the field of toxicology is hardly undermined by its not generally providing sufficient grounds for judgments of human causation.

Relative Risk Greater Than Two

In the midst of his argument for the Consensus Rule, Cheng points critically to what he calls “questionable proxies” for scientific certainty. One such proxy is the judicial requirement of risk ratios in excess of two. His short discussion appears to be focused upon the inference of specific causation in a given case, but it leads to a non-sequitur:

“Some courts have required a relative risk of 2.0 in toxic tort cases, requiring a doubling of the population risk before considering causation.⁷³ But the preponderance standard does not require that the substance more likely than not caused any case of the disease in the population, it requires that the substance more likely than not caused the plaintiff’s case.”[26]

Of course, it is exactly because we are interested in the probability of causation of the plaintiff’s case, that we advert to the risk ratio to give us some sense whether “more likely than not” the exposure caused plaintiff’s case. Unless plaintiff can show he is somehow unique, he is “any case.” In many instances, plaintiff cannot show how he is different from the participants of the study that gave rise to the risk ratio less than two.

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[1] Edward K. Cheng, “The Consensus Rule: A New Approach to Scientific Evidence,” 75 Vanderbilt L. Rev. 407 (2022) [Consensus Rule].

[2] Consensus Rule at 410 (“The judge and the jury, lacking in expertise, are not competent to handle the questions that the Daubert framework assigns to them.”)

[3] Consensus Rule at 467 (“Under the Consensus Rule, experts no longer offer their personal opinions on causation or teach the jury how to assess the underlying studies. Instead, their testimony focuses on what the expert community as a whole believes about causation.”)

[4] Consensus Rule at 467.

[5] Consensus Rule at 437.

[6] Consensus Rule at 434.

[7] Consensus Rule at 434.

[8] Consensus Rule at 422.

[9] Consensus Rule at 429.

[10] Consensus Rule at 432-33.

[11] Consensus Rule at 434.

[12] Consensus Rule at 456.

[13] Consensus Rule at 457.

[14] Consensus Rule at 459.

[15] Steven E. Nissen, M.D., and Kathy Wolski, M.P.H., “Effect of Rosiglitazone on the Risk of Myocardial Infarction and Death from Cardiovascular Causes,” 356 New Engl. J. Med. 2457 (2007).

[16] P.D. Home, et al., “Rosiglitazone Evaluated for Cardiovascular Outcomes in Oral Agent Combination Therapy for Type 2 Diabetes (RECORD), 373 Lancet 2125 (2009).

[17] Consensus Rule at 458.

[18] Jonathan M. Samet, et al., Asbestos: Selected Health Effects (2006).

[19] Consensus Rule at 445.

[20] Hall v. Baxter Healthcare Corp., 947 F. Supp.1387 (D. Or. 1996) (excluding plaintiffs’ expert witnesses’ causation opinions); In re Breast Implant Cases, 942 F. Supp. 958 (E. & S.D.N.Y. 1996) (granting partial summary judgment on claims of systemic disease causation).

[21] Consenus Rule at 424 (citing Ronald L. Wasserstein & Nicole A. Lazar, “The ASA Statement on p-Values: Context, Process, and Purpose,” 70 Am. Statistician 129, 131 (2016)).

[22] Id.

[23] Austin Bradford Hill, “The Environment and Disease: Association or Causation?” 58 Proc. Royal Soc’y Med. 295, 295 (1965). See Schachtman, “Ruling Out Bias & Confounding is Necessary to Evaluate Expert Witness Causation Opinions” (Oct. 29, 2018); “Woodside & Davis on the Bradford Hill Considerations” (Aug. 23, 2013); Frank C. Woodside, III & Allison G. Davis, “The Bradford Hill Criteria: The Forgotten Predicate,” 35 Thomas Jefferson L. Rev. 103 (2013).

[24] Consensus Rule at 444.

[25] Consensus Rule at 424 & n. 74 (citing to one of multiple court advisory expert witnesses in Hall v. Baxter Healthcare Corp., 947 F. Supp.1387, 1449 (D. Or. 1996), who suggested that toxicology would be appropriate to consider when the epidemiology was not clear). Citing to one outlier advisor is a rather strange move for Cheng considering that the “consensus” was readily discernible to the trial judge in Hall, and to Judge Jack Weinstein, a few months later, in In re Breast Implant Cases, 942 F. Supp. 958 (E. & S.D.N.Y. 1996).

[26] Consensus Rule at 424 & n. 73 (citing Lucinda M. Finley, “Guarding the Gate to the Courthouse: How Trial Judges Are Using Their Evidentiary Screening Role to Remake Tort Causation Rules,” 49 Depaul L. Rev. 335, 348–49 (2000). See Schachtman, “Rhetorical Strategy in Characterizing Scientific Burdens of Proof” (Nov. 15, 2014).