David Faigman’s Critique of G2i Inferences at Weinstein Symposium

The DePaul Law Review’s 20th Annual Clifford Symposium on Tort Law and Social Policy is an 800-plus page tribute in honor of Judge Jack Weinstein. 64 DePaul L. Rev. (Winter 2015). There are many notable, thought-provoking articles, but my attention was commanded by the contribution on Judge Weinstein’s approach to expert witness opinion evidence. David L. Faigman & Claire Lesikar, “Organized Common Sense: Some Lessons from Judge Jack Weinstein’s Uncommonly Sensible Approach to Expert Evidence,” 64 DePaul L. Rev. 421 (2015) [cited as Faigman].

Professor Faigman praises Judge Jack Weinstein for his substantial contributions to expert witness jurisprudence, while acknowledging that Judge Weinstein has been a sometimes reluctant participant and supporter of judicial gatekeeping of expert witness testimony. Professor Faigman also uses the occasion to restate his own views about the so-called “G2i” problem, the problem of translating general knowledge that pertains to groups to individual cases. In the law of torts, the G2i problem arises from the law’s requirement that plaintiffs show that they were harmed by defendants’ products or environmental exposures. In the context of modern biological “sufficient” causal set principles, this “proof” requirement entails that the product or exposure can cause the specified harms in human beings generally (“general causation”) and that the product or exposure actually played a causal role in bringing about plaintiffs’ specific harms.

Faigman makes the helpful point that courts initially and incorrectly invoked “differential diagnosis,” as the generally accepted methodology for attributing causation. In doing so, the courts extrapolated from the general acceptance of differential diagnosis in the medical community to the courtroom testimony about etiology. The extrapolation often glossed over the methodological weaknesses of the differential approach to etiology. Not until 1995 did a court wake to the realization that what was being proffered was a “differential etiology,” and not a differential diagnosis. McCullock v. H.B. Fuller Co., 61 F.3d 1038, 1043 (2d Cir. 1995). This realization, however, did not necessarily stimulate the courts’ analytical faculties, and for the most part, they treated the methodology of specific causal attribution as general acceptance and uncontroversial. Faigman’s point that the courts need to pay attention to the methodological challenges to differential etiological analysis is well taken.

Faigman also claims, however, that in advancing “differential etiologies, expert witnesses were inventing wholesale an approach that had no foundation or acceptance in their scientific disciplines:

 “Differential etiology is ostensibly a scientific methodology, but one not developed by, or even recognized by, physicians or scientists. As described, it is entirely logical, but has no scientific methods or principles underlying it. It is a legal invention and, as such, has analytical heft, but it is entirely bereft of empirical grounding. Courts and commentators have so far merely described the logic of differential etiology; they have yet to define what that methodology is.”

Faigman at 444.[1] Faigman is correct that courts often have left unarticulated exactly what the methodology is, but he does not quite make sense when he writes that the method of differential etiology is “entirely logical,” but has no “scientific methods or principles underlying it.” Afterall, Faigman starts off his essay with a quotation from Thomas Huxley that “science is nothing but trained and organized common sense.”[2] As I have written elsewhere, the form of reasoning involved in differential diagnosis is nothing other than the iterative disjunctive syllogism.[3] Either-or reasoning occurs throughout the physical and biological sciences; it is not clear why Faigman declares it un- or extra-scientific.

The strength of Faigman’s claim about the made-up nature of differential etiology appears to be undermined and contradicted by an example that he provides from clinical allergy and immunology:

“Allergists, for example, attempt to identify the etiology of allergic reactions in order to treat them (or to advise the patient to avoid what caused them), though it might still be possible to treat the allergic reactions without knowing their etiology.”

Faigman at 437. Of course, not only allergists try to determine the cause of an individual patient’s disease. Psychiatrists, in the psychoanalytic tradition, certain do so as well. Physicians who use predictive regression models use group data, in multivariate analyses, to predict outcomes, risk, and mortality in individual patients. Faigman’s claim is similarly undermined by the existence of a few diseases (other than infectious diseases) that are defined by the causative exposure. Silicosis and manganism have played a large role in often bogus litigation, but they represent instances in which a differential diagnosis and puzzle may also be an etiological diagnosis and puzzle. Of course, to the extent that a disease is defined in terms of causative exposures, there may be serious and even intractable problems caused by the lack of specificity and accuracy in the diagnostic criteria for the supposedly pathognomonic disease.

As for whether the concept of “differential etiology” is ever used in the sciences themselves, a few citations for consideration follow.

Kløve & D. Doehring, “MMPI in epileptic groups with differential etiology,” 18 J. Clin. Psychol. 149 (1962)

Kløve & C. Matthews, “Psychometric and adaptive abilities in epilepsy with differential etiology,” 7 Epilepsia 330 (1966)

Teuber & K. Usadel, “Immunosuppression in juvenile diabetes mellitus? Critical viewpoint on the treatment with cyclosporin A with consideration of the differential etiology,” 103 Fortschr. Med. 707 (1985)

G.May & W. May, “Detection of serum IgA antibodies to varicella zoster virus (VZV)–differential etiology of peripheral facial paralysis. A case report,” 74 Laryngorhinootologie 553 (1995)

Alan Roberts, “Psychiatric Comorbidity in White and African-American Illicity Substance Abusers” Evidence for Differential Etiology,” 20 Clinical Psych. Rev. 667 (2000)

Mark E. Mullinsa, Michael H. Leva, Dawid Schellingerhout, Gilberto Gonzalez, and Pamela W. Schaefera, “Intracranial Hemorrhage Complicating Acute Stroke: How Common Is Hemorrhagic Stroke on Initial Head CT Scan and How Often Is Initial Clinical Diagnosis of Acute Stroke Eventually Confirmed?” 26 Am. J. Neuroradiology 2207 (2005)

Qiang Fua, et al., “Differential Etiology of Posttraumatic Stress Disorder with Conduct Disorder and Major Depression in Male Veterans,” 62 Biological Psychiatry 1088 (2007)

Jesse L. Hawke, et al., “Etiology of reading difficulties as a function of gender and severity,” 20 Reading and Writing 13 (2007)

Mastrangelo, “A rare occupation causing mesothelioma: mechanisms and differential etiology,” 105 Med. Lav. 337 (2014)


[1] See also Faigman at 448 (“courts have invented a methodology – differential etiology – that purports to resolve the G2i problem. Unfortunately, this method has only so far been described; it has not been defined with any precision. For now, it remains a highly ambiguous idea, sound in principle, but profoundly underdefined.”).

[2] Thomas H. Huxley, “On the Education Value of the Natural History Sciences” (1854), in Lay Sermons, Addresses and Reviews 77 (1915).

[3] See, e.g.,Differential Etiology and Other Courtroom Magic” (June 23, 2014) (collecting cases); “Differential Diagnosis in Milward v. Acuity Specialty Products Group” (Sept. 26, 2013).

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