Maurice A. Deane School of Law at Hofstra University Scholarly Commons at Hofstra Law Hofstra Law Faculty Scholarship 2004 Restoring the Individual Plaintiff ot Tort Law by Rejecting ‘Junk Logic’ about Specific aC usation Vern R. Walker Maurice A. Deane School of Law at Hofstra University Follow this and additional works at: https://scholarlycommons.law.hofstra.edu/faculty_scholarship Recommended Citation Vern R. Walker, Restoring the Individual Plaintiff ot Tort Law by Rejecting ‘Junk Logic’ about Specific aC usation, 56 Ala. L. Rev. 381 (2004) Available at: https://scholarlycommons.law.hofstra.edu/faculty_scholarship/141 This Article is brought to you for free and open access by Scholarly Commons at Hofstra Law. It has been accepted for inclusion in Hofstra Law Faculty Scholarship by an authorized administrator of Scholarly Commons at Hofstra Law. For more information, please contact [email protected]. RESTORING THE INDIVIDUAL PLAINTIFF TO TORT LAW BY REJECTING "JUNK LOGIC" ABOUT SPECIFIC CAUSATION Vern R. Walker* INTRODUCTION .......................................................................................... 382 I. UNCERTANTIES AND WARRANT IN FINDING GENERAL CAUSATION: PROVIDING A MAJOR PREMISE FOR A DIRECT INFERENCE TO SPECIFIC CAUSATION .......................................................................... 386 A. Acceptable Measurement Uncertainty: Evaluating the Precisionand Accuracy of Classifications................................... 389 B. Acceptable Sampling Uncertainty: Evaluating the Population-Representativenessof Samples .................................. 396 C. Acceptable Modeling Uncertainty:Evaluating the Predictive Value of Variables ........................................................................ 405 D. Acceptable Causal Uncertainty: Explaining the Probability of Event Occurrence ..................................................................... 423 II. UNCERTAINTIES AND WARRANT IN APPLYING THE GENERALIZATION TO THE INDIVIDUAL PLAINTIFF ........................................................... 437 A. Acceptable UncertaintyAbout Plaintiff-Representativeness: Selecting an Adequately Representative Reference Group .......... 439 B. Acceptable UncertaintyAbout Assigning a Probabilityto a Specific Member of the Reference Group ..................................... 448 III. MAKING WARRANTED FINDINGS ABOUT SPECIFIC CAUSATION ....... 452 A. An IntegratedApproach to Decision-MakingAbout Acceptable Residual Uncertainty ................................................. 453 B. JudicialErrors in Reasoning About Specific Causation.............. 460 1. Judges as Factflnders and the "0.5 Inference Rule" ............ 460 2. Judges as Referees of Reasonable Inferences and Rules on Sufficiency of Evidence ..................................................... 468 3. Judges as Gatekeepers of Evidence and Rules of Adm issibility............................................................. 473 C ONCLU SION ............................................................................................. 480 * Professor of Law, Hofstra University; Ph.D., University of Notre Dame, 1975; J.D., Yale Uni- versity, 1980. The author is grateful for the financial support provided by a research grant from Hofstra University. He also wishes to thank Nicole Irvin and Gisella Rivadeneira for their research assistance. Alabama Law Review [Vol. 56:2:381 INTRODUCTION Judges have been removing the individual plaintiff from tort cases, of- ten as a byproduct of a campaign against "junk science."' In place of the individual plaintiff, they have been installing an abstract "statistical individ- ual" and adopting rules that decide cases on statistical grounds. This Article argues that, ironically, the reasoning behind these decisions and rules is too often an example of the "junk logic" that judges should be avoiding. The Article analyzes the logical warrant for findings of fact about specific causa- tion and uses that analysis to critique such rules as (1) a "0.5 inference rule" for factfinding, 2 (2) a "greater-than-50%" rule for evaluating the legal suffi- ciency of evidence, 3 and (3) certain rules of admissibility following the Su- preme Court's decisions in Daubert and Kumho Tire.4 Judges are using such rules to wrongly decide a wide variety of tort cases, from products liability cases to medical malpractice to toxic exposure cases.' This Article demonstrates that the many kinds of uncertainty inherent in warranted findings about specific causation require the factfinder to make decisions that are necessarily pragmatic, non-scientific, and non-statistical in nature. Such uncertainties are inherent in the logic of specific causation, and are not peculiar to toxic tort cases, or to epidemiologic evidence, or even to scientific evidence. The presence of significant degrees of such un- certainty makes it impossible to prove specific causation in any factual or scientific sense. Warranted findings must rest upon the common sense, practical fairness, and rough justice of the factfinder, except in categories of cases where tort policies can justify the adoption of decision rules for the entire category. Such rules, however, should not rest on the misguided sta- tistical reasoning of past cases, but on proper policy foundations. If this 1. On the campaign against "junk science," see General Electric Co. v. Joiner, 522 U.S. 136, 153, 154 n.6 (1997) (Stevens, J.,concurring in part and dissenting in part) (distinguishing the expert "weight of the evidence" reasoning in that case from "the sort of 'junk science' with which Daubert was con- cemed"); Amorgianos v. National Railroad Passenger Corp., 303 F.3d 256, 267 (2d Cir. 2002) (stating that "[tlhe flexible Daubert inquiry gives the district court the discretion needed to ensure that the court- room door remains closed to junk science while admitting reliable expert testimony that will assist the trier of fact"); and Daubert v. Merrell Dow Pharm., Inc., 43 F.3d 1311, 1321, 1322 n.18 (9th Cir. 1995) (stating that an expert's excluded testimony in that case "illustrates how the two prongs of Rule 702 [of the Federal Rules of Evidence] work in tandem to ensure that junk science is kept out of the federal courtroom"). 2. See infra Part UI.B.1. 3. See infra Part 111.B.2. 4. See infra Part ffl.B.3. 5. E.g., XYZ v. Schering Health Care Ltd., [2002] E.W.H.C. 1420 (QB), 2002 WL 1446183 (July 29, 2002) (finding against the claimants in products liability cases against manufacturers of oral contra- ceptives because they failed to prove a relative risk greater than 2.0); Fennell v. S.Md. Hosp. Ctr., Inc., 580 A.2d 206 (Md. 1990) (holding, in a medical malpractice case, that evidence of a loss of a 40% chance of survival was legally insufficient for satisfying the plaintiff's burden of proving that the defendant caused the plaintiff's death); In re Hanford Nuclear Reservation Litig., 292 F.3d 1124 (9th Cir. 2002) (holding, in cases brought against facility operators for injuries from radioactive emissions, that the district court erred in excluding plaintiffs' expert testimony, but leaving intact on remand the district court's ruling that admissible evidence on specific causation must show that exposure to the radioactive emissions at least doubled the plaintiffs' baseline risks). 20041 Restoring the Individual Plaintiff to Tort Law Article can clear away these logical misunderstandings, perhaps judges will provide better policy justifications and develop better rules. Tort law uses the term "specific causation," sometimes called "individ- ual causation," to refer to the factual issue of which particular events caused or will cause a particular injury in a specific plaintiff.6 Specific causation is distinguished from "general causation," also called "generic causation," which addresses whether there is any causal relationship at all between types of events and types of injuries.7 Specific causation is whether a spe- cific event caused or will cause a specific injury, while general causation is whether such events can (ever) cause such injuries.8 Usually, for a plaintiff to win damages in a tort case, the plaintiff must prove both general and spe- cific causation.9 A finding about specific causation can be prospective and predictive, as in: "It is unlikely that the defendant's negligent conduct, which resulted in the exposure of Jessica Jones to benzene, will cause her to develop lung cancer." Or a finding might be retrospective and explanatory, as in: "It is unlikely that the defendant's negligent conduct and Jessica Jones's resulting exposure to benzene caused her lung cancer." This Article argues that both versions, despite their temporal differences, have a similar logical structure in their warrant. Therefore, the analysis provided here applies to both pro- spective and retrospective findings of specific causation. The central epistemic problem posed by specific causation is justifying how a less-than-universal generalization about causation in groups can ever warrant a probabilistic finding about causation in a specific case.' ° When 6. E.g., DeLuca v. Merrell Dow Pharm., Inc., 911 F.2d 941,957-59 (3d Cir. 1990) (reasoning from the plaintiffs burden of proving causation by "a more likely than not standard" to a requirement that epidemiologic evidence alone would be legally insufficient evidence of specific causation unless it showed a "relative risk of limb reduction defects" of at least two); Hanford, 292 F.3d at 1129, 1133 (using the term "individual causation" to refer to the question