Discussion week 2

MULTIPLE REASONABLE BEHAVIORS CASES: THE PROBLEM OF CAUSAL UNDERDETERMINATION IN TORT LAW

Maytal Gilboa*

The University of Toronto

ABSTRACT This article introduces a signi!cant yet largely overlooked problem in the law of torts: causal underdetermination. This problem occurs when the causal inquiry of a but-for test produces not one but two results, which are contradictory. According to the !rst, the negligent defendant is the likely cause of the plaintiff’s injury, whereas according to the second, she is not. The article explains why causal underdetermination has escaped the radar of tort scholars and is perceived by courts as lack of causation. It demonstrates that the current practice in cases of causal underdetermination might lead to erroneous decisions, absolving negligent defendants of tort liability even when the evidence suggests that they are in fact the likely cause of the plaintiff’s injury. This, in turn, the article asserts, may not only lead to underdeterrence among potential defendants, but also encourage manipulative litigation strategy to escape liability in retrospect. The article then proposes solutions that contend with causal underdetermination and resolve the dif!culties that the current practice entails.

INTRODUCTION

To impose liability in negligence, a plaintiff is required to preponderantly prove three basic elements: the existence of harm, the defendant’s

* Postdoctoral fellow, University of Toronto Faculty of Law. A !rst draft of this article was writ- ten during my postdoctoral fellowship at the Federmann Center for the Study of Rationality, Hebrew University of Jerusalem. Earlier drafts of this article have been selected for presenta- tion at the 27th Annual Meeting of the American Law and Economic Association, the 33rd Annual Meeting of the European Association of Law and Economics, the 2017 meeting of the Canadian Law and Economics Association, and the 2017 meeting of the Israeli Private Law Association. I would like to thank the participants in these meetings, as well as Natalie Davidson, David Enoch, Rea Golan, Ehud Guttel, Alon Harel, Barak Medina, Omer Pelled, Omri Rachum-Twaig, Arthur Ripstein, Ohad Somech, Ernest Weinrib, Asaf Wiener, and the two Legal Theory anonymous referees for their helpful comments on earlier drafts. I wish to express a special thanks to Ariel Porat for his priceless comments and suggestions for this arti- cle at different stages of its writing. Lastly, I thank the Cegla Center for Interdisciplinary Research of the Law for their generous !nancial support.

Legal Theory, 25 (2019), 77–104. © Cambridge University Press 2019 0361-6843/19 doi:10.1017/S135232521900003X

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unreasonable behavior,1 and the existence of a causal link between the defendant’s unreasonable behavior and the plaintiff’s harm.2 The latter requirement is the focus of this article. In particular, this article examines a distinct situation in which the plaintiff cannot meet this requirement due to a unique legal pattern referred to here as “causal underdetermina- tion.” In this pattern, the causal inquiry leads to an impossible conclusion, according to which the negligent defendant is, and is not, the cause of the plaintiff’s harm at the same time. As an example of the pattern of causal underdetermination, consider the following scenario:

The physician. A physician performs an abdominal surgery to remove a tumor (“procedure A”). The physician chooses to reach the tumor by making a large incision in the abdominal wall, a clearly negligent choice. Subsequently, the patient dies. There are two alternative procedures the physician could have performed: reaching the tumor by laparoscopy from a right approach (“pro- cedure A1”), or reaching it by laparoscopy from a left approach (“procedure A2”); both alternative procedures are considered much safer (and less costly) than the procedure that was actually performed by the physician.

Exposing the patient to unreasonable risk by performing procedure A instead of either procedure A1 or A2, the physician in the above example is clearly negligent. However, according to prevailing tort law, the physician will be held liable for her negligence only if it can be established that it is her negligence that caused the patient’s death. Consider now the following facts added to the above example:

An autopsy can prove that had the physician in the example chosen to per- form the surgery by procedure A1, the patient would likely have died anyway,

1. When discussing negligent behavior, the article addresses both actions and omissions. It should be noted, however, that de!nition of omissions as causes is a matter of controversy in the literature on causation. See, e.g., David Armstrong, The Open Door: Counterfactual Versus Singularist Theories of Causation, in CAUSATION AND LAWS OF NATURE 175, 177 (Howard Sankey ed., 1999) (maintaining that “[e]very causal situation develops as it does as a result of the pres- ence of positive factors alone”); Michael Moore, For What Must We Pay? Causation and Counterfactual Baselines, 40 SAN DIEGO L. REV. 1181, 1222–1227 (2003); MICHEL S. MOORE, CAUSATION AND RESPONSIBILITY (2009), at 55, 444–451 (asserting that omission liability is not cause-based liability since negative events cannot be considered as causes). For a different view, see, e.g., H.L.A. HART & TONY HONORÉ, CAUSATION IN THE LAW (2nd ed. 1985), at 38 (stating that in the law, omissions can be perceived as factual causes, since they are “ways of describing the world . . . a real state of affairs, not just nothing”). 2. See RESTATEMENT (THIRD) OF TORTS: LIABILITY FOR PHYSICAL AND EMOTIONAL HARM (2010), at §26

(stating that conduct “is a factual cause of harm when the harm would not have occurred absent the conduct”). Under prevailing tort law, in order to impose legal liability in tort, a court must !nd both factual and proximate causation. While the former is considered to be based on questions of pure fact, hinging upon the result of the but-for test, the latter is per- ceived to be a matter of legal policy, such that it may limit the defendant’s responsibility for the damage caused. The focus of this article is on factual causation although, as is later explained at length, its theoretical framework associates both factual and policy considerations within the counterfactual inquiry.

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whereas had the physician chosen to perform the surgery by procedure A2, the patient would likely have fully recovered.

The key to inferring whether A was the cause of the patient’s death thus depends on whether one applies the counterfactual inquiry through A1 or A2. How should the law address such a situation? The physician scenario constitutes an example of causal underdetermina-

tion. This article discusses the pattern of causal underdetermination, the dif!culties that it raises, and thereafter possible solutions to resolve them through a particular counterfactual theory of causation, which expresses a contrastive view of causal relations. According to this view, causal relation is expressed by the quaternary query of what if some relevant alternative event A* (the contrastive cause) rather than A, caused B* (the plausible effect of A*) rather than B?3 In these terms, the conclusion as to whether there is causal relation depends on the existence of a difference between B and B* (i.e., on whether B ! B* >0), whereas the expression B ! B* man- ifests the difference that A actually made.4

The main dif!culty of the theory of contrastive causation is that there are countless ways to posit a contrastive cause A* to the actual cause A. Different A*s may lead to different B*s, and the factual inference regarding the causal link between A and its outcome may change accordingly. This dif!- culty is discussed and illustrated at length in the article. To demonstrate the pattern of causal underdetermination in the physician example through the lens of the theory of contrastive causation it suf!ces to acknowledge that the key to inferring whether A was indeed the cause of the patient’s death depends on the de!nition of the contrastive cause A*. The example tells us that denoting the contrastive cause by A1 would lead

to the result that the defendant’s negligence was not the cause of the patient’s death since it made no difference for the patient: had the physi- cian acted reasonably (by performing A1) instead of negligently (by

3. Jonathan Schaffer, Contrastive Causation, 114 PHIL. REV. 297, 297–299 (2005) (observing that “causation is a quaternary, contrastive relation” rather than a binary relation) [hereinafter Schaffer, Contrastive Causation]. For different approaches to the application of the theory of contrastive causation in the law, see, for example, Jonathan Schaffer, Contrastive Causation in the Law, 16 LEGAL THEORY 259 (2010) (observing that the law requires us to look for the speci!c alternate event for the actual cause, which is the hypothetical scenario in which the defendant acted lawfully instead of wrongfully) [hereinafter Schaffer, Contrastive Causation in the Law]; Alex Broadbent, Fact and Law in the Causal Inquiry, 15 LEGAL THEORY 173, 175–177 (2009) (sug- gesting the contrastive account of causation as a theoretical framework that enables us to dis- tinguish matters of fact from matters of law within the cause-in-fact inquiry). 4. The de!nition of “the cause” as “making a difference” has long been accepted by the

scholarship. See, e.g., HART & HONORÉ, supra note 1, at 29 (“[T]he cause, though not a literal intervention, is a difference from the normal course which accounts for the difference in the out- come.”); David Lewis, Causation, in CAUSATIONS AND CONDITIONALS 180, 181 (Ernest Sosa ed., 1975) (observing that a cause is something that “makes a difference, and the difference it makes must be a difference from what would have happened without it”); Schaffer, Contrastive Causation in the Law, supra note 3, at 285 (stating that the idea of contrast takes the counterfactual reasoning that causing is making a difference literally).

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performing A), it is more likely than not that the patient would have died anyway. By contrast, denoting the contrastive cause by A2 would lead us to infer that the physician’s negligence is the cause of the patient’s death, as the evidence shows that had the physician acted reasonably (by perform- ing A2), it is more likely than not that the patient would have survived. This conclusion of two coexisting con"icting but-for results is metaphysically impossible. The physician cannot be and not be the cause of the patient’s death at the same time. So, what should courts do in the face of causal underdetermination?

The article identi!es the cases in which this dif!culty may arise in negli- gence law as Multiple Reasonable Behaviors cases, or MRB cases. These cases are characterized by the following pattern: there is not one but rather several reasonable behaviors the defendant could have followed to meet her duty of care toward the plaintiff; the defendant followed neither of these reasonable behaviors (and is thus considered negligent); subsequently the plaintiff is injured. Causal underdetermination occurs in MRB cases when (at least) two of the reasonable behaviors not taken by the defendant lead to opposite conclusions as to whether the defendant’s negligence is the cause of injury, as in the physician example.

This article demonstrates that when facing this quandary, courts usually apply what is here termed a “no-causal-link solution.” According to this sol- ution, it is enough that only one of the reasonable behaviors (e.g., either A1 or A2 in the physician example) supports the conclusion of lack of a causal link between the defendant’s negligence and the plaintiff’s injury to infer that the requirement of causation was not established by the plaintiff. Consequently, in cases of causal underdetermination, negligent defendants are regularly exempted from tort liability for lack of causation. The article discusses possible explanations for the no-causal-link solution and demon- strates how it may lead to erroneous decisions, absolving defendants of tort liability even when it is more likely than not that their negligence was in fact the cause of the injury. It further explains that this mistake may have undesirable effects on defendants’ behavior, such as underdeterrence.

Having exposed the pattern of causal underdetermination in negligence cases, and the dif!culties it raises, the article proposes various solutions, reviewing !rst the suggestions made in the literature on the theory of con- trastive causation regarding the criterion for de!ning a relevant contrastive cause, A*, to any cause, A, in the legal context. It then explains why these suggestions fall short of resolving causal underdetermination in negligence cases.

The solutions developed in the article are based !rst and foremost on the understanding that the problem of causal underdetermination in negli- gence law derives from a normative—rather than a factual—situation, in which the law fails to direct the fact!nder to the speci!c relevant behavior that the defendant should have followed (instead of her actual negligent behavior). In light of this realization the article applies economic theory,

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which de!nes what is appropriate in terms of ef!ciency. Under these terms, in negligence cases, the contrastive event, A*, represents a behavior that minimizes the total costs of both the injuries and their prevention.5 The article shows that the implementation of this economic reasoning enables the singling out of a speci!c reasonable behavior in some cases of causal underdetermination. For the cases in which it does not, the article offers solutions of a different type, based on aggregation mechanisms and statisti- cal evidence, and illustrates possible ways to implement them. The article proceeds as follows. Section I sets out the theoretical and doc-

trinal framework of causal underdetermination and articulates its dif!cul- ties by using a contrastive approach to causation. To sharpen the unique features of causal underdetermination, it further distinguishes its pattern from the more familiar phenomenon of causal overdetermination. Lastly, it discusses the no-causal-link practice currently used to contend with causal underdetermination and demonstrates its problematic results. Section II discusses and illustrates possible solutions to causal underdetermination by distinguishing among three types of cases. The !rst are cases in which the fact!nder is capable of making a normative assessment and deciding which of the alternative reasonable behaviors that leads to different causal inference can be singled out as the optimal behavior in the circumstances. In the two other cases, the fact!nder is incapable of prioritizing the alter- native reasonable behaviors that lead to causal underdetermination either because she lacks the information to do so or because the normative assess- ment leads her to conclude that these alternative behaviors are equally desirable. For the latter types of cases, the article develops solutions based on aggregation mechanisms and statistical evidence, instead of nor- mative analysis. The Conclusion summarizes the discussion.

I. THE PROBLEM OF CAUSAL UNDERDETERMINATION

A. The Pattern of Causal Underdetermination

Under prevailing tort law, the existence of a causal link between a defen- dant’s negligence and a plaintiff’s injury is determined by a counterfactual inquiry known as “the but-for test.”6 Generally speaking, a counterfactual approach to causation explains the idea of causal relation between events in terms of conditionals of the form “If A had not occurred, B would not have occurred.” This simple formulation has been the source of a profusion of different approaches to counterfactual analysis.7 The causal analysis in

5. See, e.g., GUIDO CALABRESI, THE COSTS OF ACCIDENTS (1970), at 24–34 (arguing that the goal of accident law is to reduce the aggregate costs of both accidents and their prevention). 6. See supra note 2. 7. For a review of the counterfactual approach to causation, see, e.g., John Collins et al.,

Counterfactuals and Causation: History, Problems, and Prospects, in CAUSATION AND COUNTERFACTUALS 1–57 (John Collins et al. eds., 2004); L.A. PAUL & NED HALL, CAUSATION: A USER’S GUIDE (2013), at 13–24.

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this article is based on a particular counterfactual approach to causation, which examines causal relation between events through a contrastive formu- lation. According to this approach, a causal link is not a binary relation between A (cause) and B (effect). Rather, it expresses the counterfactual query of what if A* (the contrastive cause) rather than A caused B* (the probable result of A*) rather than B?8 In contrastive causation terms, the conclusion that there is a causal link between two separate events depends on the existence of a difference between B and B* (i.e., if B ! B* >0 there is such a causal link).9

The choice to articulate the arguments in the article through the contras- tive approach to causation is not coincidental. The structure of this approach corresponds with the reasoning of negligence cases, which only considers a behavior as a cause if it is proven to deviate from what counts as a reasonable behavior. Accordingly, the causal inquiry in tort law exam- ines the existence of causation according to the following counterfactual question: What would have been the outcome for the plaintiff had the defendant acted lawfully rather than negligently? A causal relation exists if and only if the evidence shows that in that case the plaintiff’s injury would have likely been prevented.10 By contrast, to negate the existence of a causal relation, a court must conclude that the plaintiff’s injury would have likely occurred even if the defendant had acted reasonably, and therefore, it is not the defendant’s negligence that caused it.11 As the article later reveals, the similarity between the causal structure of the contrastive approach and the underling reasoning of the causal inquiry in negligence cases not only helps to illuminate and articulate the legal prob- lem at the center of this article, but also provides tools for understanding its source.12

To exemplify the quaternary formulation of the contrastive approach to causation, consider the following simple illustration: I promised to water a plant; I did not water it; the plant dried out. In order to prove the existence of a causal link between the drying out of a plant and the fact that I forgot to water it, according to the contrastive approach to causation, it should be

8. See supra note 3. 9. See supra note 4. 10. As later explained, in civil law causal inferences are subject to the probabilistic require-

ment of the preponderance-of-the-evidence standard. Causation is thus only considered estab- lished when the probability of its occurrence exceeds 0.5. See infra notes 20–21 and accompanying text. 11. See, e.g., Watson v. Meltzer, 247 Ore. App. 558, 565–566 (Or. Ct. App. 2011) (observing

that in legal malpractice cases, the plaintiff must prove that, but for the defendant’s malprac- tice, he or she would have obtained a more favorable result); Chocktoot v. Smith, 280 Ore. 567, 570 (Or. 1977) (same). 12. The contrastive language accompanying the text in the article carries theoretical impor-

tance in itself, as a medium in which causal thinking is embodied. For a broader discussion on the importance of language to causal analysis, see, for example, Derek Edwards & Jonathan Potter, Language and Causation: A Discursive Action Model of Description and Attribution, 100 PSYCHOL. REV. 23 (1993).

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shown that if instead of not watering the plant (A), I had watered it (A*), it is likely that the plant would have survived (B*) instead of drying out (B). A positive difference between B and B* indicates that there is a causal link between the fact that I did not water the plant and the fact that the plant dried out. In particular, there is a causal link between A and B ! B*, as the latter term de!nes the exact difference that my omission to water the plant actually caused to its vitality. This illustration both demonstrates the relative nature of the contrastive

causal inquiry13 and reveals the unique dif!culty in inferring causal results through it. The conclusion that some action, A, made a difference, and can thus be considered a cause, can only be drawn in comparison to a contras- tive cause, A*, and its probable effect, B*.14 The problem is that there are countless ways to posit a contrastive cause. For instance, in the plant exam- ple, A* could denote the alternate behaviors of having watered the plant once a day, every day and a half, once a week, and so forth.15 Since different A*s may lead to different B*s, the factual inference regarding the causal link between my omission to water the plant and its drying out may also change.16 Thus, it may be the case that whereas de!ning the contrastive cause as my having watered the plant once a week (A1) leads to the contras- tive result that the plant would have completely dried out (B1), de!ning it as my having watered the plant once a day (A2) leads to the contrastive result that the plant would have survived (B2).17 While the former counter- factual (A1, B1) leads to the conclusion that my omission is not the cause of the plant’s drying out, since the plant would have dried out even if I had watered it,18 the latter counterfactual (A2, B2) leads to the opposite conclu- sion, i.e., that my omission to water the plant is the cause of its drying out.19

Which one is it then?

13. For a broader review of causal relativism, see, e.g., Christopher Read Hitchcock, The Role of Contrast in Causal and Explanatory Claims, 107 SYNTHESE 395, 399 (1996) (proposing to take cau- sation as an explanatory relevance relation); Peter Menzies, Causation in Context, in CAUSATION, PHYSICS, AND THE CONSTITUTION OF REALITY: RUSSELL’S REPUBLIC REVISITED 191, 192–193 (Huw Price & Richard Corry eds., 2007) (arguing that causal inference’s truth-value “can vary from one context to another, depending on how a certain contextual parameter is set”). 14. See, e.g., PETER LIPTON, INFERENCE TO THE BEST EXPLANATION (2d ed. 2004), at 42 (indicating

the necessity of contrastive dimension to infer causation). 15.  = {A1, A2, A3.., Ai}, whereas  is the set of all the possible alternate events to denote

the constructive cause, A1 = water the plant once a day; A2 = water the plant once in three days; A3 = water the plant once a week; and so forth; and A*represents the event eventually cho- sen from set Â. 16. Since the but-for result is expressed by B ! B*, whereby B is !xed, different B*s will always

lead to different but-for results. However, as explained later, in light of evidential rules such as the preponderance-of-the-evidence standard, in the law, different alternate results (B1, B2, B3.., Bn ) are not necessarily also contradictory to one another, i.e., in the law, when B1 " B2, it does not necessarily mean that B ! B1 " B ! B2. See infra notes 20–21 and accompanying text. 17. Other possible results may, of course, be that the plant lost three-quarters of its vitality,

half of it, and so forth. 18. Therefore B ! B* = 0. 19. Since B ! B*>0.

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The prominent suggestions made by scholarship in the !eld of contras- tive causation to answer this question shall be discussed in Section II. For the purpose of introducing the phenomenon of causal underdetermina- tion, to which I turn next, it suf!ces to acknowledge that the dif!culty presented by the above question is not essentially different from the one presented by the physician example that opens the article. For convenience, the example is reiterated below:

The physician. A physician performs abdominal surgery to remove a tumor. She chooses to reach the tumor by making a large incision in the abdominal wall (“procedure A”), a clearly negligent decision. Subsequently, the patient dies. There are two procedures the physician could have performed instead of A, both considered much safer and less costly than procedure A: reaching the tumor by laparoscopy from a right approach (“procedure A1”) or reaching it by laparoscopy from a left approach (“procedure A2”).

As in the plant example, here too the question of whether the negligent phy- sician is the cause of the patient’s death could be answered with both yes and no. The physician example presents a situation in which there is more than one reasonable behavior a negligent defendant could have followed to meet her duty of care toward a plaintiff. I refer to such situations as Multiple Reasonable Behaviors or MRB cases. In the example, the physician could have performed either A1 or A2. Had she done so, she would not have been considered negligent in the !rst place. Having performed neither, the fact!nder is now facing not one but two possible counterfactuals with which to apply the but-for test. In this situation, three possible but-for conclusions can be inferred: !rst, it may be the case that the two alternate procedures, A1 and A2, lead to the result that the patient’s death would have occurred anyway (regardless of the physician’s negligence). Second, the conclusion may be that had the defendant followed any of the alternate procedures, either A1 or A2, the patient’s death would not have occurred. In eitherof these two com- binations, the but-for test does not lead to opposite conclusions. A pattern of causal underdetermination occurs if and only if the but-for inquiry leads at the same time to different inferences, as in the physician example, where the evidence shows that on the one hand, had the physician performed pro- cedure A1 the patient would have died anyway, whereas on the other hand, had she performed A2 the patient would have likely survived (or vice versa).

Lastly, to infer causal underdetermination in tort law an additional condition must be met. This condition refers to the burden of proof in civil litigation, according to which the plaintiff’s claim is subject to the preponderance-of-the-evidence standard,20 also known as the P > 0.5

20. See, e.g., Dykes v. William Beaumont Hospital, 246 Mich. App. 471, 486–489 (2001) (where the Michigan Court of Appeal explicitly observed that the standard for causation is sub- ject to the preponderance-of-the-evidence standard); Merrell Dow Pharm. v. Havner, 953

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rule.21 This means that the evidence presented to support the claim of cau- sation must convince the court that it is more likely than not that the defen- dant’s negligence caused the alleged injury. In light of this rule, causal underdetermination occurs in the physician example if and only if the evi- dence shows that the chances of the patient’s death, had the physician administered the surgery by procedure A1, would be lower than 0.5, but on the other hand would exceed 0.5 had she chosen to administer it by pro- cedure A2. Figure 1 above summarizes the conditions required for the occurrence of

causal underdetermination in negligence cases, where P denotes the mag- nitude of probability that each of the reasonable behaviors not followed by a negligent defendant (A1, A2,.. Ai)22 would have led to its result (respec- tively denoted by B1, B2,.. Bi).23

B. Causal Underdetermination versus Causal Overdetermination

The previous subsection identi!ed and de!ned the pattern of causal under- determination in tort law, and particularly in negligence cases. A deeper understanding of this pattern and the problems that it entails requires us !rst to acknowledge the reasons for its almost complete absence from the vibrant, extensive debate on causation in the law.24 This absence is

FIG. 1. The Pattern of Causal Underdetermination

S.W.2d 707 (Tex. Sup. J. 1997) (where the Texas Supreme Court denied compensation in a mass tort case of children who suffered limb deformities since the plaintiffs failed to prove that the defendants increased the risk of such deformities by more than 50 percent); Dumas v. Cooney, 235 Cal. App. 3d 1593 (1991) (where the California Court of Appeal found the defendant’s late diagnosis of lung cancer negligent, but denied compensation since the prob- ability that the harm would have been avoided but for the negligence was lower than 50 percent). 21. See, e.g., ARIEL PORAT & ALEX STEIN, TORT LIABILITY UNDER UNCERTAINTY (2001), at 18. 22. As de!ned above, Â = {A1 , A2 , .. Ai }, where A* denotes the alternate behavior even-

tually chosen from the set Â. See supra note 15. 23. Where Ai ! Bi. 24. Section II discusses the few works dedicated to contrastive causation in the law that raise a

similar concern. These works, however, investigate the problem of underdetermination in the law as an example of one prominent dif!culty of the contrastive approach to causation, as dem- onstrated by the plant example below, rather than as a legal problem. They therefore miss the opportunity to develop a comprehensive discussion with regard to the source of causal

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especially noteworthy considering the scholarship’s massive engagement in exploring the seemingly mirror image of causal underdetermination in the law, namely causal overdetermination.

Overdetermination is widely discussed in the literature on causation in the law to describe factual settings in which the but-for test fails.25 The familiar pro!le of causal overdetermination in negligence cases consists of the following scenario: there are two or more defendants, each of whom has behaved wrongfully,26 and each of whose actions (or omissions)27

would have been suf!cient on their own to produce the plaintiff’s injury.28

Under these circumstances, none of the defendants can be identi!ed as the cause of the injury. To illustrate, imagine a scene in which a person is shot by two defendants who (independently) negligently !re in his direction;29

consider that each of these shots is suf!cient to cause the plaintiff’s injury. This illustration exempli!es circumstances in which the but-for test fails, since each of the defendants can claim that the plaintiff’s injury would have occurred but for his shot (for had his shot not hit the plaintiff, she would still have suffered the same outcome as a result of the other shooter’s bullet). Therefore, in such a case, the but-for test leads to the anomalous conclusion that neither of the shooters caused the plaintiff’s injury.30

Causal overdetermination is a mirror image to causal underdetermination, in which the but-for test leads to the impossible conclusion that a defendant could either be or not be the cause of the plaintiff’s injury at the same time.

In both causal overdetermination and causal underdetermination the defendant’s negligence is indisputable, and therefore the only question left to determine her liability in tort is that of causation. The difference between causal overdetermination and causal underdetermination, how- ever, is essential. As the shooters illustration exempli!es, causal overdetermi- nation may occur in factual conditions that consist of multiple causes (i.e.,

underdetermination in the law in general, and in particular in negligence cases, and to provide useful tools to resolve it. 25. Overdetermination may also be referred to as duplicative or preemptive causation situa-

tions. For elaboration on situations of factual overdetermination and their suggested solutions both metaphysically and speci!cally in the law, see, for example, Note, Rethinking Actual Causation in Tort Law, 130 HARV. L. REV. 2163 (2017), HART & HONORÉ, supra note 1, at 122– 128, 235–249; David Lewis, Causation as In!uence, 7 J. PHIL. 182 (2000); Richard W. Wright, Causation, Responsibility, Risk, Probability, Naked Statistics, and Proof: Pruning the Bramble Bush by Clarifying the Concepts, 73 IOWA L. REV. 1001, 1018–1023 (1988). 26. I.e., it is established that both defendants deviated from the standard of care, imposing

unreasonable risk on the plaintiff. 27. For further discussion regarding the “absence as causes” quandary, see supra note 1. 28. HART & HONORÉ, supra note 1, at 122–123. 29. This example is based on the famous case in the matter of Summers v. Tice, 199 P.2d 1

(Cal. 1948), with some plot changes. In Summers, the two negligent shooters in"icted two dif- ferent injuries. It was impossible to determine whose shot hit the plaintiff’s right eye, and whose shot hit the plaintiff’s upper lip. The illustration in the text presents a somewhat differ- ent scenario in which two shooters in"ict one injury. 30. The court in Summers resolved the factual dif!culty by imposing an alternative liability

rule, according to which in such circumstances both shooters should be held liable. See id. at 13–14.

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whereby A denotes more than one causal factor). By contrast, causal under- determination concerns a normative condition of multiple contrastive causes (i.e., whereby A* denotes more than one reasonable behavior). In other words, the problem of causal underdetermination is a product of a unique situation of normative overdetermination, whereby a negligent defendant could have met her duty toward a plaintiff in more than one way. It may thus occur only in MRB cases. The shift of focus to MRB cases is therefore crucial to revealing the

unique quandary of causal underdetermination. However, excluding some recent causal theorists whose works are discussed below,31 the phe- nomenon of MRB cases and its potential to produce causal underdetermi- nation in tort law are hardly present in debates on causation in the law, and accordingly, its appearance in court decisions in negligence cases is rela- tively rare. A conceivable explanation for the absence of causal underdetermination

from the causal debate may be that it simply escapes the radar of fact!nders. The question is why. A possible answer can be inferred from an observation made by Wex Malone over sixty years ago.32 Malone found that although both scholarship and courts embraced a contrastive implementation of the but-for test, in practice, fact!nders sometimes utterly ignored the but- for test’s contrastive dimension (A*, B*); in other words, they disregarded the contrastive question of what would have occurred had the defendant acted reasonably instead of being negligent. Reviewing courts’ decisions in the matter of factual causation, it becomes apparent that Malone’s obser- vation is still extant. This conclusion is evident from some courts’ state- ments, according to which, to establish causation “[a] plaintiff need not show a better result would have been obtained absent the [defendant’s] alleged negligence.”33 Since MRB cases, by de!nition, are concerned with the alternative courses of reasonable behavior not followed by the defendant, they can be exposed only in light of the two-dimensional coun- terfactual analysis of causation that consists of both the actual and the con- trastive inquiries. MRB cases would therefore remain hidden from the eyes

31. See infra notes 50–53 and accompanying text. See also PAUL & HALL, supra note 7, at 257– 258 (explicitly recognizing the special challenges derived from the relationship between causa- tion and legal liability, including the idea of “multiple possible events”). 32. Wex S. Malone, Ruminations on Cause-In-Fact, 9 STAN. L. REV. 60 (1956). 33. See, e.g., Sinclair v. Berlin, 758 N.E.2d 442 (Ill. App. 2001) (A case where the defendant

physician negligently failed to examine the plaintiff’s eyes despite her constant complaints of experiencing pain in her right eye. Subsequently, the plaintiff became blind in her right eye. Holding that the physician’s negligence was the cause of her blindness, the Appellate Court of Illinois stated that to establish causation the plaintiff was not required to prove that had the physician acted reasonably instead of negligently the result for her eye would have been bet- ter.); Morris v. Mark IV Constr. Co., 203 A.D.2d 922, 923 (N.Y. App. Div. 4th Dep’t 1994) (stat- ing that the defendants’ claims regarding the outcome that could have occurred for the plaintiff had the defendant offered any of the safety measures required by law “is not suf!cient to raise a triable issue of fact”).

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of fact!nders who are not instructed to administer a contrastive analysis of causation.

The second reason for the tendency to leave MRB cases out of the causal debate concerns the nature of the contrastive cause, A*. Though located within the counterfactual inquiry of the but-for test, conventionally per- ceived as a pure factual test, A* expresses a normative standard rather than a factual event. Speci!cally, it denotes behavior a defendant should have followed to meet her duty of care toward a plaintiff.34 For that reason, A* is usually considered and discussed as part of the question of the defen- dant’s wrongfulness, i.e., as part of the inquiry of whether the defendant deviated from the standard of care and should therefore be considered neg- ligent.35 To sharpen the difference between the latter question and the question at the center of this article, courts often focus their interrogation on whether A (the actual behavior of the defendant) is in line with either A1 or A2 (where A1 and A2 are both considered reasonable forms of behav- ior in given circumstances). By contrast, in MRB cases it is established that the defendant’s behavior, A, was not in line with either A1 or A2 and that the defendant is therefore negligent. Subsequently, the remaining question concerns the provable results of A*, !rst when it denotes A1 and second when it denotes A2. It is when these results lead to opposite conclusions on whether the negligent defendant caused the plaintiff’s injury that causal underdetermination occurs.

This article does not contend against the normative nature of contrastive cause A*.36 Rather, by focusing on its being located within the counterfac- tual inquiry of the but-for test, it suggests also accounting for its possible implications for the factual inference of causation in the law, and in

34. Accordingly, the lawful behaviors encompassing the reasonable ways the defendant did not follow is sometimes referred to by the term “theory of negligence.” See, e.g., Jones v. Alloy, 2015 N.J. Super. Unpub. LEXIS 290, 9–10, 13–14 (2015); Ykimoff v. W.A. Foote Mem’l Hosp., 285 Mich. App. 80, 87–88 (Mich. Ct. App. 2009); Posner v. Walker, 930 So. 2d 659, 667 (Fla. Dist. Ct. App. 3d Dist. 2006). 35. See, e.g., Shectman v. Brans!eld, 959 A.2d 278 (N.J. Super. 2008) (Where a defendant’s

psychiatrist allegedly failed to monitor his patient and thus did not foresee his attempting to commit suicide. The Superior Court of New Jersey observed that, considering the circum- stances, there were two possible medical approaches the psychiatrist could have followed; each of them was reasonable. Having followed one of them, the psychiatrist met the duty of care. Accordingly, the court absolved him of tort liability.). See also Saks v. NG, 890 A.2d 983 (N.J. Super. 2008) (A case in which a failed eye surgery resulted in blindness. The court held that there were two reasonable methods of anesthesia the physician could have chosen. Since the physician followed one of them, he was not held liable in tort.). 36. It should be noted that integrating normative considerations within the factual inquiry of

causation is a matter of controversy among scholars in the !eld. See, e.g., Richard Wright, Causation in Tort Law, 73 CAL. L. REV. 1735, 1740, 1803 (1985) (arguing that the causal inquiry in the law is a pure matter of factual and empirical inferences, almost always kept distinct from policy considerations). The contrastive account of causation embraced in this article presents a different view of factual causation in the law, which enables the imbrication of factual and nor- mative elements within the counterfactual inquiry, on the one hand, and distinguishing facts from normativity, on the other hand. For a similar observation regarding the theory of contras- tive causation in the law, see Broadbent, supra note 3.

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particular for causal underdetermination. As long as factual causation is a preliminary requirement for imposing legal liability, courts must both recognize the pattern of causal underdetermination and contend with it. The next subsection demonstrates how courts currently contend with causal underdetermination on the occasions that they do realize that they are facing it.

C. Causal Underdetermination as Lack of Causation

Courts often perceive causal underdetermination as lack of causation. According to this practice, it is enough if only one of the several reasonable behaviors supports the conclusion of lack of a causal link between the defendant’s negligence and the plaintiff’s injury to infer that the require- ment of causation is not met. Consequently, negligent defendants are being absolved of tort liability in such cases. This practice, here referred to as “the no-causal-link solution,” was the reason behind the court’s deci- sion in Castro v. San Diego Gas & Elec.37

Castro worked in the farm operating business. As part of his work he needed to cover roofs with plastic sheets, placing aluminum rods on top of them. One morning an accident occurred. Castro grabbed a rod that contacted a high-voltage power line and was severely injured. Subsequently, Castro sued the electric company that owned the power line. Seeking compensatory damages, he alleged that the electric company operated its high-voltage line negligently, breaching its duty of care in four different ways, each of which might have prevented the resulting injury: it could have complied with the relevant regulations regarding the power lines’ height, or removed the power line that was known as a hazard prior to Castro’s accident, or properly trained inspectors to meet the necessary requirements to prevent potential hazards, or prepared a plan aiming to detect dangerous sites of power lines so that owners of property located under such lines would be aware of the potential danger.38 As the defen- dant followed none of these reasonable alternatives,39 the jury at the trial found that the defendant was negligent. However, since the plaintiff failed to prove that it was the negligence that caused the plaintiff’s injury, the defendant company was excused from liability. Con!rming this decision, the California Court of Appeal stated that “Castro cannot prevail on his sub- stantial evidence challenge if there is substantial evidence to support the lack of causation on any one of his theories.”40 Furthermore, the Court of Appeal explicitly held that this conclusion would not have changed even if Castro had been able to successfully prove that one or more of the

37. Castro v. San Diego Gas & Elec. Co., 228 Cal. App. 4th 1280 (2010). 38. Id. at 7. 39. The exact phrase that the court used to describe the different reasonable behaviors not

followed by the defendant was “theories of negligence.” See id. at 25. For additional examples of the use of this phrase, see supra note 34. 40. Castro, 228 Cal. App. at 25.

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other reasonable behaviors mentioned would have led to an opposite but- for result (i.e., that the negligent company was the likely cause of the injury).41 As a result, the company was absolved of tort liability on grounds of lack of causation.42

Applying this practice to the physician example that opens the article, the negligent physician who chose to perform the surgery by procedure A, which is clearly riskier than either procedure A1 or A2, should be excused from tort liability, since, based on the autopsy results, had the physician per- formed the surgery reasonably, by procedure A1, the patient would likely have died anyway. Adopting the California Court of Appeal’s reasoning, this conclusion should not change even if the autopsy can prove that the other reasonable procedure not chosen by the physician, A2, would likely have led to the patient’s full recovery.

A possible explanation for this result could be based on evidential grounds and, speci!cally, on the preponderance-of-the-evidence standard. A fact!nder in the physician example may conclude that if there is one rea- sonable alternative that negates causation and another that af!rms it, then the plaintiff was not successful in proving that it is more likely than not that the patient would not have died but for the physician’s negligence. Consequently, the physician should not be held liable in tort for her negli- gence. This conclusion re"ects an approach that separates the results of each of the counterfactual inquiries (the one held with respect to the reasonable procedure A1, and the other held with respect to the reasonable procedure A2), thus perceiving each of them as either absolutely approving or absolutely negating the causal link between the physician’s negligence and the plain- tiff’s injury. However, this analysis of the causal inquiry ignores the degree to which the evidence supports the causal inference that the physician’s negligence led to the plaintiff’s injury in both of the counterfactual inqui- ries.43 The following numerical illustration demonstrates how the no-causal-link solution can lead to an erroneous decision excusing a defen- dant from tort liability even when the aggregated weight of the evidence shows that the probability that the defendant’s negligence is the cause of the alleged injury is higher than 0.5.

Consider that in the physician example the following probabilities were established by the autopsy evidence: 1) there is a 0.6 probability that the patient would have died had the physician performed the surgery by

41. Id. (citing a former Court of Appeal decision in the matter of Jonkey v. Carignan Construction Co., 139 Cal. App. 4th 26 (2006), according to which “[a]s long as a single theory of negligence is lawfully rebutted on a lack of causation theory, it matters not that another the- ory of negligence is not so rebutted”). 42. Castro, 228 Cal. App. at 24–25. 43. For the observation that in the law, causal inference depends on the nature of the evidence

available to prove what the outcome would have been had the defendant acted as she should have and on the fact!nder’s estimation of its weight, see, e.g., Mario J. Rizzo, Foreword – Fundamentals of Causation, 63 CHI.-KENT L. REV. 397, 405 (1987); David W. Robertson, Common Sense of Cause in Fact, 75 TEX. L. REV. 1765, 1774 (1997).

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A1; 2) there is only a 0.2 probability that the patient would have died had the physician performed the surgery by A2. For simplicity, assume that the “normative weight” of both procedures is similar (i.e., that they are equally appropriate).44 Aggregating the probabilities that the negligent phy- sician is the cause of the patient’s death, the conclusion is that there is an overall 0.6 probability45 that she is the cause of the unfortunate result.46 In this simple illustration, the preponderance requirement is met (0.6 > 0.5), and therefore the negligent physician should be held liable in tort. Absolving her of liability only because one of the alternate procedures would have led to the patient’s unfortunate death anyway is therefore a mistake. Moreover, by allowing negligent defendants to be absolved of tort liability

when the evidence shows that they are the likely cause of the plaintiff’s injury, the no-causal-link solution may create a problem of underdeter- rence.47 Aware of the no-causal-link practice, physicians in similar circum- stances may expect to be absolved of tort liability regardless of their negligence. Consequently, they may adopt a suboptimal level of care. Consider, for example, that the surgical procedure actually administered by the physician (procedure A) exposed the patient to a risk of 200K (prob- ability of 0.2 for harm of 1M), whereby administering either procedure A1 or A2 would have exposed the patient to a risk of 100K (probability of 0.1 for harm of 1M). In these circumstances, absolving the negligent physician of liability may cause physicians in similar cases to underestimate the risk that they impose on patients when they choose to undertake the riskier sur- gery, A. They will thus keep exposing their patients to an excessive risk of 100K in the future.48

44. This assumption is discussed at length later in the article. See infra Section II.B. 45. 1 ! (0.6 + 0.2)0.5 = 0.6. 46. The suggested aggregation of counterfactuals could be perceived as a speci!c implemen-

tation of the general idea of aggregation in the law. See Ariel Porat & Eric Posner, Aggregation and Law, 122 YALE L.J. 2 (2012) (suggesting that ef!cient deterrence could be achieved by the adoption of a general rule of aggregation of claims, instead of considering each claim separately). 47. See, e.g., David Kaye, The Limits of the Preponderance of the Evidence Standard: Justi"able Naked

Statistical Evidence and Multiple Causation, 1982 AM. B. FOUND. RES. J. 487, 494–503 (1982) (rec- ommending the preponderance-of-the-evidence standard as the most appropriate mechanism unless it leads to systematic errors associated with long-run bias toward either the plaintiff or the defendant); Saul Levmore, Probabilistic Recoveries, Restitution and Recurring Wrongs, 19 J. LEGAL STUD. 691, 693–696 (1990) (noting the problem of “recurring misses,” where there is constant bias due to which the but-for result constantly leads to the conclusion that there is a more than 0 percent but never more than 50 percent chance that a defendant in similar circumstances is the cause of an injury. In these cases, the tortfeasors are underdeterred). 48. 100K is the difference between the risk imposed on a patient by performing procedure A

and the reasonable risk that would have been imposed on her had the physician performed either procedure A1 or A2. In terms of ef!ciency, to incentivize future physicians in similar sit- uations to adopt an optimal level of care, they should be held liable only for the harm gener- ated by their negligence. See, e.g., Ariel Porat, Offsetting Risks, 106 MICH. L. REV. 243 (2007) (offering that in order to align the defendant’s liability in tort with the exact risk that her neg- ligence imposed on the plaintiff, when a negligent defendant had to choose between two risky

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Furthermore, the no-causal-link solution that redeems negligent defen- dants from tort liability in any case of causal underdetermination may gen- erate strategic behavior among defendants, motivated to increase their chances of escaping tort liability in retrospect. In particular, it may incentivize negligent defendants to search for as many non-taken reasonable precau- tions as possible. Under a rule that perceives causal underdetermination in and of itself as an indication of the absence of causation, defendants can realize that the greater the number of reasonable precautions they !nd, the greater their chance of avoiding tort liability. According to the no-causal-link solution, they need to !nd only one precaution that, if taken, would likely have led to the same result that actually occurred, to negate the requirement of causation and be excused from liability for their negligence.

II. CONTENDING WITH CAUSAL UNDERDETERMINATION

Having introduced the pattern of causal underdetermination and the prob- lems that it entails, this section moves to discuss possible solutions. For this purpose, consider again the physician example, as well as the assumption that an autopsy can prove that had the physician performed the surgery by A1, the patient would likely have died anyway, whereas had she chosen to perform the surgery by procedure A2, the patient would likely have fully recovered. The autopsy results lead to the impossible inference that the negligent physician both is the cause of the patient’s death and is not. As aforementioned, this result, derived from a speci!c legal pattern referred to here as causal underdetermination, is not essentially different from the dif!culty demonstrated by the above plant example. The latter example presented numerous alternatives that could be used as a baseline to evaluate whether my omission to water the plant, A, was the cause of its drying out, B (i.e., A* could denote either A1 = “watering the plant once a day”; A2 = “watering it every other day”; A3 = “watering the plant once a week”; and so forth). Where each of these alternatives leads to a different result, the key to inferring whether my omission was indeed the causal fac- tor of the plant’s drying out is to de!ne A* with a single value. This conclu- sion is true mutatis mutandis in regard to resolving the physician example. It all comes down to de!ning A* with a single behavior. The question is, how?

To begin answering this question, one should !rst consider the sugges- tions made in the literature on the theory of contrastive causation, concern- ing the requirements for de!ning the contrastive cause, A*, in the legal context. It should be noted that what follows is not a comprehensive survey of the context-sensitive analysis of causation outside the legal arena, or of

options, courts should offset damages by the risk that was not imposed on the plaintiff from the reasonable option).

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counterfactual approaches to causation other than the contrastive view.49 As this article focuses on the problem of causal underdetermination resulting from the contrastive structure of the causal examination in negligence law (with the hope that its examination will also contribute to the enhanced study on the context sensitivity of causal statement in general), the overview will sketch the principal proposals made in the legal literature on the def- inition of contrastive cause, starting with Jonathan Schaffer’s proposal that in the legal arena, the contrastive cause should denote whatever is cod- i!ed in the law.50

Schaffer explains that this observation complies with Jane Stapleton’s view, according to which in order to test whether a defendant’s breach of duty was involved by a connection of necessity to the plaintiff’s injury, we should ask what would have been the outcome in the hypothetical compar- ator world in which the defendant had acted just enough to comply with her duty as mandated by the law.51 Adjusting Stapleton’s reasoning to the con- trastive approach to causation, so Schaffer explains, the fact!nder would use the idea of reasonability as a !lter device providing by the law to de!ne the speci!c contrastive cause A*. However, Schaffer’s proposal is inade- quate to resolve the problem of causal underdetermination. As this prob- lem occurs in MRB cases, in which by de!nition the law allows fact!nders to de!ne A* by more than one behavior, his observation leaves the problem intact. A different proposal for de!ning the contrastive cause, A*, in the law is

Alex Broadbent’s suggestion that A* should denote an “appropriate foil” in which the defendant meets her “mere duty of care,” but not “in some particular way beyond what the duty itself requires.”52 Broadbent’s sugges- tion takes us a step forward toward the understanding that the contrastive cause is the element through which the causal inquiry becomes normatively sensitive to the context within which it is being held. The analysis suggested in the next section draws on this understanding, observing and demonstrat- ing the way in which the contrastive cause requires fact!nders to embed a (legal) normative analysis within the (philosophical) causal analysis. For the purpose of this section, however, it suf!ces to admit the failure of Broadbent’s proposal of de!ning the contrastive cause to resolve the

49. The best known counterfactual approach to causation is Lewis’s possible worlds view, inspired by the development of possible worlds semantics in modal logic. A very simplistic way to describe this view is that it holds a counterfactual true if and only if it is true in the closest possible world to the actual world. See David Lewis, Causation, 70 J. PHIL. 556, 556–567 (1973). For the position that the similarity relevance to decide which world is closer to the actual world is context related, see, e.g., Menzies, supra note 13, at 139, 139–142, 154, 172 (stating that “causal statements in different contexts require different similarity relations” and positing this view ver- sus Lewis’s view that for each claim of causal relation there is a unique kind of similarity that should determine the corresponding counterfactuals). 50. Schaffer, Contrastive Causation, supra note 3, at 315. 51. Jane Stapleton, Choosing What We Mean by “Causation” in the Law, 73 MO. L. REV. 433, 451

(2008). 52. Broadbent, supra note 3, at 189.

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pattern of causal underdetermination in tort law. Similarly to Schaffer’s def- inition of the contrastive cause in the legal context, Broadbent’s proposal does not contend with the core feature of MRB cases, which is the multiplic- ity of particular ways that a defendant could have complied with her duty of care toward a plaintiff.53 It therefore also leaves open the question of how to determine which of these alternate reasonable ways should be the one to de!ne A*. To resolve the quandary of causal underdetermination, either by de!ning A* with a single behavior or by any other means, other solutions must be considered.

Prior to discussing such solutions, however, three cases of causal underde- termination should be distinguished: the !rst consists of circumstances that enable fact!nders to resolve causal underdetermination by making a norma- tive evaluation and accordingly to prioritize reasonable behaviors that the negligent defendant did not follow. These cases of causal underdetermina- tion consist of what is here termed “normative certainty.” The other two types of causal underdetermination involve circumstances in which a fact- !nder is unable to decide which of the reasonable behaviors not followed by the negligent defendant is more desirable. These cases of causal underde- termination consist of what is here termed either “normative equivalency” or “normative uncertainty.” In the former, fact!nders are capable of norma- tively evaluating the reasonable behaviors that lead to opposite causal infer- ences, but nevertheless, cannot prioritize among them, as they !nd them equally appropriate. By contrast, in the latter cases, fact!nders cannot distin- guish which of the reasonable behaviors not followed by the negligent defen- dant is normatively superior to the others because they lack the information to do so. The solutions discussed and demonstrated in this section are tai- lored to contend with each of these cases of causal underdetermination.

It should be noted that although the normative evaluation below is based on terms of ef!ciency, the observation regarding the distinction between the three cases of causal underdetermination is relevant, mutatis mutandis, to other legal theories as well.54

A. Cases of Normative Certainty: The Most Appropriate Behavior Solution

This subsection focuses on situations in which causal underdetermination can be avoided by the fact!nder’s normative judgment regarding the desir- ability of the lawful behaviors the defendant could have followed to meet her duty toward the plaintiff. Such a normative evaluation enables a fact- !nder to prioritize in retrospect the alternate behaviors the negligent

53. For further critiques on Broadbent’s proposal, see Schaffer, Contrastive Causation in the Law, supra note 3, at 293–295. 54. A change of normative consideration may lead to a change in de!ning A* (which

denotes the behavior the defendant should have followed), which in turn may lead to a change of but-for result. The exploration of differences in but-for outcomes resulting from de!ning A* in accordance with different normative perspectives is not within the purposes of this article.

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defendant could have followed ex ante in the speci!c MRB case that she (the fact!nder) faces. When such a normative prioritization can be made, it is suggested that A* should denote the most appropriate behavior, i.e., the alternate behavior that, in the speci!c circumstances, is in best com- pliance with the fact!nder’s normative view. This de!nition of A*, also termed here “the normative solution,” can be adjusted to any normative view. In this article appropriateness is evaluated in light of ef!ciency consid- erations. In terms of ef!ciency, in negligence cases, A* should de!ne an alternate behavior that meets two conditions: !rst, its costs should be less than the expected cost of the injury. This condition is articulated by the Learned Hand formula applied by courts to determine whether a defen- dant was negligent, B < PL, where B stands for the burden of precaution, P for the probability of injury, and L for the loss.55 Second, there is no other procedure that would entail a smaller sum of total costs accrued by both the injury and its prevention. In keeping with these conditions, A* would represent the behavior that minimizes the total costs of both the inju- ries and their prevention.56

A fact!nder may de!ne A* in accordance with the normative solution hereby illustrated in terms of ef!ciency, on two conditions: !rst, she must have the necessary information regarding the risks and costs of any of the different alternate behaviors of the speci!c MRB case. Such information is required in order to make a normative assessment regarding the alternate behavior’s desirability given the circumstances. Second, the fact!nder must be capable of prioritizing the alternate behaviors based on her normative analysis, i.e., she must be able to single out a speci!c behavior that is most appropriate given the circumstances. MRB cases that meet these two conditions are consistent with what is here termed “normative certainty.” To illustrate such a case, suppose that in the physician example, the evi-

dence shows both that procedures A1 and A2 are safer and less costly than procedure A performed by the physician and that procedure A1 entails lower total costs (i.e., of both expected risk and costs of operation) com- pared to procedure A2. In these circumstances, the ef!cient procedure is A1. Accordingly, A1 should be the procedure that de!nes A*. Denoting A* with a single behavior allows the but-for test to express only

one counterfactual. In the physician example, this counterfactual is articu- lated by the following question: What would have been the outcome for the plaintiff had the physician chosen to administer procedure A1 instead of A?

55. According to Hand formula, if an injury could be avoided for less than it costs, then the individual should take the precautions, rather than allowing the injury to occur. Otherwise she will be considered negligent. The Hand formula was articulated by Judge Learned Hand in United States v. Carroll Towing Co., 159 F.2d 169 (1947), and was later endorsed by courts as well as the Restatement of Torts. See RESTATEMENT (THIRD) OF TORTS: LIABILITY FOR PHYSICAL HARM (2010), at §3 cmt. e (suggesting that negligence can be asserted by a risk-bene!t test identical to the Hand formula). 56. See, e.g., CALABRESI, supra note 5.

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Thus, assuming that an autopsy can prove with a probability greater than 0.5 that operating with A1 would likely have resulted in the patient’s death any- way, the causal result should be that the negligent physician is not the cause of the patient’s death. The negligent physician should therefore be absolved of tort liability for lack of causation. By focusing on a single coun- terfactual, the normative solution enables the fact!nder in the physician example to avoid the quandary of causal underdetermination.

The normative solution could have been a useful tool in the famous case of Haft v. Lone Palm Hotel.57 In that case, a father and son drowned to death in a hotel swimming pool. The California Health and Safety Code explicitly enabled pool operators, such as the defendant hotel, to choose one of two alternatives: they could either provide lifeguard services or post a warning sign alerting swimmers to the fact that there was no lifeguard at the pool. Having been stipulated in the Safety Code, the lifeguard and the warning sign alternatives were both considered reasonable safety pre- cautions. By choosing either one of them, the hotel would not have been considered negligent in the !rst place. Having followed neither, the hotel was indisputably negligent, and the question that remained was whether it is more likely than not that this negligence was the cause of the dece- dents’ deaths by drowning.

Haft illustrates an MRB case that may lead to causal underdetermination. The parties in Haft were well aware of such a possibility: the plaintiffs real- ized that the failure to post a warning that there was no lifeguard was not a likely cause of the deaths, because the decedents may have been aware of the absence of a lifeguard and nonetheless entered the pool. Therefore, they argued that the causal inquiry should be administered in light of the statutory alternative of providing lifeguard services.58 For the same reason, the defendant argued that the counterfactual analysis should be adminis- tered from the point of view of the absence of a warning sign, rather than the absence of a lifeguard.59 The counterarguments of the parties in Haft suggested the existence of opposite but-for inferences, namely, a causal underdetermination.

So, what should have been the but-for result had each of the reasonable behaviors stipulated in the Safety Code (either a lifeguard or a warning sign) led to an opposite factual inference? Interestingly, both the California Court of Appeal and the California Supreme Court were successful in avoid- ing this question. The Court of Appeal held that the plaintiffs had not suc- ceeded in proving that neither a warning sign nor a lifeguard would have changed the unfortunate result. Since the plaintiff did not meet the requirement of causation, the Court of Appeal found that the negligent

57. 83 Cal. Rptr. 312 (Cal. Ct. App. 1970). The Court of Appeal decision was later reversed by the California Supreme Court. See Haft v. Lone Palm Hotel, 3 Cal. 3d 756 (Cal. Ct. Sup. 1970). 58. See Haft, 83 Cal. Rptr. at 316. 59. Id.

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hotel should not be held liable for the father’s and son’s deaths by drown- ing.60 This decision was reversed by the California Supreme Court based on evidentiary grounds. Writing the unanimous decision of the court, Justice Tobriner held that the absence of a lifeguard denied the victims not only a safety measure, but also a witness to the accident. It was therefore decided that the burden of proof would shift to the defendants, who failed to dis- charge this burden. Ariel Porat and Alex Stein suggest that the shift of burden in Haft can be

justi!ed based on the doctrine of evidential damage.61 This doctrine enables the imposition of liability on defendants whose negligence aggra- vated factual uncertainty in a way that undermined the plaintiff’s ability to establish the requirement of causation. The doctrine of evidential dam- age, designed to contend with evidentiary hurdles detrimental to the plain- tiff’s ability to prove causation, however, should not be mistaken for a tool to resolve the potential quandary of causal underdetermination raised by the parties’ counterarguments in Haft. This quandary is the result of the unique normative features of MRB cases. True, had the defendant not been negli- gent in the !rst place, this quandary would not have arisen at all. However, the defendant’s negligence is not the source of causal underdetermina- tion.62 The normative solution suggested above is more appropriate for resolving causal underdetermination than is the evidential-damage doc- trine, since it addresses the source of its dif!culty, which is normative, rather than its factual symptoms. Following the normative solution suggested here, the courts in Haft could

have avoided the quandary of causal underdetermination by de!ning A* as the reasonable behavior that minimizes the total costs of both the drowning injuries and their prevention.63 This goal is achieved when each pool oper- ator seeks to minimize the total costs of her expected injury and the costs of its prevention.64 In Haft, the law permitted replacing a lifeguard with a warning sign. Clearly, the latter alternative would reduce the costs of pre- cautions to the minimum. However, to induce future pool operators to at least follow this least expensive alternative, a fact!nder should de!ne A* as “providing a lifeguard.” This conclusion is based on the assumption that applying the but-for test with the lifeguard alternative is more likely to lead to the but-for result that a pool operator’s omission to provide a safety measure is the cause of a drowning event.65 Accordingly, pool

60. Id. at 321, 326. 61. Ariel Porat & Alex Stein, Liability for Uncertainty: Making Evidential Damage Actionable, 18

CARDOZO L. REV. 1891, 1908–1909 (1997). 62. Unless it is the defendant’s negligence that obscured the facts of the case and thus

caused what is here termed normative uncertainty. See infra Section II.B. 63. See supra note 56 and accompanying text. 64. Id. See also STEVEN SHAVELL, ECONOMIC ANALYSIS OF ACCIDENT LAW (1987), at 6–9. 65. For a similar observation, see Levmore, supra note 47, at 705–706 (explaining that apply-

ing the but-for test by comparing the actual result to the probable result of posting a warning sign may produce a causal link in 10 percent of similar cases).

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operators will likely be held liable in tort for any injury that occurs following their negligence. Aware of this result, pool operators will be induced to at least post a warning sign in the absence of a lifeguard.

De!ning A* as the single reasonable behavior of providing a lifeguard is only the !rst stage of the MRB causal inquiry. A fact!nder still needs to infer whether, in the speci"c case that she faces, providing such services would likely have prevented the decedents’ deaths by drowning. Only a pos- itive answer to that question con!rms that the pool operator’s negligence was indeed the cause of the deaths. Having reached a single but-for result, it is apparent that the normative solution is a useful key to avoiding causal underdetermination in cases where the law stipulates several reasonable alternatives to meet a speci!c duty.

However, as aforementioned, the normative solution can only be applied if a fact!nder has both enough information to normatively evaluate the dif- ferent alternate behaviors of the MRB case that she faces and the ability to effectively prioritize these behaviors based on such evaluation (i.e., to single out the most appropriate behavior). The next subsection illustrates cases of causal underdetermination in which at least one of these conditions is not met.

B. Normative Equivalency and Normative Uncertainty

To illustrate a possible setting of causal underdetermination that consists of what is here de!ned as “normative equivalency,” consider again the physi- cian example, with a minor change:

The Physician Example. Version 2. The physician still makes the negligent choice of performing the surgery by procedure A, instead of by the less risky and less costly treatments A1 or A2. However, this time procedures A1 and A2 incur similar costs and are equally safe. As in the former version, the autopsy result can prove that had the physician performed the surgery by A1, the patient would likely have died anyway, whereas had she chosen to perform the surgery by procedure A2, the patient would likely have fully recovered.

Here too a fact!nder faces a case of causal underdetermination. Furthermore, this version of the physician example also demonstrates cir- cumstances in which a fact!nder can make a normative assessment regard- ing the desirability of the reasonable behaviors that the negligent defendant could have followed to meet her duty toward the plaintiff under the circum- stances. She has the information needed to evaluate the risks and costs of each of the reasonable procedures not administered by the physician. However, contrary to the former version of the example, in this version, based on her assessment, the fact!nder perceives the procedures not fol- lowed by the physician as equally desirable; neither A1 nor A2 stands out as “the most appropriate behavior” in the circumstances. In ef!ciency terms, the second version of the physician example demonstrates a case

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in which there is not one, but two optimal levels of care. In these circum- stances, a normative evaluation is not a suf!cient means to de!ne A* with a single value. Both A1 and A2 may denote A*. As the autopsy results show that either of these alternatives would have led to a different outcome for the patient, the problem of causal underdetermination persists. The same conclusion appears in the following, slightly different, version of the physician example:

The Physician Example. Version 3. The circumstances are similar to the two ear- lier versions of the example, with a minor change: it is impossible to know which of the alternate procedures, A1 or A2, entails smaller risks or lower costs under the circumstances. Here too, the autopsy results prove that had the physician chosen to perform the surgery by procedure A1, it is likely that the patient would have died anyway, whereas had she chosen to perform the surgery by procedure A2, the patient would likely have fully recovered.

This version of the physician example illustrates a possible setting of causal underdetermination that consists of what is here de!ned as “normative uncertainty,” whereby a fact!nder lacks the information to conclude which one of (at least) two reasonable behaviors not followed by a negligent defendant is normatively preferable under the circumstances. The normative uncertainty demonstrated in the latter version of the phy-

sician example may arise, for instance, with respect to new or experimental medical methods that have not yet produced the necessary evidence to infer the potential costs and risks associated with their implementation. It may also result from an unresolved controversy over the expected risks associ- ated with two or more familiar medical methods (when both are considered reasonable, but it is impossible to determine which of them would minimize the costs of the expected harm and its prevention). The difference between the third and the second versions of the physi-

cian example is the reason the fact!nder is not able to prioritize the desir- ability of the alternate procedures, A1 and A2, i.e., to retrospectively decide upon a single optimum level of care, in these circumstances. Whereas in the third version, which demonstrates a case of normative uncertainty, a fact- !nder cannot infer which of these procedures should be considered the ef!cient treatment for factual reasons, in the second version, which describes normative equivalency, the fact!nder is incapable of singling out the ef!cient treatment for normative reasons (she perceives them both as equally appropriate). Be that as it may, if the number of reasonable treatments that lead to two opposite but-for inferences cannot be reduced to one, the problem of causal underdetermination persists. The fact!nder in both versions may conclude both that the negligent physician is the likely cause of the patient’s death and that she is not. The next subsection proposes that in cases of both normative equivalency

and normative uncertainty, where causal underdetermination cannot be

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avoided by normative means, fact!nders should turn instead to different solutions based on either an aggregation mechanism or statistical evidence.

C. Aggregation Mechanism and Statistical Evidence Based Solutions

Where (at least) two of the reasonable behaviors a negligent defendant did not follow lead to opposite conclusions, in the absence of normative cer- tainty that enables a fact!nder to single out the most appropriate behavior in the circumstances, the problem of causal underdetermination persists. This subsection introduces two possible solutions for causal underdetermi- nation in such cases. The !rst solution is based on an aggregation mecha- nism,66 previously demonstrated in Section I.67 This solution resolves causal underdetermination by allowing the but-for inquiry to take into account the degree of proof (roughly estimated in probabilistic terms) that the outcome would have been different had the defendant acted reasonably instead of negligently, with respect to each “reasonable theory of negligence”68 of the MRB case. In the physician example, this solution takes into account the aggregation of both the probability that the outcome would have been different had the physician administered procedure A1 and the prob- ability that the outcome would have been different had she administered procedure A2, based on the autopsy results. The second solution is termed here the “dominant behavior solution.”69 According to this solution, A* should denote the alternate behavior that statistics can prove is the domi- nant practice70 out of the several MRB options.

To demonstrate these solutions, consider again the physician example. Assume that statistics can show that in similar cases 60 percent of all physi- cians perform the surgery by A1 and, respectively, only 40 percent perform it by A2. The analogy in such a case is tossing a biased coin, with expected results of falling on A1 in 60 out of 100 "ips and falling on A2 in 40 out of 100 "ips. Assume further that the autopsy results can show that there is a 0.6 probability that the patient would have died had the physician performed

66. For elaboration on the idea of aggregation in the law, see Porat & Posner, supra note 46. 67. See supra notes 44–46 and accompanying text. 68. As aforementioned, the behaviors encompassing the reasonable approaches the defen-

dant did not follow is sometimes referred to by the term “theory of negligence.” See supra note 34. 69. This solution is based on the reasoning of David Kaye’s suggested maximum-likelihood

rule. See Kaye, supra note 47, at 508–513. 70. It should be noted that de!ning A* according to probabilistic data inferred from statis-

tical evidence may be controversial. Some scholars believe that naked statistical evidence should not count as an indicator for proportional liability. See, e.g., ALEX STEIN, FOUNDATIONS OF EVIDENCE LAW (2005), at 238–241 (arguing that naked statistical evidence allocates the risk of error between the parties in an unequal fashion and thus violates the basic notion of equality in civil litigation); Chris William Sanchirico, Character Evidence and the Object of Trial, 101 COLUM. L. REV. 1227, 1259–1263 (2001) (arguing that leaning on data regarding statistical evi- dence, such as the defendant’s prior criminal conduct, may create distorted incentives for ef!- cient behavior ex ante).

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the surgery by A1, and only a 0.2 probability that the patient would have died had the physician performed the surgery by A2. Lastly, consider that the court estimates the patient’s loss of life at 1 million dollars. Given this information, the resulting difference between the actual un-

fortunate result (the patient’s death) and the aggregated probabilities that this result would have occurred regardless of the physician’s negligence is: 1 ! (0.6 ! 0.6 + 0.4 ! 0.2) = 0.56. Accordingly, the fact!nder can conclude that there is a 0.56 probability that the physician’s negligence is the cause of the patient’s loss of life. It should be noted that this result does not imply that causation in itself is

a matter of probability.71 Rather, it means that the plaintiff could prove her causal claim up to a certain degree—0.56 probability in this case. Accordingly, we could say that the plaintiff has successfully met the preponderance-of-the-evidence standard required to establish her claim for the existence of a causal link between the defendant’s negligence and the patient’s death.72

Having preponderantly established the requirement of causation, a court can impose liability on a negligent physician, holding her legally respon- sible for the proven results of her negligence. There are two different ways to impose liability in these circumstances: either according to the preponderance-of-the-evidence standard or according to a proportional liability reasoning.73 To demonstrate both alternatives, consider again the 0.56 probability that the physician’s negligence is the cause of the patient’s loss of life. As this level of proof complies with the preponderance- of-the-evidence standard (0.56 >0.5), one option is to conclude that the physician should be held 100 percent liable for the patient’s loss of life and that the sum of compensation should be therefore set at 1M.74 Alternatively, the physician’s liability could be limited in accordance with the probability that her negligence caused the patient’s loss of life. In that case, the required compensation would be set at 560K out of 1M. Another possible implementation of the aggregation solution is to attri-

bute to both reasonable procedures, A1 and A2, equal weight, regardless

71. Discussing the nature of causal law is not within the scope of this article. 72. There could be a distinction between the magnitude of proof regarding the existence of

causation, based on the evidence, and the materialized belief regarding the existence of cau- sation based on this estimation. Richard Wright, for example, argues that the latter belief is not a matter of degree but rather a binary decision as to the existence of causation. See Richard Wright, Proving Causation: Probability versus Belief, in PERSPECTIVES ON CAUSATION 195, 205–212 (Richard Goldberg ed., 2011). The suggested analysis does not aim to contend against this observation. It is possible, for example, that the offered analysis would lead a fact!nder to believe that there is a causal link between the physician’s negligence and the patient’s death as a matter of fact. 73. Be that as it may, the choice of liability regime is a matter of legal policy rather than

causation. 74. This implementation expresses an all-or-nothing reasoning, according to which when the

probability that the negligent defendant is the cause of the harm exceeds 0.5, the defendant should be held 100 percent liable for that harm.

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of any statistical evidence that may prove any other probabilistic division between them. This probabilistic equivalency may express either a state of normative equivalency or a policy akin to the idea of tossing a fair coin.75

Whatever the reason, attributing equal weight to the alternate procedures in the physician example will lead to the conclusion that it is more likely than not that the negligence is the cause of the patient’s death, in light of the following aggregation:

1 ! (0.6 + 0.2)0.5 = 0.6. According to this result, there is a 0.6 probability that the negligent physician is the cause of the patient’s loss of life. Accordingly, the court may decide to either base its decision on the prepon- derance standard and hold the negligent physician 100 percent liable for the patient’s loss of life, and thus set the sum of compensation at 1M, or limit her liability in accordance with the probability that her negligence caused the patient’s loss of life and set the compensation at 600K.

A second solution for causal underdetermination in the absence of nor- mative certainty is to have A* denote the “dominant behavior” based on the statistical evidence. In light of this solution, given the aforementioned statis- tics in the physician example, A* should denote A1 since there is a greater chance that a reasonable physician in the defendant’s shoes would have chosen to perform the surgery with it than with A2. According to the above numerical example, in this case, the probability that the physician’s negligence caused the patient’s death is lower than 0.5.76 Consequently, the physician in our example would be absolved of tort liability. Alternatively, allowing an option of proportional liability, in lieu of the pre- ponderance rule, the physician’s liability can be restricted according to the probability that her negligence caused the patient’s loss of life.77 In this case, she will be required to compensate the plaintiff in the amount of 400K (out of the 1M estimation of the patient’s loss of life).78

It should be noted that although both the aggregation solution and the dominant behavior solution enable resolving causal underdetermination in cases of normative equivalency and normative uncertainty, the former may be perceived as preferable from an economic perspective. This is mainly because the aggregation solution enables the imposition of liability on defendants in proportion to the damage that they were proved to have in"icted on others. A similar numerical result can be achieved through

75. For a related argument of a !fty-!fty split between the litigating parties, see, for example, John E. Coons, Approaches to Court Imposed Compromise: The Uses of Doubt and Reason, 58 NW. U. L. REV. 750, 759 (1963) (arguing that in some cases of factual uncertainty, courts should impose a compromise by dividing the sum claimed between the parties, assuming that the court assigns equal weight to every alternative); Gideon Parchomovsky et al., Of Equal Wrongs and Half Rights, 82 N.Y.U. L. REV. 738 (2007) (suggesting that an equal division between litigating parties is jus- ti!ed in cases where other solutions may create windfall gains or losses). 76. According to the autopsy results, had the physician administered the surgery by A1 there

is a 0.6 probability that the patient would have died anyway. 77. See Kaye, supra note 47, at 509. 78. B ! B1 = 1 ! 0.6 = 0.4; 0.4 ! 1M.

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the familiar proportional liability rule, which aims to ensure that defen- dants will internalize exactly the expected losses that they create.79

However, the aggregation solution is preferable to the proportional liability rule from a causal perspective: the traditional rule of proportional liability is known for its ability to establish a necessary inference when factual uncer- tainty prevents a plaintiff from establishing the requirement of causation under the preponderance rule.80 Therefore, it allows a plaintiff to recover for risks the defendant in"icted upon her without proving that the defen- dant actually caused the resulting injury. By contrast, the aggregation solu- tion proposed here provides doctrinal and theoretical grounds for a proportional liability result, while keeping with the framework of the but-for test, namely, without waiving the preliminary requirement of preponder- antly establishing a causal link between the defendant’s negligence and the resulting injury.

CONCLUSION

This article explored a legal pattern in negligence law termed causal under- determination. In this pattern the but-for test leads at the same time to two opposite conclusions: according to one the defendant’s negligence is the cause of the injury, while according to the other she is not. Causal under- determination can only occur in Multiple Reasonable Behavior cases, in which there are several reasonable behaviors the defendant could have fol- lowed to meet her duty toward the defendant, she had followed none of them, thus acted negligently, and subsequently, a harm occurred. When asking whether the unfortunate outcome would have been different had the defendant acted reasonably instead of negligently, each of the reason- able behaviors might lead to a different conclusion. When this happens, the unique situation of causal underdetermination occurs. The article discussed and demonstrated the pattern of causal underdeter-

mination, the dif!culties it raises, and its possible solutions through a par- ticular approach to the counterfactual view of causation, namely, the contrastive theory of causation. The quaternary relation formed by this the- ory corresponds with the test of causation applied in the law in general, and

79. Generally speaking, proportional liability determines recovery by multiplying the plain- tiff’s total damages by the percentage chance that the defendant caused the damages. 80. See, e.g., Richard Delgado, Beyond Sindell: Relaxation of Cause-in-Fact Rules for Indeterminate

Plaintiffs, 70 CALIF. L. REV. 881 (1982) (suggesting relaxing the rules of causation in indetermi- nate plaintiffs tort cases and allowing distribution of damages based on allocation schemes of proportional recovery); David Rosenberg, The Causal Connection in Mass Exposure Cases: A “Public Law” Vision of the Tort System, 97 HARV. L. REV. 849 (1984) (arguing that in mass exposure cases the court should award each potential victim compensation in proportion to the likelihood that her injury was caused by the defendant’s wrongful exposure); Daniel A. Farber, Toxic Causation, 71 MINN. L. REV. 1219 (1986) (same); Glen. O. Robinson, Probabilistic Causation and Compensation for Tortious Risk, 14 J. LEGAL STUD. 779, 779–780 (1985) (observing that in many tort cases causation is inherently hard to prove, particularly when the injury is the result of a combination of several causes and isolating one of them as the cause is a dif!cult task).

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speci!cally in negligence cases, which examines whether the result would have been different (B– B*>0) had the defendant acted reasonably (A*) instead of negligently (A). (where B denotes the actual harm and B* the probable harm that would have occurred had the defendant acted reason- ably). Given the similarity between the structure of the causal inquiry applied in negligence cases and the causal structures of the contrastive the- ory of causation, the article not only used the contrastive theory as a descrip- tive method, but also sought to learn from the suggestions made in the literature on contrastive causation with respect to the challenge of de!ning the relevant contrastive event for any actual event. It then explained why these suggestions are inadequate to resolve the problem of causal underde- termination in negligence cases. Subsequently, the article suggested differ- ent solutions, based on the understanding that the source of causal underdetermination in the law is !rst and foremost normative. In particular, it is the result of distinct circumstances in which the law is unable to !lter a certain reasonable behavior with which the causal inquiry should be held.

Following this insight, the proposed solutions in the article were based on whether a normative analysis can lead the fact!nder to prioritize one rea- sonable behavior over the other MRB alternatives. The normative analysis was demonstrated in the article through an economic theory of negligence law, distinguishing between cases where a fact!nder is capable of deciding which of the multiple reasonable behaviors re"ects the optimum level of care and cases where a fact!nder cannot single out the optimal behavior in the circumstances, either because the reasonable behaviors that lead to opposite conclusions are equally desirable or because the fact!nder lacks the information needed to prioritize them. For the latter sorts of cases, the article offered different solutions, based on an aggregation mechanism and statistical evidence.

It is my hope that the article not only exposes the dif!culties of causal underdetermination in negligence law and offers viable solutions to con- tend with it, but also provides a possible answer to Jonathan Schaffer’s pon- dering as to the way in which contrasts are generated from the context, at least with respect to the legal arena.81 The article’s answer to this concern is that in the law, the contrastive cause depends on normative analysis. Therefore, the context sensitivity of the theory of contrastive causation in the law is expressed in the idea that the contrastive cause is the window through which the causal theory integrates with legal theory.

81. Schaffer, Contrastive Causation, supra note 3, at 320.

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