Negligence - Harms traceable to multiple causes

Learning Outcomes

This article explains how negligence law handles harms traceable to multiple causes on the MBE, including:

• Identifying when an exam fact pattern presents concurrent, successive, or alternative causes, and translating those factual variations into distinct causation analyses.
• Comparing the ordinary but-for test with the substantial factor test, and selecting the correct test when multiple sufficient or multiple necessary causes appear in MBE questions.
• Applying alternative liability and related burden-shifting doctrines when several negligent defendants are before the court but only one actually caused an indivisible injury.
• Determining when joint and several liability applies, when liability is only several, and how contribution and indemnity reallocate loss among multiple tortfeasors.
• Distinguishing alternative liability, concerted action, res ipsa loquitur against multiple defendants, and market share liability, and spotting which theory the bar examiners are testing.
• Evaluating whether harm is divisible or indivisible, how damages should be apportioned across successive or concurrent defendants, and when a later negligent act aggravates an existing injury.
• Integrating plaintiff’s contributory negligence or comparative fault into multi-defendant problems, calculating the impact on recoverable damages while keeping actual and proximate causation analytically separate.

MBE Syllabus

For the MBE, you are required to understand how negligence law addresses situations with multiple potential causes of harm, with a focus on the following syllabus points:

• Recognizing when the ordinary “but for” test of actual causation is inadequate because multiple sufficient causes exist.
• Applying the substantial factor test in concurrent-causation problems.
• Distinguishing alternative liability, concerted action, res ipsa loquitur against multiple defendants, and market share liability.
• Understanding when joint and several liability applies and when liability is only several.
• Knowing how apportionment of damages works when harm is divisible versus indivisible.
• Integrating plaintiff’s contributory negligence or comparative fault into multi-defendant negligence problems.

Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, remember to look more closely at that area during your revision.

1. When two defendants act independently, and either act alone would have been sufficient to cause the plaintiff’s harm, which test is used to determine actual causation?
   1. But-for test
   2. Substantial factor test
   3. Proximate cause test
   4. Market share liability
2. If two hunters negligently fire in the plaintiff’s direction, but only one bullet causes injury and it is impossible to tell whose shot hit the plaintiff, what doctrine may apply?
   1. Res ipsa loquitur
   2. Alternative liability
   3. Comparative negligence
   4. Last clear chance
3. In a negligence case with multiple defendants, when is joint and several liability most likely to apply?
   1. When each defendant’s act is a but-for cause of harm
   2. When the harm is indivisible and caused by concurrent acts
   3. When the plaintiff is contributorily negligent
   4. When the harm is divisible

Introduction

Negligence rarely occurs in a vacuum. Many exam fact patterns involve several actors, multiple risk-creating events, or successive accidents. The core questions are:

• Is each defendant an actual cause of the harm?
• Is the harm indivisible or can it be separated among defendants?
• How is liability allocated among negligent parties?

To answer these questions you must be comfortable moving beyond the simple “D hits P” model and applying special causation and allocation doctrines.

Key Term: Concurrent Causes
Two or more acts or events that operate together to produce an indivisible injury. Sometimes either act alone would have been sufficient to cause the harm; other times neither is sufficient by itself, but together they are.

Key Term: Alternative Liability
A doctrine that shifts the burden of proof on causation to multiple negligent defendants when: all were negligent, only one actually caused the plaintiff’s indivisible injury, the plaintiff cannot identify which one, and all possible tortfeasors are before the court.

Multiple Causes in Negligence

In a standard single-defendant case, actual causation is tested by the but-for test: but for the defendant’s breach, would the harm have occurred? When more than one cause is present, three broad patterns appear on the MBE:

• Multiple sufficient causes: Each defendant’s act, by itself, would have produced the harm (e.g., two independently set fires that merge and destroy a house).
• Multiple necessary causes: No single act is sufficient, but together they cause the harm (e.g., two drivers whose combined impacts injure the plaintiff).
• Alternative causes: Several defendants were negligent, but only one actually injured the plaintiff and we cannot tell which.

The law adjusts the causation analysis to keep negligent defendants from escaping liability purely because of evidentiary uncertainty.

The Substantial Factor Test

When two or more defendants act independently and each act alone would have been sufficient to cause the plaintiff’s harm, the ordinary but-for test fails. If you ask, “But for D1’s conduct, would the harm have occurred?”, the answer is “Yes, because D2’s conduct alone would have caused it,” and vice versa. To avoid the absurd result that nobody is a cause, courts use the substantial factor test.

Key Term: Substantial Factor Test
A causation test used when multiple independent acts could each have caused the harm; a defendant is liable if their conduct was a substantial factor in bringing about the injury.

A defendant’s conduct is a substantial factor if it was more than trivial or negligible in producing the harm. In the classic “two fires” scenario, each negligently set fire that is large enough on its own to destroy the property is a substantial factor.

On the MBE:

• Look for language such as “either act alone would have been sufficient” or “two independent causes, each capable of causing the harm.”
• Apply the substantial factor test instead of the but-for test.
• If both defendants’ acts were substantial factors in causing an indivisible harm, each is liable for the entire loss (subject to contribution among them).

Substantial factor problems almost always lead into joint and several liability.

Key Term: Joint and Several Liability
A rule under which each of multiple defendants is liable for the entire indivisible harm when their concurrent acts were each a substantial factor in producing it. The plaintiff may recover all damages from any one liable defendant.

Application and Consequences

If both defendants’ acts were substantial factors, each is liable to the plaintiff for the full amount of damages. Behind the scenes, defendants can seek:

• Contribution: One defendant who pays more than their fair share can recover the excess from other tortfeasors.
• Indemnity: In some relationships (e.g., employer–employee), one defendant may be entitled to full reimbursement from another.

The MBE will rarely ask you to calculate contribution; your task is to recognize that the plaintiff can recover 100% from any jointly and severally liable defendant unless the question expressly adopts a pure several-liability regime.

Alternative Liability and Burden Shifting

Sometimes, multiple defendants are negligent, but only one actually caused the harm, and even with reasonable investigation the plaintiff cannot prove which one. A classic example is Summers v. Tice: two hunters negligently fire in the plaintiff’s direction; only one pellet hits the plaintiff, but we cannot tell whose shot it was.

Without a special doctrine, the plaintiff would lose against both defendants for failure to prove actual causation. To avoid that unfair result, courts use alternative liability.

As noted above, alternative liability has strict requirements:

• All defendants acted negligently toward the plaintiff (breach + duty).
• The injury was caused by one (and only one) of them.
• The plaintiff, despite due diligence, cannot determine which one.
• All possible negligent actors are joined as defendants.

If these conditions are met, the burden of proof on causation shifts to the defendants. Each defendant must prove that they did not cause the harm; if they cannot, both (or all) may be held liable.

Alternative liability is an important MBE doctrine and appears in fact patterns involving:

• Two negligent shooters (hunters, police officers, etc.).
• Two negligent drivers where only one actually makes contact, but it is unclear which.

Do not confuse alternative liability with:

• Concerted action: where defendants agree to engage in tortious conduct together; in that case each is an actual cause under traditional principles.
• Market share liability: a minority doctrine used primarily in DES drug cases, allocating liability based on each manufacturer’s share of the relevant market. Market share liability is different in structure and rarely tested in depth on the MBE.

Joint and Several Liability

As multiple-defendant problems are tested, the bar exam also expects you to know how responsibility is allocated among defendants.

Under the traditional rule:

• If multiple defendants’ concurrent negligence causes an indivisible injury, each is jointly and severally liable for the entire injury.
• The plaintiff can recover the entire judgment from any one tortfeasor.
• That defendant can then seek contribution from others in a separate action.

Joint and several liability typically applies in the following settings (drawn from common exam patterns):

• Two or more tortfeasors whose negligence combines to produce an indivisible harm.
• Tortfeasors acting in concert (e.g., drag racing).
• Alternative liability cases.
• Res ipsa loquitur applied against multiple defendants.
• Situations where both an employer and employee are liable (vicarious liability plus direct negligence).

Some jurisdictions today have modified or abolished joint and several liability, limiting each defendant’s responsibility to their proportionate share of fault, especially when that share is below a statutory threshold. Unless the question states otherwise, assume the traditional joint and several rule applies on the MBE.

Apportionment of Damages: Divisible vs Indivisible Harm

Whether defendants are jointly and severally liable depends on whether the harm is divisible or indivisible.

Key Term: Apportionment of Damages
The process of dividing responsibility for damages among multiple defendants based on the portion of harm each caused, used when the plaintiff’s injury is divisible.

If the plaintiff’s harm can be reasonably separated into distinct components, each linked to a particular defendant, courts apportion damages:

• Divisible harm: P suffers a broken arm in one accident and a broken leg in another. Each defendant is liable only for the injury they caused.
• Indivisible harm: P suffers a single back injury from two nearly simultaneous collisions that cannot be medically separated. Both defendants are jointly and severally liable for the full back injury.

On the MBE:

• If the facts describe clearly distinct injuries matched to different defendants, think apportionment and several liability.
• If the facts say the harm cannot be separated, or if it is logically a single injury (e.g., one burned house, one death, one lost limb), treat as indivisible and apply joint and several liability.

Successive tortfeasors raise related issues:

• The original negligent actor is usually liable for additional harm caused by foreseeable medical malpractice. The doctor is also liable for the aggravation; they may be jointly and severally liable for the worsened condition.
• If a later, distinct accident independently aggravates the plaintiff’s condition, courts may attempt to apportion, but if apportionment is impossible, both may be held jointly and severally liable.

Plaintiff’s Negligence: Contributory Negligence and Comparative Fault

Multiple-cause questions often involve not just multiple defendants, but also a negligent plaintiff. You must integrate the relevant defense regime into the analysis.

Key Term: Contributory Negligence
A common-law rule (now followed in only a few jurisdictions) under which any negligence by the plaintiff that contributes to the accident is a complete bar to recovery.

In a pure contributory negligence jurisdiction:

• If the plaintiff’s negligence is a factual cause of the harm, the plaintiff is barred from recovery, even if defendants are far more negligent.
• The last clear chance doctrine may allow recovery if the defendant had the last clear opportunity to avoid the accident but failed to do so.

Key Term: Comparative Fault
A modern system under which the plaintiff’s own negligence reduces, but does not necessarily bar, recovery in proportion to the plaintiff’s percentage of fault. Often called comparative negligence.

Most jurisdictions have adopted comparative fault, either:

• Pure comparative negligence: the plaintiff can recover even if more at fault than defendants, but recovery is reduced by the plaintiff’s percentage of fault.
• Modified comparative negligence: the plaintiff is barred if more at fault than the defendant(s) (or, in some jurisdictions, if equally at fault).

In multi-defendant comparative negligence settings (important on the MBE):

• The plaintiff’s percentage of fault is compared with the combined fault of all defendants.
• Example: P is 40% at fault; D1 and D2 are 30% each. P can recover 60% of her damages.
• If P is 60% at fault; D1 and D2 are 20% each (40% combined), a modified comparative jurisdiction where plaintiffs cannot be “more at fault than defendants combined” will bar P’s recovery.

Comparative fault affects the amount the plaintiff can collect from each defendant but does not change the basics of actual causation, alternative liability, or the substantial factor test.

Worked Example 1.1

Two factories, A and B, independently discharge pollutants into a river. Either discharge alone would have been sufficient to kill the fish population. The plaintiff sues both for damages to his fishing business. What test applies, and who is liable?

Answer:
The substantial factor test applies. Each discharge is independently sufficient to kill the fish, so the but-for test would wrongly suggest each is not a cause (“but for A, the fish would have died from B; but for B, they would have died from A”). Under the substantial factor test, both discharges are substantial factors in producing one indivisible harm—the loss of the fish population. Both factories are liable for the entire harm under joint and several liability (subject to contribution between them).

Worked Example 1.2

Three companies manufacture a drug. The plaintiff develops an illness after taking the drug but cannot prove which company’s product she used. All companies used the same formula and were negligent in failing to warn of risks. What doctrine may apply?

Answer:
The doctrine of alternative liability may apply if all possible tortfeasors are before the court and only one drug actually caused the plaintiff’s illness. Because all three manufacturers were negligent toward this plaintiff and the drug from one (but only one) caused her harm, the burden of proof on causation can shift to the defendants. Each manufacturer must prove its product was not the one ingested. If none can do so, all may be held liable, typically jointly and severally, with apportionment between them handled by contribution.

Worked Example 1.3

Two drivers, D1 and D2, negligently collide, causing a fire that spreads and destroys P’s house. It is impossible to determine whether D1 or D2’s car started the fire, but both were negligent. What result?

Answer:
This is not an alternative liability problem because both drivers’ negligence combined to create the risk (and each was a but-for cause of the collision that led to the fire). The house fire is an indivisible harm, and each driver’s negligence was a substantial factor in bringing it about. Both D1 and D2 are jointly and severally liable for the entire loss. P may recover the full amount from either driver; that driver can then seek contribution from the other.

Worked Example 1.4

P is injured in two car accidents two weeks apart. In the first, D1 negligently rear-ends P, causing a moderate back injury. In the second, D2 negligently sideswipes P, significantly worsening the back injury. Medical experts can separate the original injury from the aggravation. How are damages allocated?

Answer:
The harm is divisible. D1 is liable for the original back injury caused by the first accident; D2 is liable for the aggravation caused by the second accident. Each defendant is liable only for the portion of harm they caused, and damages are apportioned accordingly. Joint and several liability is unnecessary because the injuries can be reasonably separated.

Worked Example 1.5

In a contributory negligence jurisdiction, P is walking while texting in violation of a safety statute. D, speeding, hits P in a crosswalk. The statute prohibiting texting while driving was enacted to reduce accidents caused by texting drivers, not pedestrians. D argues contributory negligence bars P’s claim because P violated a statute. What result?

Answer:
D’s contributory negligence defense should fail. Although P violated a statute, negligence per se applies only if P is in the class the statute protects and suffers the type of harm the statute aims to prevent. Here the statute protects other road users from texting drivers; it does not aim to protect speeding drivers from texting pedestrians. P’s texting was not an actual cause of the collision—the speeding D hit a pedestrian lawfully in a crosswalk. Therefore, even in a contributory negligence jurisdiction, P’s recovery is not barred.

Exam Warning

On the MBE, if you see a fact pattern stating that either defendant’s act alone would have caused the harm, do not use the ordinary “but for” test. Apply the substantial factor test, treat each act as an actual cause, and consider joint and several liability.

Revision Tip

When the plaintiff cannot prove which of several negligent defendants caused the harm, look for facts indicating that all were negligent, only one caused the harm, causation is genuinely uncertain, and all possible tortfeasors are in the case. Those are signals for alternative liability and burden shifting—not for dismissing the claim.

Key Point Checklist

This article has covered the following key knowledge points:

• The but-for test is inadequate when multiple sufficient causes exist; in those cases, the substantial factor test governs actual causation.
• The substantial factor test is central in concurrent-cause cases where either act alone would have caused an indivisible harm.
• Alternative liability shifts the burden of proof on causation to defendants when all are negligent, only one caused the harm, and the plaintiff cannot identify which one.
• Joint and several liability generally applies when multiple concurrent acts produce an indivisible injury; each defendant is liable for the full harm.
• Damages are apportioned only if harm is divisible; otherwise, apportionment occurs between defendants through contribution, not by limiting the plaintiff’s recovery.
• Plaintiff’s own negligence is handled through contributory negligence or comparative fault, affecting the amount (or availability) of recovery but not the basic causation analysis.

Key Terms and Concepts

• Concurrent Causes
• Alternative Liability
• Substantial Factor Test
• Joint and Several Liability
• Apportionment of Damages
• Contributory Negligence
• Comparative Fault