Negligence - But for and substantial causes

Learning Outcomes

This article examines the causation element of negligence as tested on the MBE, clarifying both actual and proximate cause and showing how to apply them to multiple-choice fact patterns. It explains when the traditional but-for test is satisfied, how to analyze situations where the but-for test appears to fail, and why courts employ the substantial factor test in cases involving concurrent or multiple sufficient causes. It discusses alternative-cause scenarios and burden-shifting, as well as special medical-malpractice problems such as loss-of-chance claims. It further analyzes how foreseeability limits liability through proximate cause, emphasizing the proper identification of foreseeable plaintiffs, types of harm within the scope of the risk, and the operation of the eggshell plaintiff rule. The article also reviews how to classify intervening events, distinguishing ordinary intervening causes that leave liability intact from superseding causes—especially unexpected criminal or intentional acts—that cut off liability. Throughout, the focus is on spotting common MBE causation patterns, eliminating distractor answer choices that confuse factual and proximate cause, and selecting the response that best reflects modern doctrine.

MBE Syllabus

For the MBE, you are required to understand causation in negligence, with a focus on the following syllabus points:

• Apply the 'but-for' test to determine actual cause.
• Recognize situations involving concurrent causes where the 'but-for' test is inadequate.
• Apply the substantial factor test in concurrent cause scenarios.
• Analyze proximate cause based on the foreseeability of the harm and the plaintiff.
• Distinguish between direct and indirect causation.
• Identify intervening forces and determine if they are superseding causes that cut off liability.
• Understand the 'eggshell plaintiff' rule regarding the extent of harm.
• Recognize special causation issues such as alternative causes and loss-of-chance in medical malpractice.

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. To establish actual cause in a negligence action, the plaintiff generally must show that the defendant's breach was the:
   1. Sole cause of the injury.
   2. Proximate cause of the injury.
   3. 'But-for' cause of the injury.
   4. Substantial factor in causing the injury.
2. Two motorcyclists simultaneously rev their engines very loudly next to an elderly pedestrian with a known heart condition, causing the pedestrian to suffer a fatal heart attack. Either motorcyclist's noise alone would have been sufficient to cause the heart attack. In a suit against one of the motorcyclists, what test should be used to determine actual cause?
   1. The 'but-for' test.
   2. The foreseeability test.
   3. The substantial factor test.
   4. The risk-utility test.
3. A driver negligently runs a red light, striking a car lawfully proceeding through the intersection. The driver of the struck car is taken to the hospital, where a doctor negligently administers the wrong medication, causing further injury. Is the driver who ran the red light liable for the injuries caused by the doctor's negligence?
   1. Yes, because the driver's negligence was the 'but-for' cause of all subsequent injuries.
   2. Yes, because subsequent medical negligence is generally considered a foreseeable intervening cause.
   3. No, because the doctor's negligence was a superseding cause.
   4. No, unless the driver could specifically foresee the doctor's negligent act.

Introduction

Once duty and breach have been established in a negligence claim, the plaintiff must prove causation. Causation consists of two distinct components, both of which must be proven: actual cause (also known as cause-in-fact or factual cause) and proximate cause (legal cause). Actual cause links the defendant's breach to the plaintiff's injury, asking if the breach actually caused the harm. Proximate cause examines whether the harm was a foreseeable consequence of the breach and whether liability should fairly attach.

On the MBE, many negligence questions are decided entirely on causation. A defendant may be clearly negligent, and the plaintiff may obviously be injured, but if causation is missing, the answer is “no liability.” It is therefore important to separate:

• Factual causation questions: "Would this injury have occurred if the defendant had not been negligent?"
• Proximate cause questions: "Is this the sort of harm, to this sort of plaintiff, that makes the defendant's conduct negligent in the first place?"

Key Term: Actual Cause
The requirement that the defendant's negligent conduct was a necessary antecedent to the plaintiff's injury; the injury would not have occurred 'but for' the defendant's act, or the act was a substantial factor in causing the injury.

Key Term: Proximate Cause
The legal requirement that the plaintiff's injury must have been a foreseeable result of the defendant's negligent conduct, and that no superseding intervening cause broke the chain of causation.

Actual Cause (Cause-in-Fact)

The primary test for determining actual cause is the 'but-for' test. However, the MBE also expects you to recognize situations where the but-for test breaks down and must be supplemented by the substantial factor test or by burden-shifting rules.

Key Term: But-For Test
A method of proving factual causation that asks whether the injury would have occurred in the absence of the defendant’s negligent act. If the harm would not have occurred but for the defendant’s conduct, that conduct is an actual cause.

The 'But-For' Test

Under this test, the plaintiff must show that, but for the defendant's negligent act, the injury would not have occurred. If the injury would have happened even without the defendant's negligence, then the defendant's conduct is not the actual cause.

The but-for test is applied using a “negative” hypothetical: mentally remove the defendant’s conduct and ask whether the injury still would have occurred at roughly the same time and in the same way. If the answer is “yes,” the conduct is not a factual cause.

Worked Example 1.1

Driver negligently fails to stop at a stop sign and enters an intersection. Pedestrian, who was properly crossing the street, is struck and injured by Driver's car. Would Pedestrian have been injured if Driver had stopped at the sign?

Answer:
No. But for Driver's negligent failure to stop, Pedestrian would not have been injured. Driver's negligence is the actual cause of Pedestrian's injury.

Note that the but-for test does not require the defendant to be the sole cause. It is enough that the defendant’s conduct was a necessary part of the causal story. Other background conditions (e.g., the plaintiff’s presence in the intersection) can also be but-for causes without relieving the defendant of responsibility.

Preexisting Conditions and Acceleration of Harm

The but-for test also applies when the plaintiff already has a preexisting condition:

• If the plaintiff would have suffered the same harm at the same time even without the defendant’s negligence, actual cause is not satisfied.
• If the defendant’s negligence accelerated the harm (e.g., caused death earlier than it would have occurred), most courts treat the defendant as an actual cause.

On the MBE, facts will usually be drafted to point in one direction. Look for phrases like “would have died within minutes anyway” (suggesting no but-for causation) versus “would have survived for several years” (suggesting acceleration and actual cause).

Problems with the 'But-For' Test: Concurrent and Multiple Causes

The 'but-for' test proves inadequate in certain multi-defendant situations.

Key Term: Concurrent Causes
Two or more acts or forces that combine to produce a single harm. They may operate simultaneously or in close sequence.

• Multiple Sufficient Causes (Overdetermined Harm): Where two or more defendants (or a defendant and a natural force) commit separate negligent acts, each of which alone would have been sufficient to cause the plaintiff's injury, the 'but-for' test fails. Applying it to either defendant individually suggests that the injury would have occurred even without that specific defendant's negligence (because the other cause would still have produced it).

Key Term: Multiple Sufficient Causes
A causation situation in which several separate acts are each sufficient, on their own, to bring about the plaintiff’s harm; the harm is “overdetermined.”

In such cases, strictly insisting on but-for causation would let every negligent defendant escape liability simply because there happens to be another sufficient cause.

The Substantial Factor Test

To address the multiple sufficient causes problem, courts use the substantial factor test. Under this test, if a defendant's negligent act was a substantial factor in bringing about the plaintiff's injury, the defendant is an actual cause (even if not the 'but-for' cause). An act is a substantial factor if it contributed materially to the harm in a non-trivial way.

This test is typically applied when multiple forces combine simultaneously (or in close temporal proximity) to cause the harm, and each force alone would have been sufficient.

Key Term: Substantial Factor Test
An alternative to the 'but-for' test used in concurrent cause situations; asks whether the defendant's conduct was a substantial factor in bringing about the plaintiff's harm.

Worked Example 1.2

Two campers in different parts of a forest negligently fail to extinguish their campfires. Each fire spreads independently. The two fires merge and burn down Plaintiff's cabin. Expert testimony establishes that either fire alone would have been sufficient to destroy the cabin. Plaintiff sues one of the campers. Is that camper's negligence an actual cause?

Answer:
Yes. Although the 'but-for' test fails (the cabin would have burned down anyway due to the other fire), the camper's negligence was a substantial factor in causing the destruction. Under the substantial factor test, the camper is an actual cause.

On the MBE, whenever you see two independently sufficient forces that merge (two fires, two shooters, two loud noises causing harm), you should think “substantial factor” and “multiple sufficient causes,” not ordinary but-for causation.

Alternative Causes and Burden Shifting

A different problem arises when only one of several negligent defendants actually caused the harm, but it is impossible to tell which one did so.

Classic pattern:

• Two or more defendants act negligently in similar ways.
• Only one of them causes the injury.
• The plaintiff cannot show which defendant was the actual cause.

Many courts (and the MBE) follow a burden-shifting approach: once the plaintiff shows that each defendant was negligent and one of them caused the harm, the burden of proof shifts to the defendants to prove they did not cause the injury. If they cannot, all negligent defendants may be held liable.

This is sometimes referred to as the “alternative causation” doctrine (modeled on Summers v. Tice). It is still an actual-cause analysis, but the evidentiary burden is adjusted.

Special Causation Problem: Loss of Chance

Medical malpractice questions sometimes raise a loss-of-chance issue: the defendant’s negligence did not create the preexisting disease or condition, but it reduced the plaintiff’s chance of survival or a better outcome.

Key Term: Loss of Chance
A doctrine, recognized in some jurisdictions, allowing a plaintiff to recover where a healthcare provider’s negligence reduces the patient’s probability of survival or better outcome, even if the original chance of recovery was below 50%.

Under a traditional but-for approach, a patient whose pre-negligence chance of survival was less than 50% often cannot prove that the doctor’s negligence probably caused the death (because the patient likely would have died anyway). Loss-of-chance doctrine modifies this by:

• Treating the lost probability itself as a compensable injury, and
• Allowing proportional damages (e.g., recovery equal to the percentage reduction in chance times the value of full damages).

On the MBE:

• Unless the question explicitly states that the jurisdiction follows a loss-of-chance approach, assume the ordinary but-for standard applies.
• If the question describes a jurisdiction that allows recovery for loss of chance, focus on whether the defendant’s negligence more likely than not reduced the plaintiff’s chance of a better outcome and caused the loss of that chance.

Worked Example 1.3

A patient with advanced cancer has a 40% chance of survival with prompt chemotherapy. Doctor negligently delays treatment by several months, reducing the patient’s survival chance to 10%. The patient dies. In a jurisdiction that does not recognize loss-of-chance, can the patient’s estate establish actual cause under the ordinary but-for test?

Answer:
Probably not. Because the patient had less than a 50% chance of survival even with proper care, the estate cannot show that the doctor’s negligence probably caused the death (the patient was more likely than not going to die anyway). Without a loss-of-chance doctrine, actual cause fails.

If the question instead specifies a jurisdiction that recognizes loss-of-chance, the estate could recover damages based on the 30% reduction in survival probability.

Proximate Cause (Legal Cause)

Even if the defendant's conduct is the actual cause of the plaintiff's harm, liability will attach only if the conduct is also the proximate cause. Proximate cause is a legal limitation on liability, essentially asking whether the harm was a foreseeable result of the defendant's negligence and within the scope of the risk that made the conduct negligent.

Key Term: Foreseeability (in Proximate Cause)
The requirement that the general type of harm and the class of persons injured be reasonably predictable consequences of the defendant’s conduct.

Foreseeability Test

The dominant test for proximate cause is foreseeability. The central questions are:

• Was the type of harm suffered by the plaintiff a foreseeable result of the defendant's negligent conduct?
• Was the plaintiff a foreseeable victim—within the “zone of danger” created by the negligence?

If both the type of harm and the plaintiff are foreseeable, proximate cause is usually satisfied, even if the manner in which the harm occurred was somewhat unusual.

Key Term: Foreseeable Plaintiff (Zone of Danger)
A person who is within the range of danger created by the defendant’s negligent conduct and who can reasonably be expected to suffer the type of harm risked by that conduct.

Unforeseeable Type of Harm

If the type of harm that occurred was not foreseeable, the defendant is generally not liable, even if their negligence was the actual cause. Very bizarre, freakish chains of events can break proximate cause because the actual harm falls outside the scope of the risk.

Extent of Harm (Eggshell Plaintiff Rule)

The extent of the harm need not be foreseeable.

Key Term: Eggshell Plaintiff Rule
The principle that a tortfeasor takes the plaintiff as they find them and is liable for the full extent of the injury, even if the plaintiff’s preexisting vulnerability makes the harm much worse than a normal person would have suffered.

If the defendant's negligence causes some physical harm of the type that is foreseeable (e.g., a minor impact), but the plaintiff suffers unusually severe injuries due to a pre-existing vulnerability (the "eggshell skull"), the defendant is liable for the full extent of the injuries. This is tested frequently on the MBE: once liability is established, peculiar fragility does not limit the defendant’s responsibility.

Foreseeable Plaintiff: The Palsgraf Problem

The classic case is Palsgraf: a defendant may owe a duty only to persons within the zone of danger. For MBE purposes:

• A plaintiff far outside the area of risk, injured by an unexpected chain of events, is usually treated as an unforeseeable plaintiff and cannot recover.
• A plaintiff close to the dangerous activity who is harmed by the general type of risk created by the defendant’s negligence is a foreseeable plaintiff.

Direct vs. Indirect Causation: Intervening Forces

• Direct Cause: If the harm results directly from the defendant's negligence without any external intervening force, proximate cause is usually established if the harm was of a foreseeable type. Courts are generous to plaintiffs here.

• Indirect Cause (Intervening Forces): Often, an intervening force comes into play after the defendant's negligent act and combines with it to cause the injury. The key question is whether the intervening force was foreseeable.

Key Term: Intervening Cause
A force or event that comes into play after the defendant's negligent act and contributes to causing the plaintiff's injury.

Key Term: Superseding Cause
An unforeseeable, intervening cause that breaks the chain of proximate causation between the defendant's initial negligent act and the plaintiff's ultimate injury, relieving the defendant of liability for that injury.

Foreseeable Intervening Forces

If the intervening force was a normal or foreseeable consequence of the situation created by the defendant's negligence, it does not cut off liability. The defendant remains the proximate cause.

Common foreseeable intervening forces tested on the MBE:

• Subsequent medical malpractice (negligent treatment of injuries caused by defendant).
• Negligence of rescuers ("danger invites rescue").
• Normal responses or reactions to the defendant's act (e.g., crowd panic, fleeing from danger).
• Subsequent diseases or accidents caused by the weakened condition resulting from the original injury (e.g., a plaintiff falls again because of a weakened leg injured in the first accident).

The initial tortfeasor is liable for all such reasonably foreseeable consequences.

Unforeseeable Intervening Forces (Superseding Causes)

If the intervening force was unforeseeable and produced an unforeseeable result, it is generally deemed a superseding cause, which breaks the chain of proximate causation and relieves the defendant of liability for the harm caused by that force.

Examples often superseding (unless made foreseeable by the facts):

• Independent, highly unusual criminal acts of third parties.
• Deliberate intentional torts by third parties.
• Extraordinary acts of God or natural forces that are highly unexpected in the context.

However, when the defendant’s negligence itself creates or increases the risk of such conduct (e.g., leaving keys in a car in a high-crime area, failing to provide security in a known dangerous location), criminal acts may be considered foreseeable, and thus not superseding.

Worked Example 1.4

Driver negligently speeds through a residential area, loses control, and crashes into a utility pole, knocking it down. The impact causes sparks that ignite dry leaves nearby, starting a small fire. An arsonist, unrelated to the driver, sees the small fire and decides to add gasoline, causing the fire to spread rapidly and burn down Plaintiff's house. Is Driver's negligence the proximate cause of the destruction of Plaintiff's house?

Answer:
Likely no. While Driver's negligence was the actual cause ('but-for' the crash, the pole wouldn't have fallen, no sparks, no initial fire), the arsonist's act of intentionally adding gasoline and greatly expanding the fire is an unforeseeable criminal act of a third party. This act would be considered a superseding cause, breaking the chain of proximate causation from Driver's initial negligence to the destruction of the house.

Worked Example 1.5

Store negligently fails to repair a broken exterior light in a parking lot located in a high-crime neighborhood, despite numerous prior assaults there. One evening, Customer is assaulted and robbed in the darkened lot. The assailant is never caught. Is the criminal attack a superseding cause that relieves Store of liability?

Answer:
No. Given the history of assaults and Store’s knowledge of the high-crime conditions, the risk of criminal attack was precisely the kind of harm that made the failure to provide lighting negligent. The assault is therefore a foreseeable intervening criminal act, not a superseding cause, and Store can be held liable.

Direct vs. Indirect: Putting It Together

When analyzing proximate cause on the MBE:

• Ask whether there were any intervening events between the negligence and the injury.
  • If none: direct cause; proximate cause usually satisfied if harm type is foreseeable.
  • If yes: analyze whether the intervening force is foreseeable or superseding.
• Do not confuse actual cause (but-for / substantial factor) with proximate cause (foreseeability, scope of risk). Both must be satisfied.

Exam Warning

Do not confuse actual cause ('but-for' or substantial factor) with proximate cause (foreseeability). Both must be proven for negligence liability. An act can be the actual cause but not the proximate cause if the resulting harm or the manner in which it occurred was unforeseeable, or if a superseding cause cuts off liability.

Key Point Checklist

This article has covered the following key knowledge points:

• Causation in negligence requires proof of both actual cause and proximate cause.
• The primary test for actual cause is the 'but-for' test, which uses a “negative” hypothetical.
• The substantial factor test applies to concurrent causes where the 'but-for' test fails because multiple sufficient causes overdetermine the harm.
• In alternative-cause situations, some courts shift the burden to negligent defendants to disprove causation.
• Loss-of-chance doctrine, where recognized, modifies causation in medical malpractice by treating reduced probability of a better outcome as a compensable injury.
• Proximate cause is based primarily on the foreseeability of the type of harm and the class of plaintiff.
• The extent of harm need not be foreseeable (eggshell plaintiff rule); defendants take plaintiffs as they find them.
• Liability generally requires a foreseeable plaintiff within the zone of danger.
• Foreseeable intervening forces (e.g., medical malpractice, rescues, normal reactions, subsequent accidents) do not break the chain of proximate causation.
• Unforeseeable intervening forces, especially independent criminal acts or intentional torts, may be superseding causes that cut off liability—unless the defendant’s negligence made such acts foreseeable.

Key Terms and Concepts

• Actual Cause
• But-For Test
• Substantial Factor Test
• Concurrent Causes
• Multiple Sufficient Causes
• Loss of Chance
• Proximate Cause
• Foreseeability (in Proximate Cause)
• Foreseeable Plaintiff (Zone of Danger)
• Eggshell Plaintiff Rule
• Intervening Cause
• Superseding Cause