General principles - Causation

Learning Outcomes

This article explains core causation doctrines in tort law for MBE-style negligence questions, including:

• Distinguishing factual (actual) causation from proximate (legal) causation and recognizing how each appears in multiple-choice fact patterns.
• Applying the “but for” test in ordinary single-cause problems and the substantial factor test in multiple sufficient cause scenarios such as merging fires or concurrent collisions.
• Analyzing proximate cause using foreseeability, direct versus indirect cause, and the effect of intervening events on the scope of liability.
• Evaluating when intervening causes remain within the scope of risk and when they become superseding causes that cut off liability, especially with criminal acts, medical negligence, and acts of nature.
• Resolving causation issues with multiple or indeterminate defendants, including alternative causes, burden shifting, joint and several liability, and concerted action.
• Applying the alternative causes approach and understanding how market-share or industry-wide liability may be tested in simplified form.
• Using the eggshell plaintiff rule to determine the proper extent of damages once the type of injury is foreseeable.
• Spotting fact patterns in which causation fails despite clear negligence and avoiding common MBE traps, such as insisting on a single “real” cause or conflating factual and proximate cause.

MBE Syllabus

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

• Factual (actual) cause: the "but for" test in ordinary cases.
• Factual cause in complex situations: substantial factor test, multiple sufficient causes, and alternative causes.
• Proximate (legal) cause: foreseeability and the scope of risk created by the defendant’s conduct.
• Intervening and superseding causes, including negligent rescue and medical treatment, criminal acts, and acts of nature.
• Causation with multiple or indeterminate defendants, including joint and several liability and concerted action.
• The eggshell plaintiff rule and its relationship to foreseeability and damages.
• Typical exam pitfalls where causation fails despite clear negligence.

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. Which of the following best describes the "but for" test in tort causation?
   1. The defendant's act must be the only cause of the harm.
   2. The harm would not have occurred but for the defendant's act.
   3. The defendant's act must be the direct cause of the harm.
   4. The defendant's act must be the most important cause of the harm.
2. In a case with two fires, each sufficient to destroy a house, both started by different defendants, what test is most appropriate to establish causation?
   1. But for test
   2. Substantial factor test
   3. Proximate cause test
   4. Foreseeability test
3. Which of the following is most likely a superseding cause that breaks the chain of proximate causation?
   1. Negligent medical treatment after the initial injury
   2. A foreseeable rescue attempt
   3. An unforeseeable criminal act by a third party
   4. The plaintiff's own contributory negligence
4. Under the "eggshell plaintiff" rule, a defendant:
   1. Is liable only for foreseeable injuries
   2. Is liable for all injuries, even if the extent is unforeseeable
   3. Is not liable if the plaintiff had a pre-existing condition
   4. Is liable only for physical injuries

Introduction

Causation is a core requirement in tort law. To recover for negligence, and for many other tort theories (including strict liability and some intentional torts), a plaintiff must prove that the defendant's conduct caused the harm.

Causation has two distinct elements:

• Factual (actual) causation: Did the defendant’s conduct in fact contribute to producing this harm?
• Proximate (legal) causation: Is the harm sufficiently related to the defendant’s conduct, in terms of foreseeability and policy, to justify holding the defendant liable?

Key Term: Cause in Fact
The factual connection between the defendant’s conduct and the plaintiff’s harm; the idea that the harm would not have occurred in the same way without the defendant’s conduct.

The MBE frequently combines these elements with duty and breach in fact patterns involving multiple actors, preexisting conditions, or intervening events. A systematic approach is important:

1. Identify factual cause.
2. Analyze proximate cause.
3. Watch for multiple causes and intervening events.
4. Apply the eggshell plaintiff rule to the scope of damages.

Factual Causation: The "But For" and Substantial Factor Tests

The first step is to determine whether the defendant's conduct was a factual cause of the plaintiff's harm.

Key Term: But For Test
A test for factual causation asking whether the harm would have occurred in the absence of the defendant's conduct.

Single-cause and ordinary multiple-cause cases In most negligence problems, use the "but for" test:

• Ask: Would the harm have occurred when and as it did, but for the defendant’s negligent act or omission?
• If no, factual causation is established.
• If yes (the harm would have happened anyway at the same time and in the same way), that defendant is not an actual cause.

For example, negligent brake repair is not a factual cause of injuries from a meteorite striking the car. The harm would have occurred regardless of the brake repair.

On the MBE, do not demand that the defendant be the only cause; factual causation is satisfied if the defendant’s negligence was a necessary condition of the injury, even if other forces also contributed.

Multiple sufficient causes and the substantial factor test In some cases, the "but for" test fails because each of several independent acts would have been sufficient to cause the harm by itself.

Key Term: Multiple Sufficient Causes
A causation situation in which two or more independent acts, each alone sufficient to cause the harm, operate together so that no single act is a "but for" cause.

Key Term: Substantial Factor Test
A test for factual causation used when multiple acts combine to cause harm, holding a defendant liable if their conduct was a substantial factor in bringing about the harm.

If either act alone would have caused the harm, asking whether the harm would have occurred “but for” any one defendant’s negligence yields “yes” for each, which would incorrectly exonerate all defendants. In such cases, courts and the Restatement use the substantial factor (or “multiple sufficient causes”) doctrine:

• Each defendant is an actual cause if their conduct was a substantial factor in producing the harm.
• Each may be held liable, typically with joint and several liability in traditional regimes.

Multiple and Indeterminate Causes

Sometimes the difficulty is not that there are multiple sufficient causes, but that it is unclear which of several negligent defendants actually caused the harm.

Key Term: Alternative Causes Approach
A method where, if multiple defendants acted negligently but only one caused the harm, the burden shifts to each defendant to prove they were not the cause.

Classic pattern: two or more defendants are negligent; only one caused the injury; and the plaintiff cannot prove which one. In this setting:

• The plaintiff must show that each defendant was negligent and that the harm was caused by one of them.
• The burden of proof then shifts to the defendants to show they did not cause the harm.
• If no defendant can exonerate themselves, courts may impose joint and several liability on all.

This is sometimes extended in “market share” or industry-wide liability cases, but the MBE focus is the basic alternative causes rule (think of multiple negligent hunters and a single bullet).

Key Term: Concerted Action
A situation where two or more tortfeasors act together pursuant to a common plan or in conscious parallel conduct that results in harm, making each liable for the entire harm.

If defendants are acting in concert (for example, street racing or jointly dropping a heavy object from a roof), each is treated as a cause of the entire harm, even if it is impossible to attribute precise portions of damage to each.

Key Term: Loss of Chance Doctrine
A doctrine in some medical malpractice cases allowing recovery for the reduction in a patient’s probability of survival or recovery caused by negligent treatment, even when the patient initially had less than a 50% chance of survival.

Traditional but-for causation can bar recovery where a victim’s baseline survival chance is under 50%. Under loss-of-chance, the plaintiff can recover the value of the lost probability (for example, losing a 20% chance of survival to a 5% chance). Some jurisdictions follow this; others do not, so be guided by the facts given in the question.

Exam Tip: The presence of multiple actors never means there is only one “real” cause. More than one defendant can be a cause in fact of the same harm.

Proximate (Legal) Cause: Foreseeability and Intervening Causes

Even if factual causation is present, the defendant is only liable if the harm is a proximate result of their conduct.

Key Term: Proximate Cause
The legal limitation on liability, requiring that the harm be within the scope of the risks that made the defendant’s conduct negligent, i.e., a foreseeable result of that conduct.

The general rule:

• A defendant is liable for all harmful results that are the normal incidents of and within the increased risk created by their conduct.
• If the type of harm is foreseeable, the defendant is generally liable, even if the exact manner or extent of harm is unusual.

Key Term: Direct Cause Case
A case in which there is an uninterrupted chain of events from the defendant’s act to the plaintiff’s injury, with no intervening forces.

Key Term: Indirect Cause Case
A case in which an intervening force comes into play after the defendant’s act and contributes to the plaintiff’s injury.

In direct cause cases, the analysis is straightforward: if some harm of this kind was foreseeable from the defendant’s negligence, they are liable, even if the chain of events was odd or the timing was unusual.

In indirect cause cases, the key is the effect of intervening events.

Intervening and Superseding Causes

Sometimes, another event occurs after the defendant's act and contributes to the harm.

Key Term: Intervening Cause
Any force or event that occurs after the defendant’s act and contributes to the plaintiff’s harm.

If the intervening event is one of the foreseeable risks created by the defendant’s conduct, it does not break the chain of causation; the defendant remains liable.

Key Term: Superseding Cause
An unforeseeable intervening event that breaks the chain of proximate causation, relieving the defendant of liability for subsequent harm.

A superseding cause is:

• Independent of the defendant’s conduct,
• Not within the scope of risks created by the defendant, and
• So unforeseeable or extraordinary that it cuts off liability.

Common foreseeable intervening causes (usually not superseding):

• Negligent medical treatment for the original injury.
• Negligent rescue efforts.
• Efforts to escape danger or protect persons/property.
• “Reaction” forces (crowd panic, swerving to avoid danger).
• Subsequent disease or accident resulting from a weakened condition.

Common potential superseding causes:

• Extraordinary acts of nature (e.g., lightning strikes, freak storms) that are unrelated to the risk created by the defendant.
• Independent, unforeseeable criminal acts or intentional torts by third parties, especially where the defendant’s conduct did not increase the risk of such acts.
• Highly unusual victim behavior completely outside the risks created by the defendant.

On the MBE, criminal or intentional misconduct by a third party is often superseding, unless the defendant’s negligence increased the risk of that very crime (for example, negligently leaving a building unsecure in a high-crime area).

Multiple and Indeterminate Causes (Revisited)

When more than one defendant may have caused the harm, and it is unclear whose act was responsible, courts may:

• Use the substantial factor test in multiple sufficient cause cases (for example, merging fires).
• Apply the alternative causes approach, shifting the burden of proof to defendants.
• Impose liability based on concerted action where defendants acted together.

All of these doctrines are tools to avoid unfairly denying recovery where negligence is clear, but the precise mechanics of causation are uncertain.

The Eggshell Plaintiff Rule

A defendant takes the plaintiff as they find them. If the plaintiff has a pre-existing vulnerability, the defendant is liable for the full extent of the harm, even if it is greater than a typical person would suffer.

Key Term: Eggshell Plaintiff Rule
The principle that a defendant is liable for all harm actually caused, even if the extent or severity of injury is unforeseeable due to the plaintiff's pre-existing condition or unusual susceptibility.

This rule interacts with proximate cause as follows:

• The type of harm must be foreseeable (for example, bodily injury from negligent driving).
• Once that type of harm occurs, the defendant is liable for all consequences, even if the extent or specific complications are unforeseeable (e.g., hemorrhage due to hemophilia, unusual psychological reaction).

Do not confuse foreseeability of type of harm (proximate cause) with foreseeability of extent of harm (eggshell rule).

Worked Example 1.1

A driver negligently runs a red light and hits a pedestrian. The pedestrian suffers a broken leg, and due to a rare bone disease, the injury leads to permanent disability. Is the driver liable for the full extent of the harm?

Answer:
Yes. Under the eggshell plaintiff rule, the driver is liable for all consequences of the foreseeable type of harm (bodily injury from a collision), even though the extent of the injury and resulting disability were unforeseeable.

Worked Example 1.2

Two factories negligently start separate fires. Each fire alone would have destroyed a nearby house. The fires merge and the house is destroyed. Can both factories be held liable?

Answer:
Yes. The substantial factor test applies in this multiple sufficient cause scenario. Each factory's negligence was independently sufficient to destroy the house and was a substantial factor in the actual destruction, so both can be liable.

Worked Example 1.3

A defendant injures a plaintiff, who is then given negligent medical treatment, making the injury worse. Is the defendant liable for the aggravated harm?

Answer:
Yes. Negligent medical treatment is a foreseeable intervening cause. It does not supersede the original negligence; the original defendant remains liable for the full harm, including aggravation by medical negligence.

Worked Example 1.4

A store negligently leaves a spill on the floor. A customer slips, injures her ankle, and while being carried to a chair, a friend stumbles and drops her, worsening the injury. The store argues the friend’s stumble is a superseding cause. Is the store liable for the full harm?

Answer:
Yes. The friend’s attempt to help is a foreseeable rescue/aid reaction to the store’s negligence. It is an intervening, not superseding, cause. The store remains liable for the entire injury.

Worked Example 1.5

A nightclub negligently fails to repair broken exterior lights and does not employ security despite repeated warnings about assaults in the alley. A patron leaving at night is attacked by a criminal. Is the nightclub likely liable for the injuries?

Answer:
Likely yes. Although criminal acts are often superseding causes, here the criminal assault was a foreseeable risk of operating an inadequately lit, unsecured premises in a known high-risk area. The attack is within the scope of the risk created by the nightclub’s negligence.

Worked Example 1.6

Two drivers are speeding toward an intersection. One runs a red light; the other runs a stop sign from the cross street. They collide, and one car is pushed onto the sidewalk, injuring a pedestrian. The red-light runner claims the other driver’s negligence is the “real” cause. Is that a good defense?

Answer:
No. Both drivers’ negligence can be factual causes of the pedestrian’s harm. Each is a substantial factor in the collision and resulting injury and can be held liable. The existence of another negligent actor does not negate the first driver’s liability.

Worked Example 1.7

Two drivers simultaneously clip an unlit motorcycle that has been abandoned in the middle of the road by its owner. Either collision alone would have “totaled” the motorcycle. The owner sues both drivers for the loss of the motorcycle. Can he recover?

Answer:
The drivers’ negligence (driving without lights) is a substantial factor in the destruction of the motorcycle, even though the owner’s own negligence in leaving it unlit also contributed. Each driver is a factual cause under the substantial factor test and may be liable, subject to reduction for the owner’s comparative fault.

Exam Warning

On the MBE, do not confuse factual causation with proximate cause. Even if the defendant's act was a factual cause, liability may be cut off if the type of harm was not within the foreseeable risks, or if an unforeseeable superseding cause intervened. Conversely, if a defendant’s negligence clearly created the risk of the type of harm that occurred, the presence of other contributing causes usually does not defeat causation.

Revision Tip

When analyzing causation, always ask in order:

1. Would the harm have occurred when and as it did but for the defendant's act (or was their conduct a substantial factor in multiple sufficient cause cases)?
2. Was the type of harm a foreseeable result of the defendant’s conduct, or did an unforeseeable superseding cause break the chain?
3. If the plaintiff is unusually vulnerable, remember the eggshell plaintiff rule for the extent of damages.

Key Point Checklist

This article has covered the following key knowledge points:

• Causation in tort law requires both factual (actual) cause and proximate (legal) cause.
• The "but for" test is used for factual causation in ordinary cases; the substantial factor test applies when multiple acts are each sufficient to cause the harm.
• In alternative causes cases, where multiple negligent defendants and only one cause the harm, the burden may shift to defendants to disprove causation.
• Concerted action can make each participating defendant liable for the entire harm, even if the exact contribution is unclear.
• Proximate cause limits liability to harms within the scope of risks that made the defendant’s conduct negligent; foreseeability of the type of harm is key.
• Foreseeable intervening causes (such as negligent rescue, negligent medical care, reaction forces, and related diseases or accidents) usually do not break the causal chain.
• Unforeseeable, extraordinary intervening events (such as independent, unexpected criminal acts or unusual acts of nature) may be superseding causes that cut off liability.
• The eggshell plaintiff rule requires defendants to bear full liability for all harm actually caused, even if the extent of injury is unforeseeable due to a pre-existing vulnerability.
• Multiple actors can simultaneously be actual and proximate causes; the existence of other negligent causes does not automatically absolve any defendant.
• Common MBE traps include demanding that the defendant be the sole cause, confusing factual cause with proximate cause, and mislabeling foreseeable interventions as superseding causes.

Key Terms and Concepts

• Cause in Fact
• But For Test
• Multiple Sufficient Causes
• Substantial Factor Test
• Alternative Causes Approach
• Concerted Action
• Loss of Chance Doctrine
• Proximate Cause
• Direct Cause Case
• Indirect Cause Case
• Intervening Cause
• Superseding Cause
• Eggshell Plaintiff Rule