Study guide
16.1 Cause in Fact & Multiple Actors
California First-Year Law Students' Examination Baby Bar
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1 Introduction & FYLSE Examination Guide
1.1 FYLSE Exam Structure, Passing Standard & Administration 1.2 Study Method, Pacing & MBE-Style Technique
2 Contracts: Mutual Assent, Capacity & Contract Formation
2.1 The Offer & Objective Theory of Assent 2.2 Termination & Irrevocability of Offers 2.3 Acceptance & the Mailbox Rule 2.4 Mirror Image Rule & UCC 2-207 Battle of the Forms 2.5 Mistake in Contract Formation 2.6 Fraud, Duress, Undue Influence & Illegality 2.7 Capacity to Contract
3 Contracts: Consideration, Estoppel & Implied Obligations
3.1 Consideration & the Bargained-For Exchange 3.2 Past & Moral Consideration, Pre-Existing Duty & Modification 3.3 Promissory Estoppel & Detrimental Reliance 3.4 Implied-in-Fact Contracts & Quasi-Contract 3.5 Implied Covenant of Good Faith, Fair Dealing & Indefinite Terms
4 Contracts: Third-Party Beneficiaries, Assignment & Delegation
4.1 Third-Party Beneficiary Contracts 4.2 Assignment of Rights 4.3 Delegation of Duties
5 Contracts: Statute of Frauds, Parol Evidence & Interpretation
5.1 Statute of Frauds: Contracts Within the Statute 5.2 Satisfying the Statute: Writing, Signature & Exceptions 5.3 The Parol Evidence Rule 5.4 Contract Interpretation & Canons of Construction
6 Contracts: Performance, Conditions, UCC Warranties & Breach
6.1 Conditions: Express, Constructive & Excuse 6.2 Substantial Performance vs. Perfect Tender 6.3 Cure, Rejection, Revocation & Installment Contracts 6.4 UCC Article 2 Warranties & Risk of Loss 6.5 Anticipatory Repudiation & Insecurity
7 Contracts: Remedies, Discharge & Changed Circumstances
7.1 Expectation Damages & Their Limitations 7.2 Reliance, Restitution & Liquidated Damages 7.3 Specific Performance & Injunctive Relief 7.4 UCC Article 2 Buyer's and Seller's Remedies 7.5 Impossibility, Impracticability & Frustration of Purpose 7.6 Discharge by Agreement: Rescission, Novation & Accord and Satisfaction
8 Criminal Law: Principles of Criminal Liability & Mens Rea
8.1 Actus Reus, Voluntary Acts & Omissions 8.2 Criminal Causation & Intervening Causes 8.3 Common Law & Model Penal Code Mens Rea 8.4 Mistake of Fact, Mistake of Law & Concurrence
9 Criminal Law: Inchoate Offenses & Accomplice Liability
9.1 Criminal Attempt & Impossibility 9.2 Solicitation & the Merger Doctrine 9.3 Conspiracy & Merger Doctrine 9.4 Accomplice Liability: Actus Reus & Mens Rea 9.5 Scope of Accomplice Liability, Withdrawal & Parties to Crime
10 Criminal Law: Homicide & Crimes Against the Person
10.1 Common Law Murder & Malice Aforethought 10.2 Degrees of Murder, Premeditation & Deliberation 10.3 The Felony Murder Rule & Its Limitations 10.4 Voluntary & Involuntary Manslaughter 10.5 Non-Homicide Crimes Against the Person
11 Criminal Law: Property Crimes, Theft & Habitation
11.1 Common Law Larceny 11.2 Embezzlement & False Pretenses 11.3 Robbery & Extortion 11.4 Receiving Stolen Property 11.5 Burglary 11.6 Arson
12 Criminal Law: Defenses: Capacity, Justification & Excuse
12.1 Insanity: The Four Legal Tests 12.2 Intoxication, Diminished Capacity & Infancy 12.3 Self-Defense & Defense of Others 12.4 Defense of Property, Habitation & Law Enforcement Privileges 12.5 Duress & Necessity 12.6 Entrapment & Mistake as an Excuse
13 Torts: Intentional Torts to Persons & Property
13.1 Intent, Motive & Incapacity in Intentional Torts 13.2 Transferred Intent 13.3 Battery & Assault 13.4 False Imprisonment 13.5 Trespass to Land 13.6 Trespass to Chattels & Conversion
14 Torts: Privileges & Defenses to Intentional Torts
14.1 Consent: Express, Implied & Capacity 14.2 Exceeding & Vitiating Consent 14.3 Self-Defense & Defense of Others (Tort) 14.4 Defense of Property & Recapture of Chattels 14.5 Public & Private Necessity 14.6 The Shopkeeper's Privilege
15 Torts: Negligence — Duty of Care & Breach
15.1 Duty of Care & the Foreseeable Plaintiff 15.2 Affirmative Duties to Act & Special Relationships 15.3 Standard of Care: The Reasonable Person & Children 15.4 Professional Negligence, Informed Consent & Negligence Per Se 15.5 Land Possessor Liability & Premises Duties 15.6 Breach of Duty & Res Ipsa Loquitur
16 Torts: Causation, Defenses, Multiple Tortfeasors, Wrongful Death & Damages
16.1 Cause in Fact & Multiple Actors 16.2 Proximate Cause & Intervening Forces 16.3 Contributory, Comparative Negligence & Assumption of Risk 16.4 Joint and Several Liability, Contribution & Indemnity 16.5 Vicarious Liability & Non-Delegable Duties 16.6 Wrongful Death & Survival Actions 16.7 Statute of Limitations & Claim Accrual 16.8 Tort Damages: Compensatory, Nominal, Punitive & Mitigation
17 Torts: Strict Liability & Products Liability
17.1 Strict Liability for Animals 17.2 Abnormally Dangerous Activities & Scope of Risk 17.3 Strict Products Liability & Defect Categories 17.4 Products Liability Under Negligence, Warranty & Misrepresentation 17.5 Defenses to Products Liability
18 Torts: Emotional Distress, Defamation, Privacy & Economic Torts
18.1 Intentional Infliction of Emotional Distress 18.2 Negligent Infliction of Emotional Distress 18.3 Common Law Defamation: Elements, Libel & Slander 18.4 Constitutional Limits, Privileges & Defenses to Defamation 18.5 Invasion of Privacy 18.6 Nuisance & Economic Torts
16.1 Cause in Fact & Multiple Actors
Key Takeaways Explain This Topic AI
The but-for test asks whether the harm would have occurred absent the defendant's conduct, and it governs the ordinary single-defendant case.
Where two independently sufficient forces each would have caused the harm, the but-for test fails and the substantial factor test applies.
Under alternative liability, drawn from Summers v. Tice, where two or more defendants act negligently and only one caused the harm, the burden shifts to each defendant to exculpate himself.
Market share liability, drawn from Sindell, apportions damages among manufacturers of a fungible product by market share where the specific producer cannot be identified.
Concert of action makes all participants in a joint negligent enterprise liable for the entire harm regardless of who directly caused it.
Last updated: September 2026
Cause in Fact & Multiple Actors
Exam Snapshot: Causation represents the dual-stage bridge linking the defendant's breach of duty to the plaintiff's compensable damages. On the California FYLSE (Baby Bar), examinees frequently commit the fatal analytical error of collapsing these two inquiries into a single discussion or confusing their distinct functions. Remember: Cause-in-Fact (Actual Cause) is an empirical, factual question asking did the defendant's breach physically bring about the harm? , whereas Proximate Cause (Legal Cause) is a normative policy question asking should the defendant be held legally answerable for those physical consequences? A plaintiff must independently prove both actual cause and proximate cause by a preponderance of the evidence to establish negligence liability.
1. Actual Cause: The "But-For" Test (Sine Qua Non)
The baseline, universal common-law standard for determining cause-in-fact in tort law is the "But-For" Test (also termed the rule of sine qua non —"without which not"): Plaintiff's Harm Would NOT Have Occurred BUT FOR Defendant's Negligent Breach
- ├── YES ──▶ Cause-in-Fact Established (Proceed to Proximate Cause)
- └── NO ──▶ No Actual Cause ──▶ Prima Facie Case Fails (Unless an Exception Applies)
- Mechanics of the But-For Inquiry
To satisfy the but-for test, the plaintiff must prove that it is more probable than not (greater than 50% probability) that the plaintiff's physical injury or property damage would not have occurred in the absence of the defendant's negligent act or omission. The fact-finder conducts a mental thought experiment:
- Mentally isolate and remove the defendant's negligent conduct;
- Substitute the conduct that a reasonably prudent person would have exhibited under the same circumstances; and
Determine whether the plaintiff's injury would still have transpired.
The Mandatory "Negligent Aspect" Requirement
A signature examiner trap on the FYLSE involves scenarios where the defendant was undeniably acting negligently at the time of the incident, but the negligent aspect of the defendant's conduct did not causally contribute to the plaintiff's harm. It is never enough that a defendant was acting negligently in the abstract; the physical harm must flow directly from the specific hazard that made the actor's conduct negligent in the first place.
Classic FYLSE Hypothetical: A commercial delivery driver operates a delivery van at 45 mph in a posted 25 mph residential school zone. While speeding down the street, an unprecedented, sudden wind gust snaps a massive, healthy oak branch from an adjacent public park. The branch falls vertically onto the roof of the delivery van, crushing a passenger. If the driver had been traveling at the lawful speed limit of 25 mph, the van would have arrived at that exact physical location two minutes later, and the branch would have fallen harmlessly onto empty pavement. Is the driver's speeding an actual cause of the passenger's injury?
Legal Analysis: No. Although the driver was negligent in speeding, the speed was merely a coincidental cause in placing the vehicle at that physical coordinate in time and space. Speeding creates foreseeable risks of collisions with pedestrians, bicycles, or other vehicles due to increased braking distance; it does not increase the risk of being struck from above by falling tree limbs. Subtracting the negligence (supposing a prudent driver happened to be at that spot traveling lawfully) would have yielded the identical injury. The speeding was not a cause-in-fact of the passenger's catastrophe.
Concurrent Necessary Causes
The but-for test readily accommodates multi-actor situations where multiple negligent acts combine to produce an injury that none of the acts alone could have produced ( concurrent necessary causes ):
If Motorist A negligently speeds through an intersection and Motorist B negligently fails to check a blind spot, and their simultaneous collision deflects Motorist A's sedan onto the sidewalk, striking Pedestrian P, the but-for test applies to both motorists.
But for Motorist A's speeding, the collision would not have occurred. But for Motorist B's failure to observe the blind spot, the impact would not have occurred. Both drivers are causes-in-fact, and both are subject to joint and several liability.
2. The Substantial Factor Test: Concurrent Independent Sufficient Causes
While the but-for test governs standard tort actions, it suffers from a fatal conceptual breakdown in one critical recurring scenario: concurrent independent sufficient causes . Two Independent Forces / Fires
- ├── Fire A (Defendant's Negligence) ──▶ Sufficient alone to destroy Timber Ranch
- └── Fire B (Other Origin) ──▶ Sufficient alone to destroy Timber Ranch
- │
- ▼
- Fires Merge ──▶ Timber Ranch Destroyed
- [Under 'But-For' Test: Neither is liable (Ranch would have burned anyway)]
- [Under 'Substantial Factor' Test: BOTH are held to be Causes-in-Fact]
- The Breakdown of the But-For Test
Imagine two independent actors, Defendant 1 and Defendant 2, who negligently ignite separate fires on opposite sides of a valley. Each fire is sufficiently large, hot, and aggressive to completely incinerate Plaintiff's timber ranch. The two fires merge before reaching the ranch, forming a single catastrophic blaze that consumes the property:
If Plaintiff sues Defendant 1 under the strict but-for test, Defendant 1 argues: "Even if I had exercised perfect care and lit no fire, Plaintiff's ranch would have burned to the ground at the exact same moment from Defendant 2's fire; therefore, my fire is not a but-for cause."
Defendant 2 makes the identical argument regarding Defendant 1's fire.
Under a rigid but-for test, both tortfeasors would escape all liability, leaving an entirely blameless plaintiff without a remedy despite being devastated by two culpable wrongdoers.
The Substantial Factor Doctrine & Kingston v. Chicago & N.W. Ry.
To avert this grave injustice, the common law established the Substantial Factor Test ( Kingston v. Chicago & Northwestern Railway Co. , Wis. 1927; Restatement (Second) of Torts § 432(2) ):
The Rule of Law: Where two or more independent negligent acts combine to produce a single indivisible injury, and each act alone would have been sufficient to bring about the entire harm, the but-for test is discarded. Instead, each defendant's conduct is deemed an actual cause-in-fact if it was a substantial factor in producing the harm.
Application in Kingston: A railroad negligently sparked a fire that merged with another fire of unknown human origin. The merged fire destroyed the plaintiff's property. The court held that the railroad could not escape liability by pointing to the concurrent fire. Because the railroad's fire was an independently operating, active force that was a substantial factor in the destruction, the railroad was held liable for the entire damage award.
Modern Toxic Exposures: The substantial factor test serves as the bedrock of toxic tort litigation, particularly asbestos and industrial chemical exposure claims ( Borel v. Fibreboard Paper Products Corp. ). Where a shipyard worker contracts mesothelioma after twenty years of working around thirty different asbestos suppliers, the worker cannot prove which specific manufacturer's fiber triggered cellular mutation under a but-for standard. The law allows the plaintiff to prove that exposure to a particular defendant's defective product was a substantial factor contributing to the aggregate exposure that caused the disease.
3. Alternative Liability: Summers v. Tice & Burden Shifting
A distinct causal conundrum arises when multiple actors breach their legal duty of care, but only one actor's conduct physically inflicted the plaintiff's harm, yet the plaintiff cannot prove which one was the actual cause. Summers v. Tice Paradigm
- ├── Hunter A shoots negligently in Plaintiff's direction
- ├── Hunter B shoots negligently in Plaintiff's direction
- └── EXACTLY ONE birdshot pellet enters Plaintiff's eye
- │
- ▼
Both Defendants breached duty; only ONE caused harm.
[Plaintiff cannot prove which defendant fired the single pellet]
COURT: Shifts Burden of Proof to Defendants to exonerate themselves.
If neither can prove who fired it ──▶ BOTH held Jointly and Severally Liable.
The Landmark Holding in Summers v. Tice
In Summers v. Tice (Cal. 1948), plaintiff Summers went quail hunting with defendants Tice and Simonson. The three hunters formed an open triangle. Tice and Simonson knew Summers was ahead of them in the brush. A quail flushed, and both Tice and Simonson simultaneously fired their shotguns in Summers's direction, breaching their duty of ordinary care. Summers was struck in the eye by a single birdshot pellet and in the lip by another. Both shotguns used identical 12-gauge birdshot ammunition. It was ballistically impossible for Summers to establish which hunter fired the shot that destroyed his eye.
The Four Mandatory Elements of Alternative Liability
To invoke alternative liability, the plaintiff must establish four cumulative elements:
All Defendants Acted Tortiously: Every defendant before the court breached the standard of care. If two people were hunting, but only one shot negligently while the other exercised reasonable care, alternative liability cannot apply.
All Potential Wrongdoers Are Joined: The plaintiff must name and bring all tortious actors before the court. If three hunters fired negligently, the plaintiff cannot sue only two and invoke alternative liability.
Harm Caused by Exactly One (or Fewer Than All): The nature of the physical injury establishes that only one of the negligent acts caused the damage, but the innocent plaintiff is in no position to identify the specific culprit.
Contemporaneous Conduct: The negligent acts occurred in a unified, contemporaneous transaction creating identical risks.
Procedural Mechanism & Joint Liability Outcome
Once these four elements are established, the court shifts the burden of proof on causation to the defendants to affirmatively prove: "It was not my shot or action that injured the plaintiff."
If a defendant cannot carry this burden and exonerate themselves, the court treats both (or all) defendants as joint tortfeasors held jointly and severally liable for the plaintiff's entire damage award.
4. Market Share Liability: Sindell v. Abbott Laboratories
When the dilemma of unidentified tortfeasors expands to an entire commercial industry involving hundreds of manufacturers and thousands of victims over decades, alternative liability fails because the plaintiff cannot join all possible wrongdoers. To resolve this crisis, the California Supreme Court fashioned Market Share Liability ( Sindell v. Abbott Laboratories , Cal. 1980). Market Share Liability Paradigm
- ├── Ingestion of Fungible Drug (DES) during pregnancy decades earlier
- ├── Drug causes rare cancer in adult daughters
- ├── Impossible to identify specific manufacturer (identical generic chemical formula)
- └── PLAINTIFF JOINS Substantial Share of the Manufacturing Market
- │
- ▼
- COURT: Each manufacturer held liable for percentage of judgment
proportional to its share of the relevant commercial market.
The DES Crisis & The Rationale for Market Share Liability
From 1947 to 1971, pharmaceutical companies manufactured and marketed diethylstilbestrol (DES), a synthetic estrogen prescribed to pregnant women to prevent miscarriages. Decades later, daughters exposed to DES in utero developed clear cell adenocarcinoma (a rare cancer) and reproductive anomalies. Because DES was manufactured from a uniform, generic chemical formula and prescribed interchangeably without brand identification, plaintiffs could not identify which specific company manufactured the pills ingested by their mothers twenty-five years earlier.
Mandatory Prerequisite Elements for Market Share Liability
Fungible Product: The product must be completely fungible—identical chemical composition, identical design, and carrying an identical, uniform defect across all manufacturers.
Inability to Identify Specific Producer: The plaintiff must demonstrate that the inability to identify the specific manufacturer is not the plaintiff's fault, but the result of the generic nature of the product and the passage of time.
Substantial Share of the Market Joined: The plaintiff must join in the lawsuit a substantial percentage (typically 75% to 80% or more) of the manufacturers who produced the product in the relevant geographic and temporal market.
Apportionment and Doctrinal Limits
Liability Apportionment: Each defendant manufacturer is held severally liable only for the percentage of the plaintiff's damages that corresponds to its percentage share of the market at the time of exposure.
Right of Exoneration: A defendant manufacturer can escape liability entirely if it proves it did not produce or market DES in the plaintiff's geographic region, or did not market the drug during the specific year in question.
Doctrinal Limitation: Courts nationwide have strictly limited market share liability to fungible pharmaceuticals like DES. It is routinely rejected in actions involving asbestos (different fibers, varying toxicity levels), lead paint (different chemical formulations over decades), and firearms (non-fungible ballistic signatures).