Negligence: Breach, Res Ipsa Loquitur, and Negligence Per Se

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Negligence: Breach, Res Ipsa Loquitur, and Negligence Per Se

Key Takeaways Explain This Topic AI

Breach is the failure to conform to the applicable standard of care; courts and the bar use the Hand formula (B < P × L) and custom evidence to evaluate reasonableness.

Negligence per se borrows a statutory standard when the plaintiff is within the class the statute protects and the harm is the type the statute was designed to prevent; in most states it establishes duty and breach conclusively, but in California it creates a rebuttable presumption (Evid. Code § 669).

Res ipsa loquitur permits an inference of breach when the accident is of a type that does not ordinarily occur absent negligence and the instrumentality was within the defendant's exclusive control.

Violation of a statute may be excused when compliance would be more dangerous or compliance was impossible despite reasonable care; a statute can also be evidence of breach even when negligence per se does not apply.

Custom is relevant but not conclusive — an entire industry can be negligent (The T.J. Hooper); professional custom, by contrast, generally sets the standard for malpractice.

Last updated: June 2026

Proving Breach: The Hand Formula and Custom

Breach is the failure to behave as the reasonably prudent person would under the circumstances. The challenge is proof. One analytical tool is Judge Learned Hand's formula from United States v. Carroll Towing : a defendant breaches when the burden of precaution (B) is less than the probability of harm (P) multiplied by the magnitude of the loss (L) — i.e., B < P × L . If a cheap precaution would have averted a likely, serious injury, failing to take it is unreasonable.

This is a way of articulating, not replacing, the reasonable-person inquiry, and bar examiners reward candidates who weigh the burden of avoidance against the foreseeable risk.

Custom is admissible to show what is reasonable, but it is not dispositive (outside professional malpractice). The famous holding of The T.J. Hooper is that an entire industry can lag behind reasonable prudence — tugboats customarily lacked radios, yet failing to carry one was still negligent. Custom thus cuts both ways: compliance with custom is evidence of due care, and departure from custom is evidence of breach, but neither settles the question. The exception is professional malpractice , where the custom of the profession in good standing does generally set the binding standard, proven by expert testimony.

Where direct evidence of how the defendant behaved is unavailable, plaintiffs turn to two doctrines that supply the standard or permit an inference: negligence per se and res ipsa loquitur.

Negligence Per Se — and California's Rebuttable Presumption

Negligence per se lets the plaintiff borrow a criminal statute or regulation to supply the standard of care, replacing the flexible reasonable-person inquiry with a fixed legislative rule. Two threshold requirements (the 'class of person / class of risk' test):

The plaintiff is within the class of persons the statute was designed to protect; AND

The injury is of the type of harm the statute was designed to prevent.

Example: a statute requiring railroad gates is meant to protect travelers from collisions with trains. If the defendant violates it and a traveler is struck by a train, negligence per se applies. But if a pedestrian is injured because the open gate fell on her, the harm is outside the protected risk and the doctrine does not apply.

The California twist: In most jurisdictions, an unexcused statutory violation conclusively establishes duty and breach (the jury then decides only causation and damages). California is different: under Evidence Code § 669 , statutory violation creates a rebuttable presumption of negligence . The defendant may rebut by showing she did what might reasonably be expected of a person of ordinary prudence acting under similar circumstances and desiring to comply with the law. Always flag this distinction on a California essay.

Excused violations (recognized in California and the Restatement): a violation is excused when (a) compliance would be more dangerous than violation, (b) compliance was impossible or beyond the actor's control, (c) the actor neither knew nor should have known of the occasion for compliance, (d) there was an emergency not of the actor's making, or (e) the actor exercised reasonable care to comply. Even where negligence per se fails (no protected class, or excused), the statute may still be admissible as some evidence of breach .

Res Ipsa Loquitur — Letting the Accident Speak

When a plaintiff cannot prove exactly what the defendant did wrong, res ipsa loquitur ('the thing speaks for itself') permits the factfinder to infer breach from the nature of the accident. The traditional elements:

The accident is of a type that does not ordinarily occur in the absence of negligence ;

The instrumentality was within the exclusive control of the defendant (or the defendant had the right and responsibility to control it); and

The injury was not due to any voluntary action or contribution by the plaintiff .

Classic example: a barrel of flour rolls out of a warehouse window and strikes a passerby ( Byrne v. Boadle ); barrels do not ordinarily fall absent negligence, and the warehouse controlled the premises. The famous Ybarra v. Spangard case extended res ipsa against multiple defendants (an entire surgical team) where an unconscious patient was injured and could not identify which member was negligent — a California decision that relaxes the exclusive-control element when the defendants together controlled the plaintiff's care.

Procedural effect: Res ipsa does not shift the burden of proof. In most states it merely creates a permissible inference , letting the plaintiff survive a directed motion and reach the jury — the jury may, but need not, find negligence. It does not entitle the plaintiff to a directed verdict.

Doctrine What it supplies Effect

Negligence per se The standard of care (from a statute) Conclusive elsewhere; rebuttable presumption in CA (§ 669)

Res ipsa loquitur An inference of breach (from circumstances) Permissible inference; survives directed verdict

MBE trap: Res ipsa is unavailable if the plaintiff can show the specific negligent act, or if the instrumentality was equally accessible to others (no exclusive control). Watch for fact patterns where a third party could have tampered with the instrumentality.