See also: Tort, Tort law, Negligence

Res ipsa loquitur, sometimes abbreviated to res ipsa, is literally translated as "the thing speaks for itself." It is a doctrine applied in tort cases (particularly when the true circumstances that gave rise to the injury are exclusively known by the defendant) in which the only way in which the plaintiff's injury would have occurred is by the defendant's negligence. Res ipsa applies when (1) plaintiff's injury was caused by an "instrumentality or condition" that was in the defendant's exclusive control, (2) under the circumstances, the plaintiff's injury would not normally have occurred if the defendant had not acted negligently, (3) the plaintiff was not responsible for the event that caused the injury.

I. Exclusive Control of Defendant

Basically, for res ipsa to apply, the defendant must be the only person who could control the instrumentality that caused the injury at the relevant time. Some courts apply this requirement narrowly, requiring that the defendant be in control of the instrumentality at the time of the plaintiff's injury. A less restrictive rule requires that the defendant be in control of the instrumentality at the time any negligence would have occurred. Under the first, narrower, rule, res ipsa would not apply to a defendant surgeon who negligently performed an operation that led to an injury to the patient only after the patient was no longer in hospital, as the conditions that led to the patient's injury were no longer under the "exclusive control" of the surgeon. The second rule, which requires only exclusive control at the time the negligence would have occurred would apply res ipsa, because the physician had exclusive control over the relevant conditions at the time the operation was negligently performed.

This requirement has been criticised by no less than the author of the standard work on torts, Dean Prosser:
[R]es ipsa loquitur was reduced to a formula - a catchword easy to repeat as a substitute for consideration of the evidence.

Unhappily the proof of facts by facts is not capable of reduction to a formula; it has an inconvenient habit of pending always upon the facts. Text writers have much to answer for in this world. The strict and literal application of Wigmore's formula has led to such absurd results as the Rhode Island case in which, in the defendant's department store; the plaintiff sat down in a chair that collapsed, and a directed verdict for the defendant was affirmed upon the ground that both "user" and "control" of the chair were in the plaintiff "at the time of the injury." (Footnote omitted.)
Prosser, Res Ipsa Loquitur in California, 37 Calif. L. Rev. 183, 187 (1949).

Backlash to decisions such as the Rhode Island decision cited by Prosser have led to a substantial dilution of this requirement. Now, the plaintiff in that case would likely be able to recover under res ipsa, as the chair was in the control of the defendant at the time that it was negligently placed where customers would likely sit on it.

II. Implied Negligence

Res ipsa loquitur will only apply if, in the normal course of events, the only way the plaintiff's injury would likely have occurred is by negligence on the part of the defendant. In other words, if the defendant had exercised normal caution, the injury would not normally have occurred. This inference of negligence can be supported by process of elimination, although causes other than negligence on the part of the defendant need not be completely ruled out - it is only necessary to show that negligence of some kind on the part of the defendant is the most likely explanation for the injury.

III. Injury not Caused by Plaintiff's Conduct

Usually, if the plaintiff did not act negligently in a way that would have contributed to the injury, res ipsa will apply.

IV. Effect

Usually, res ipsa is ultimately a question of fact. The court will determine whether to instruct the jury on the application of res ipsa loquitur, but it will be up to the jury (or judge in a bench trial) to decide whether the inference created by res ipsa is sufficient to find for the plaintiff. Sometimes, if the plaintiff provides sufficient evidence to give rise to res ipsa, courts have required the defendant to provide explanatory evidence that negates the inference of negligence; however, in most cases, the jury will weigh the inference of res ipsa loquitur against the defendant's explanation.

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