The one bite rule is one of several methods of establishing the liability of a defendant in a dog bite case. Other methods are set forth in Legal Rights of Dog Bite Victims in the USA.
The first time that a canine attacks a human being, the paramount legal issue is the extent to which the state having jurisdiction adheres to the English common law pertaining to injuries inflicted by domestic animals. The common law shielded the owner of a domestic animal from civil liability to the first victim of each of his animals. This absolving principle came to be known variously as the "one bite rule," the "first bite rule," or the "first bite free" rule.
However, it has been repeatedly stated in the court decisions that the name "one-bite rule" is a misnomer, in that the rule applies to any injury whether or not it was caused by a bite, and that proof of the dangerous propensity of the animal does not require the existence of a prior bite even in a biting case.
The rationale of the one bite rule was that domestic animals by definition were not injurious, and therefore liability could be predicated only on the defendant's knowledge that a particular animal had a propensity to behave in manner that was injurious to humans. Again, the rule applied to any type of injury, whether or not a bite.
The rule provided not only a shield for the animal's owner but also a sword for its victim, because it justified compensating any victim -- after the first one -- who was injured by the same dangerous propensity. The owner, keeper or harborer of the dog thus was held strictly liable when his domestic animal injured a subsequent person the same way it hurt a prior one, provided that the owner had knowledge or "scienter" of the animal's dangerous propensity.
English common law strict liability for canine inflicted injuries therefore was founded on the defendant's scienter (i.e., knowledge) of his dog's dangerousness. For that reason, this is often referred to as the "scienter cause of action," or as "common law strict liability." Since the gist of the tort "is the keeping of a thing known to be dangerous, one who keeps or harbors an animal owned by another may be liable, if he has such knowledge." Prosser or Torts, chapter 10, section 57, page 441.
The Restatement (Third) of Torts, section 23, comment (b), sets forth the rationale of holding dog owners strictly liable for keeping vicious dogs:
Ownership of animals such as dogs and cats is widespread throughout the public; therefore, the limited risks entailed by ordinary dogs and cats are to a considerable extent reciprocal. Accordingly, the case on behalf of strict liability for physical harms that all such ordinary animals might cause is weak. However, even though animals in such categories generally entail only a modest level of danger, particular animals may present significant and abnormal dangers.... Even if ... retention [of such an animal] is itself proper, an abnormally dangerous animal is by definition unusual; owning such an animal is an activity engaged in by a few that imposes significant risks on others within the community. In these circumstances, strict liability is fairly imposed.
The one-bite rule underlies civil and criminal actions in modern America. When a person is bitten by a dog, 18 American states (listed in Legal Rights of Dog Bite Victims in the USA) use the one-bite rule to determine legal liability. The other 32 and the District of Columbia have abrogated or modified the one-bite rule by so-called "dog bite statutes." (The list of those states also is in Legal Rights of Dog Bite Victims in the USA.)
§ 509. HARM DONE BY ABNORMALLY DANGEROUS DOMESTIC ANIMALS.
Except as stated in § 517, a possessor of a domestic animal which he has reason to know has dangerous propensities abnormal to its class, is subject to liability for harm caused thereby to others, except trespassers on his land, although he has exercised the utmost care to prevent it from doing the harm.
The Restatement, quoted above, uses the phrase "has reason to know." The significance of it is explained in Restatement (Second) of Torts § 12 (1965):
§ 12. Reason to Know; Should Know
(1) The words "reason to know" are used throughout the Restatement of this Subject to denote the fact that the actor has information from which a person of reasonable intelligence or of the superior intelligence of the actor would infer that the fact in question exists, or that such person would govern his conduct upon the assumption that such fact exists.
(2) The words "should know" are used throughout the Restatement of this Subject to denote the fact that a person of reasonable prudence and intelligence or of the superior intelligence of the actor would ascertain the fact in question in the performance of his duty to another, or would govern his conduct upon the assumption that such fact exists.
The one bite rule is important even in the states that have a dog bite statute, where the wording of the statute does not apply to the incident in question. The dog bite statutes vary greatly. They might or might not apply to keepers and harborers as well as owners, and to injuries by means other than biting. They might provide for full compensation, or payment of just medical bills, or payment of medical bills and other economic losses (like loss of income), or double compensation under some circumstances. Some do not apply during the day, or if the dog owner posted a "bad dog sign," or if the victim had provoked the dog weeks before. Even though they are worded as strict liability laws, moreover, the court decisions permit a variety of defenses that vary from state to state. If the dog bite statute does not apply to a particular defendant, or does not provide sufficient compensation, the victim can still assert the scienter cause of action, because it is always available in every state.
The scienter cause of action can be directed against not only the dog's owner, but also its harborer or keeper:
A person, although not the owner of a vicious dog, may make himself liable to others by knowingly keeping or harboring the dog upon his premises, after knowledge of his vicious propensities, and this is true even when such keeping is without the consent and against the wishes of the animal's owner. ... The owner of an animal is the person to whom it belongs. Whether or not a person is a keeper depends upon the peculiar facts and circumstances of each individual case. A man may own an animal and yet not be its keeper. The word 'keeper' is equivalent to 'the person who harbors.' Harboring means protecting. So one who treats a dog as living at his house, and undertakes to control his actions is the owner or keeper within the meaning of the law; but the casual presence of an animal on his premises, if not so treated, does not constitute him such owner or keeper. (3 C.J.S. 1266, § 165.)
Contributory negligence and comparative negligence are not defenses against the scienter cause of action. This is expressed in Restatement (Second) of Torts, section 515, comment (b):
Since the strict liability of the possessor of an animal is not founded on his negligence, the ordinary contributory negligence of the plaintiff is not a defense to such an action. The reason is the policy of the law which places the full responsibility for preventing the harm upon the defendant. Thus where the plaintiff merely fails to exercise reasonable care to discover the presence of the animal, or to take precautions against the harm which may result from it, his recovery on the basis of strict liability is not barred.
Trespass is not a defense against the scienter cause of action in all states. Trespass was not a defense at the common law. (Dog Owners' Liability: Statutory Effects, Duke Law Journal, Vol. 1960, No. 1 (Winter, 1960), pp. 146-149.) The case of Radoff v. Hunter (1958) 158 Cal.App.2d 770, for example, held that keeping a guard dog on one's premises, without making the dog's presence obvious or warning of its presence, shall result in liability toward all persons that the dog injures, whether such person are considered invitees, licensees or trespassers. In the Radoff case, the plaintiff was a trespasser who was bitten by the defendants' watch dog, which was kept on a chain in a parking lot behind the defendants' store. The dog often could not be seen and no sign warned of its presence. Ruling that trespass was not a defense to the scienter cause of action, the court stated that "the keeping of the trained watchdog, under conditions where at times he was concealed from view and where no warning was given of his presence, was similar to maintaining a trap whereby invitees, licensees or trespassers might be injured."
In addition to civil laws, criminal laws which apply to the owners, keepers and harborers of dogs are generally based upon the one-bite principle, in that criminal liability usually does not result from dog attacks unless the dog previously engaged in similar behavior.
It should be noted that the one-bite rule applies to cases involving any domestic animal. For example, an owner's liability for injuries by his cat would be determined by the one-bite rule. In the absence of a strict liability statute or ordinance in the jurisdiction where the incident happened, the factual issue would be whether the cat was known to its owner, harborer or keeper to inflict injuries on people or property a certain way. If so, and if the cat were to do it again in that way, then that person would be liable under the one-bite rule.