A defendant in a negligence suit typically tries to negate one of the elements of the plaintiff's cause of action. In other words, the defendant introduces evidence, for example, that he or she did not owe a duty to the plaintiff, exercised reasonable care, did not cause the plaintiff's damages, and so forth. In addition to negating one or more of these elements, a defendant may rely on one of a few doctrines that may eliminate or limit liability based on alleged negligence. Two of the more common doctrines are comparative fault and assumption of the risk.

Comparative Fault

Under traditional tort law, a defendant could avoid liability by proving contributory negligence on the part of the plaintiff. Contributory negligence occurs when a plaintiff's conduct falls below a certain standard necessary for the plaintiff's protection, and this conduct cooperates with the defendant's negligence in causing harm to the plaintiff. Where the plaintiff's negligence for his or her own protection is the cause-in-fact and proximate cause of the plaintiff's damages, then the doctrine of contributory negligence would bar recovery.

Contributory negligence has led to harsh results in some cases, and the vast majority of states have replaced the doctrine with an alternative called comparative negligence. The doctrine of comparative negligence reduces a plaintiff's recovery by the percentage in which the plaintiff is at fault for his or her damages. A majority of states have modified this rule, barring a plaintiff from recovering if the plaintiff is as much at fault (in some states) or more at fault (in other states) than the defendant.

Assumption of the Risk

Another defense that traditionally has barred recovery for a plaintiff applies when a plaintiff has assumed the risk involved in an obviously dangerous activity but proceeded to engage in the activity anyway. In order for this doctrine to apply, the plaintiff must have actual, subjective knowledge of the risk involved in the activity. The plaintiff must also voluntarily accept the risk involved in the activity. An example might involve an amusement park ride that flips passengers completely upside-down. A passenger who saw the ride and knew what would happen on the ride assumed the risks associated with the ride. On the other hand, a plaintiff does not assume the risk of something unexpected related to the ride, such as where a loose bolt causes the ride to throw the plaintiff in a violent manner.