Comparative fault—also known as comparative negligence—is a rule of tort law for allocating liability when two or more parties to an accident or liability incident are at least somewhat at fault.

For example, in a case in which both the plaintiff and the defendant were negligent, the jury may be asked to apportion the liability or responsibility for the accident (and the resulting damages) between the parties—usually as a percentage of each party's negligence—and the plaintiff’s recovery against the defendant may be offset or reduced by the amount of the plaintiff’s negligence. This rule may be referred to as pure comparative fault.

Other states have enacted a modified comparative fault statute or law that reduces a plaintiff’s recovery by the plaintiff’s percentage of fault, but bars a plaintiff from any recovery if the plaintiff is 50% or more at fault for the cause of the accident.

And in South Dakota, the comparative fault system uses a slight/gross negligence system and only analyzes the comparative fault if the plaintiff’s negligence is slight and the defendant’s negligence is gross. Under this slight/gross negligence system, if the plaintiff’s negligence is more than slight and the defendant’s negligence is less than gross, the plaintiff is barred or prohibited from any recovery.

Comparative fault laws vary from state to state and may change or evolve at any time—whether they are located in court opinions or in statutes.

In Massachusetts, the rule of comparative fault, also known as comparative negligence, is applied in cases where both the plaintiff and the defendant share some degree of fault for an accident or injury. Massachusetts follows a modified comparative fault system. Under this system, a plaintiff's recovery is reduced by their percentage of fault. However, if the plaintiff is found to be 50% or more at fault for the accident, they are barred from recovering any damages from the defendant. This means that as long as the plaintiff's fault is less than 50%, they can still recover damages, albeit reduced in proportion to their degree of fault. This approach aims to allocate damages in a way that reflects the respective responsibility of each party for the incident.