Alternative liability is a legal principle that a court may apply in a negligence case when multiple defendants could have caused plaintiff’s injury but the plaintiff cannot prove which one.
Most classes will teach alternative liability as a way to shift the burden of proving causation from the plaintiff to defendants – – each defendant must prove he did NOT cause plaintiff’s injury otherwise defendants are jointly and severally liable to the plaintiff.
Courts apply alternative liability in circumstances where all of the possible tortfeasors are present but the plaintiff cannot show which of the tortfeasors committed the act that caused his injury. Under these circumstances, the court may say that unless any of the defendants can prove he was not responsible for the injury, each is liable to the plaintiff.
Some legal scholars will analyze this on a deeper level and question whether it is really tied to causation.
The case most commonly associated with alternative liability is Summers v Tice. In that case a hunter was injured by two defendants who carelessly fired their shotguns at the plaintiff. One of the defendants shot the plaintiff in the eye but the plaintiff could not prove which defendant was responsible. You might see alternative liability applied in modern cases where, say, multiple medical teams treated a plaintiff but there is no way to prove which defendant caused his injury. As in Summer v Tice, a court is likely to hold that each defendant is jointly and severally liable.
Courts have developed sophisticated ways to deal with cases where multiple defendants might have caused injuries including liability based on market share. Liability based on market share may be applied where not all defendants are present in the court and it is impossible to know which defendant was liable for the injuries. For example, let’s say ten different companies manufactured a dangerous drug and the plaintiff was injured by the drug. The plaintiff does not know which defendant made the product which injured him. A court might apportion liability based on a defendant’s share of the market for the drug.