It’s no secret that courts in the United States are overworked. Processing cases can take years, and Americans are a famously litigious people. Courts use several legal doctrines to help limit the number of cases they have to deal with and to expedite the process of dealing with the ones they do have. Two of those doctrines were traditionally called res judicata and collateral estoppel, but courts today increasingly refer to them as claim preclusion and issue preclusion, respectively.
Res Judicata/Claim Preclusion
Res judicata is a Latin phrase meaning something adjudicated. This doctrine prevents someone from filing successive lawsuits over the same legal claim when a prior lawsuit between the same parties has resulted in a final judgment on the merits. This is so even when a plaintiff underestimated the extent of her injuries in a prior lawsuit and wants to sue again to make up the difference. But it’s not just enough that there be a prior judgment. The prior judgment must have been “on the merits.” For example, if a court dismisses a lawsuit because the defendant lived in another state and wasn’t subject to the court’s jurisdiction, then that wouldn’t be a judgment on the merits. It has nothing to do with the dispute between the parties or their legal rights, and so it wouldn’t make sense to bar future lawsuits based on that same claim.
Collateral Estoppel/Issue Preclusion
Collateral estoppel is like res judicata in some respects, but it differs in a number of other important respects. Collateral estoppel prevents the parties to a lawsuit from relitigating an issue that was decided in a prior lawsuit involving the person against whom collateral estoppel is used. For instance, suppose that John, Sally, and Ken are parties to a contract under which John will buy a truck from Sally and Ken. But Sally and Ken are only required to sell their truck after John fixes the roof on their house. Suppose John sues Sally and claims he is entitled to the truck. Sally counters that he is not, because his repairs were deficient. The court rules in favor of Sally. In this situation, John would not be able to then sue Ken and claim that he had repaired the roof as required, because that issue was already decided against him in his lawsuit against Sally. The doctrine of collateral estoppel protects the courts against having to rule on the same issue repeatedly when that issue has already been decided in a prior lawsuit involving at least some of the same parties.
Res judicata and collateral estoppel can be harsh doctrines, but they also help to keep cases moving smoothly through the court system. This can reduce the strain on courts, and allow them to deal with more pressing cases.