Ten years ago, Florida started a revolution.

That state's Supreme Court threw out the old "all-or-nothing" rule governing damage suits as an approach "almost totally regarded as unjust," and state after state has fallen in line behind. When, just before Christmas, Iowa's high court followed the trend, it became the 39th state to move to a more modern way of looking at such litigation. But the more modern way means that a lot of companies are paying up in product liability or other tort actions that a decade ago they could have beaten.

The old rule from common law is called the "contributory negligence defense." It says, in essence, that if the company or person being sued for causing an injury can show that the plaintiff contributed to the accident or its results--even in a slight way--the defendant is not at all liable.

In its place, jurisdictions are substituting what lawyers dub "comparitive negligence." As Iowa Justice Mark McCormick explains it, "In its pure form, the doctrine of comparative negligence assigns responsibility for damages in proportion to a party's fault in proximately causing them. The rule thus operates to reduce rather than bar recovery." A jury might decide, for instance, that a manufacturer is 85 percent responsible for an injury because the product it produced is hazardous, but the victim bears 15 percent of the blame because he should have been more careful. That kind of fine distinction may seem beyond the ability of mere mortals, but is no different from the kind of number assigning--how many dollars is a finger worth, how much pain did an accident victim suffer--that jurors do all the time.

The "pure" approach that McCormick was describing is what is now the law in Iowa and most other jurisdictions where judges, rather than legislators, have imposed comparitive negligence--states that include not just the pioneering Florida but also California, Illinois, and Michigan. It is also the approach taken by the National Conference of Commissioners on Uniform State Laws, a body that tries to get the various states to take similar positions on controversial legal issues. But in fact, only Washington has enacted a law substantially similar to the NCCUSL model.

When the lawmakers write the change, they have most often tried to make sure that comparitive negligence won't stick someone little at fault with a huge payment to the more guilty party. Under the "pure" form, that can happen in an accident where the person primarily responsible ends up with extensive injuries. A jury finding that the other party is only 10 or 15 percent responsible can still produce a six-figure award in a bad enough case, and the legislators want to avoid the headlines such a horrible ruling would produce.

Nebraska and South Dakota have decreed that the comparitive negligence approach comes into play only when the plaintiff's contribution to the accident is "slight" and the defendant's fault is "gross." Some two dozen other states follow an approach pioneered by Wisconsin: plaintiffs collect only when their negligence is less than that of the corporation or individual they are suing.

Support for the change is by no means universal. Businessmen would be wrong to assume that the 11 holdout states will necessarily fall in line. Iowa joined the list by a close 5-4 vote.

As Justice James H. Carter, one of the four Iowa dissenters, notes, "all-or-nothing situations with respect to litigation damages in other areas of the law are commonplace." And whenever a state abandons an old legal concept, problems come trailing in its wake. Carter asks, for instance, what will happen in cases where others contributed to the accident, but are not parties to the suit, or have no money to pay their share.

California, for example, has had to wrestle with whether under comparitive negligence an old rule that a person whose "wanton and willful misconduct" causes an accident has to pay full damages is still good law. (The answer: no.)

But, as Justice McCormick argues, comparitive negligence "is demonstrably superior" to the older approach, and in that case respect for precedent "does not require blind imitation of the past or adherence to a rule merely because it was laid down in the time of Henry IV."