Torts are sometimes committed by two or more persons acting in concert. Under a long established common law rule known as joint and several liability, any of these "joint tort-feasors" could be held liable for the full damage caused not only by his acts, but by the acts of those with whom he was working in concert. This doctrine allows plaintiffs to recover their full damages when one or more of the defendants are insolvent or cannot be located. Yet as one can easily see, when a person can be held liable for harm done by another party, predicting one’s own liability is little more than a random guess.
Thus the doctrine of joint and several liability was applied only to a narrow class of cases. The first was those in which the defendants truly acted in concert – to use a simple example, a case where two men stand together and take turns throwing rocks at your windows. The other class was an even smaller group of cases in which the defendants acted separately, but in which the action of any one defendant would have caused the entire harm even if the other defendants had not contributed to the harm, as when two people carelessly start separate fires, each of which burns half of the plaintiff’s house, but either of which would have destroyed the house on its own.
In the modern liability system, however, the emphasis is on compensating plaintiffs, not determining fault. Thus the goal is to have as many potential defendants held liable as possible. That way, the plaintiff is likely to collect from somewhere. As a result, courts have expanded the definition of acting in concert to include independent actors who just happen to be in the same place at approximately the same time. And where that is not enough, courts have simply changed the rules and forced defendants, not plaintiffs, to prove whodunit.
An extreme example of this judicial magic is McCoy v. DeLiefde.  Four hunters went out, and when game was flushed, three shot, with one hitting and wounding McCoy. There was no doubt that the defendants had no intention of harming McCoy, and certainly had no conspiracy to do so. In fact, McCoy admitted that he knew which two of the three defendants had not caused him any harm. Yet the Court still imposed joint liability, on the theory that the defendants had a tacit conspiracy to be out hunting together, and that this somehow made each liable for harm caused by the others.
In products liability cases, Michigan courts have been a bit more subtle, but joint and several liability has still been used to hold manufacturers liable for harm that they did not cause or conspire to cause.
One common tactic has been to shift the burden of proof from the plaintiff to the defendant. This is done precisely in those cases in which the Court knows that the defendants cannot prove they did not cause the harm, even though all know that one or more of the defendants did not cause the harm.
In Green v. Union Optical Center, Inc.,  the plaintiff was unable to remember, or to produce evidence, as to whether the shattered eyeglass lens that caused the harm was purchased from Union Optical or from NuVision. The Court’s response was to shift the burden of proof to the defendants to show that they were not the cause of the harm. Here, this decision may have been defensible, for the two manufacturers had both failed to maintain records, required by law, keeping track of all lens sales. Thus it could be said that the plaintiff was prejudiced in producing proof by the defendant’s failure to live up to statutory obligations.
However, from Green it was a short step to Abel v. Eli Lilly.  In creating its afore-mentioned "DES unique" theory, the court again shifted the burden of proof onto the defendants, although the defendants had not violated any sort of statutory record-keeping duty. Since the defendants were obviously more able than the plaintiffs to know to whom sold the drug in question (in fact, they were probably less able to know), the court was, "clearly motivated not so much by legal reasoning, as by considerations of social policy."  It is, perhaps, unfair to be too critical of the courts in the DES cases, which involved a difficult set of facts in which it was clear that the plaintiffs had suffered harm as a result of taking DES, but it was not possible to match plaintiffs to the defendants whose product they had taken. Under the circumstances, perhaps "considerations of social policy" was the best way to decide.  Yet, even accepting this, it is clear that the courts did not consider the desirable social policy of competitive markets and affordable liability insurance in its decision. 
When joint and several liability is used to force a defendant manufacturer to pay for harm it did not cause, it can only be justified as redistribution, or social insurance. A manufacturer’s liability depends less on its own behavior and the characteristics of its product than on the products and behavior of other manufacturers over whom it has no control, and the amount that a defendant must pay under the doctrine depends on who else is available to pay. The doctrine is a prime cause of uncertainty in the insurance industry. 
In sum, judicial policy where joint tort-feasors are concerned has frequently been to rely on an insurance rationale to determine liability, without regard for actual causation. This makes liability insurance rate setting little more than guesswork, and as a result insurers add a margin for error to their premiums, or, in some cases, decide not to take an unknown risk.
Recognizing the fundamental unfairness of forcing a party to pay for damage it did not cause, the Michigan legislature in 1986 reformed joint and several liability rules to limit liability to a defendant’s percentage of fault. However, for inexplicable reasons, the new rule was not applied to products liability actions.  Thus the state passed up a major opportunity to restore sanity to insurance markets.