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The so-called doctrine of res ipsa loquitur has been a mystery since its birth more than a century ago. This Article helps solve the mystery. In practical effect, res ipsa loquirtur, though usually thought of as a tort doctrine, functions as a rule of trial practice that allows jurors to rely on circumstantial evidence surrounding an accident to find the defendant liable. Standard jury instructions in negligence cases, however, fail to inform jurors that they are permitted to rely upon circumstantial evidence in reaching a verdict. Why, then, is another, more specific circumstantial evidence charge necessary or desirable?

We describe and evaluate the arguments that have been made in support of and in opposition to the res ipsa instruction. One theory is that jurors are confused in performing their task when given only standard instructions; the charge, therefore, clarifies their task, thereby improving the quality of their decisionmaking. A competing theory is that the instruction biases jurors in favor of plaintiffs, thereby degrading the quality of the decisionmaking. Our theoretical analysis concludes that the boas explanation is stronger. We reach this conclusion by applying for the first-time modern learning on cognition to the res ipsa instruction.

To support our theoretical conclusion, we report the results of experiments designed o determine the effects of the instruction. All these experiments were intended first to confirm that the res ipsa instruction has an effect and second to confirm or refute our theoretical conclusion that the instruction biases rather than clarifies. While the empirical results did not demonstrate bias, they also failed to show the absence of bias. Moreover, we found no evidence that the instruction reduces confusion. Our conclusion is that the charge has no positive effect, and either may create a bias or, at best, is meaningless.

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