We have been reviewing Ron Allen and Mike Pardo’s excellent paper on “relative plausibility,” and as we mentioned in our previous post, we commend their relative plausibility theory of juridical proof for its simplicity. In brief, their theory is that jurors in civil and criminal cases compare and evaluate competing explanations of the evidence presented at trial. In civil cases, where the standard of proof is the preponderance of the evidence, the side with the most plausible explanation of the evidence, i.e. the side who offers the best explanation of the evidence, wins. In criminal cases, where the standard of proof is beyond a reasonable doubt, the defense wins if it offers any plausible explanation of the evidence consistent with the accused’s innocence.

Next, Allen and Pardo consider various criticisms of their theory of proof on pp. 19-45 of their paper. Although we agree with Allen and Pardo that most of these criticisms are unsound, their relative plausibility theory is still vulnerable to one major problem, a problem we dubbed in a previous post as *the unknown probability space problem*. In the words of Allen and Pardo (p. 26): “we reject the assumption that the plaintiff’s explanation [in civil cases] must fill more than half of the possible probability space.” (Indeed, they *have* to reject this assumption because their project is to banish formal probability theory from the courtroom.) They present a simple, stylized argument to support their conclusion (ibid.): “Return again to our example where a [juror] concludes that a plaintiff’s explanation is 0.4 likely and a defendant’s explanation is 0.2 likely…. [W]hen the remaining 0.4 is unknown, a [relative plausibility] account (plaintiff wins) better accords with the goals of accuracy and equalizing the risk of error.”

Alas, this conclusion is flat out wrong from a descriptive perspective, and it is misguided as a normative matter. After all, to paraphrase Detective Sherlock Holmes: the improbable could always be true. But that is not how the law of proof works. The preponderance standard in civil cases not only requires the plaintiff to prove that his story is more likely to be true than the defendant’s story; it also requires the plaintiff to prove that his story is more likely to be true, period. Why? Because we care about accuracy! What about equalizing the risk of error? What about it? By definition, it is the plaintiff in civil cases and the prosecutor in criminal ones who bears the risk of error, since the burden of proof is on the moving party. Why must the plaintiff or prosecutor bear this risk? Two reasons: (1) because we care about accuracy, and (2) because we also want to avoid false positives, especially in criminal cases. Given these goals, any system of proof that does not require the moving party to prove his case beyond the 0.5 threshold in civil cases or beyond a higher threshold in criminal prosecutions will generate more errors (namely, false positives) than an alternative system that does require the moving party’s proof to exceed such thresholds.

In any case (pun intended), there is a deeper problem with Allen and Pardo’s relative plausibility theory. They have proposed their comparative/explanatory theory as an alternative to conventional probabilistic accounts of proof, but in reality ** plausibility is just another word for probability**! (In fact, Allen and Pardo concede as much on p. 20: “According to our account, legal fact-finding does indeed aim at what are essentially probabilistic conclusions.”) Why, then, do Allen and Pardo want to banish formal probability from the law of proof? Is most of this dispute, as Allen and Pardo say in a different context (p. 42), “just rhetoric”? Stay tuned; we will return full circle to reconsider the reasons why Allen and Pardo reject the subjective probability theory of proof …

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