In our previous post, we restated Ron Allen and Mike Pardo’s “relative plausibility” theory of legal proof (or what Allen and Pardo also call the “explanatory account” of evidence), an approach meant to displace probabilistic or mathematical theories of legal evidence. Briefly, according to Allen and Pardo, jurors don’t engage in highfalutin probabilistic reasoning when they decide civil or criminal cases. Instead, they simply compare and contrast the stories of the plaintiff and the defendant, and the party who offers the best explanation of the evidence—i.e. the party with the best* story—wins. (*Of course, the meaning of “best” in any given case will depend on the burden of proof used to decide the case, such the preponderance standard in civil trials or the reasonable doubt standard in criminal ones.)
Next, after presenting the details of their relative plausibility theory of legal proof (pp. 13-15), Allen and Pardo go on to explain the virtues of their theory on pp. 15-19 of their paper. In brief (p. 16), their relative plausibility theory avoids the several traps that probabilistic theories of evidence fall into, such as “(1) the need for numbers to compare with the standard of proof; (2) lack of fit with how fact-finders evaluate and reason from evidence; (3) inconsistency with the legal doctrine and jury instructions (the conjunction problem); and (4) inconsistency [regarding] the policy goals underlying standards of proof.” (We summarized these various problems in our May 31 blog post.) We won’t rehash their technical arguments here, because the main advantage of their relative plausibility theory is its elegant simplicity.
To put it bluntly, more simpler or parsimonious explanations are generally better than complex or convoluted ones, a fundamental principle called “Occam’s Razor” in the philosophy of science literature (see image below). Moreover, the relative plausibility theory of legal proof is not only simple and parsimonious; it also makes the most intuitive sense, so we generally agree with Allen and Pardo that jurors will weigh the evidence presented at a trial holistically and that they will favor the side whose story is more persuasive, or to borrow Allen and Pardo’s terminology, whose story offers the best explanation of the evidence. And, by the way, we agree with them 100% that defendants who fail to present an alternative story do so at their own peril. (Legally speaking, defendants are not required to present a competing story, unless they are alleging an affirmative defense, because it is the plaintiff who bears the burden of proof in most cases.)
Alas, the only potential “problem” (at least for Allen and Pardo) with the relative plausibility theory is that it is, at the end of the day, just another probabilistic theory of evidence! In truth, this is not a problem for us, since we not only welcome the use of probability theory in law; we openly embrace the subjective theory of probability developed by Frank Ramsey and Bruno de Finetti. It is, however, a problem for Allen and Pardo, since they want to banish probability theory from the courtroom. In our next blog post, we will switch gears and consider the weaknesses of the relative plausibility theory …