* This “micro-law review article” was revised and updated on 25 Feb. 2014–
Like Richard Epstein , prior probability agrees that “reasonableness,” standing alone, is an open-ended and indeterminate legal standard. Yet, at the same time, this legal standard permeates many areas of the law, especially torts and contracts. The law of torts, for example, imposes on all actors a duty to act reasonably (i.e. a duty to avoid creating unreasonable risks). Contract law, too, incorporates reasonableness in many of its rules. Reasonableness, in short, is a well-worn road in the path of the law.
We recently had an epiphany, however, during our frequent travels on this well-worn road. (We teach torts, and one of our best friends teaches contracts.) Our revelation is that “reasonableness” is just a code word or euphemism for random adjudication. In summary, our argument has two steps. First, notice that reasonableness and juries go hand-in-hand in most Anglo-American legal systems. Whether a given person has acted reasonably is ordinarily a question for the jury to decide.
So, why are we equating a jury’s finding of reasonableness or non-reasonableness to a random decision method, like a coin toss or the spin of a roulette wheel? Our answer to this question takes us to the second step of our argument: Because no one can forecast the actual outcome of a coin toss until after the coin has landed, just as no one can accurately forecast a jury verdict until after the verdict has been rendered, particularly in those cases in which the issue of reasonableness is in dispute.  In other words, both a coin toss and the application of the reasonableness standard are purely probabilistic events. Or in the immortal words of Col. Nathan R. Jessep (played by Jack Nicholson) in A Few Good Men: “You want to investigate me? Roll the dice and take your chances …”
Therefore, when a judge instructs a jury to decide whether a given defendant was reasonable or not, or whether the State has proven its case beyond a reasonable doubt, what the judge is really doing is no different than tossing a coin or using some other random mechanism for deciding this question.  The jury is not required to explain its decision. It’s just required to decide, and different juries will often reach different decisions based on the same or similar facts. (Note: we do not mean criticize juries. Instead, we are criticizing legal rules based on “reasonableness.”)**
 For a critique of the risk-utility balancing version of the reasonableness standard, see Richard Epstein, “The Risks of Risk/Utility,” Ohio State Law Journal, vol. 48 (1987), pp. 469-477.
 The existence of a “legal process prediction market,” however, might in fact be able to produce accurate forecasts of jury trials, but this particular idea is something we will blog about in more detail in future posts.
 Of course, we are only talking about a small number of events, since only a tiny fraction of all cases (civil and criminal, State and federal) go to trial. See, e.g., Marc Galanter, “The Vanishing Trial,” Journal of Empirical Legal Studies, vol. 1, no. 3 (Nov. 2004), pp. 459-570.
** Why can’t most law review articles and comments be this short?