What does “reasonable doubt” mean?

Not what we’d like it to mean, according to Rob MacCoun.

Rob MacCoun reports that in simulated-jury experiments, the probability that a “juror” will come into the deliberation with a verdict for or against someone is largely independent of the standard of proof as given in the judge’s instructions: on the same evidence, a juror is just as likely to find that someone is “guilty beyond a reasonable doubt” as to find that the person is “responsible [for an accident] by a preponderance of the evidence.”

But the enunciated standard of proof matters: it determines how willing people are to change their minds. An experimental subject whose first impression is that a defendant is guilty is likely to change his mind if randomly put in a mock jury with one or two “jurors” who argue for a “not guilty” verdict, but that same subject will stand pat on his finding of “responsible by a preponderance of the evidence.”

So “reasonable doubt” is defined in the mock-juror’s mind not epistemologically, but socially: something is subject to “reasonable doubt” as long as an apparently reasonable person doubts it, while it’s true by a “preponderance of the evidence” unless a majority disagrees.

Note the scary implication: twelve jurors, each of whom thinks that someone is probably, but not certainly, guilty will tend to find that person guilty beyond a reasonable doubt.

Author: Mark Kleiman

Professor of Public Policy at the NYU Marron Institute for Urban Management and editor of the Journal of Drug Policy Analysis. Teaches about the methods of policy analysis about drug abuse control and crime control policy, working out the implications of two principles: that swift and certain sanctions don't have to be severe to be effective, and that well-designed threats usually don't have to be carried out. Books: Drugs and Drug Policy: What Everyone Needs to Know (with Jonathan Caulkins and Angela Hawken) When Brute Force Fails: How to Have Less Crime and Less Punishment (Princeton, 2009; named one of the "books of the year" by The Economist Against Excess: Drug Policy for Results (Basic, 1993) Marijuana: Costs of Abuse, Costs of Control (Greenwood, 1989) UCLA Homepage Curriculum Vitae Contact: Markarkleiman-at-gmail.com