A Guide to Exploiting Cognitive Science Research that Has Confirmed the “Weak Evidence Effect”

Authored by: Paul L. Colby

Abstract

A highly important development for trial advocacy has taken place in the field of cognitive science, which has gone unnoticed.  Over the last two decades, cognitive science researchers have slowly but surely verified the “weak evidence effect,” which holds that a lawyer can harm his case by presenting evidence that supports his case but is weak.  This Article offers guidance, lacking in the cognitive science literature, for trial advocates to exploit this newly-verified phenomenon.

 What qualifies me to provide this guidance?  In 1990, along with Robert H. Klonoff, I published a trial treatise entitled Sponsorship Strategy: Evidentiary Tactics for Winning Jury Trials (Winning Jury Trials).  To my knowledge, Winning Jury Trials is the first published work to categorically identify the weak evidence effect as a phenomenon, and it did so years before cognitive science empirically verified the effect (or, apparently, took any note of it).  Indeed, the Yale Law Journal ran two lengthy book reviews of Winning Jury Trials, faulting me and my co-author for not waiting for empirical verification before sharing our discovery of this curious evidentiary effect with the trial advocacy community.  Winning Jury Trials also anticipated cognitive science on a number of more specific evidentiary propositions that researchers in that field have subsequently managed to verify, such as that the weak evidence effect increases the weaker that the evidence is as compared to the stronger evidence it accompanies.  Now that empirical studies have definitively verified the core propositions of Winning Jury Trials, including its announcement of the weak evidence effect, as this Article demonstrates, the principles announced in that text provide a unique and powerful basis for lawyers to exploit this newly-verified phenomenon.

Winning Jury Trials was written to serve as a handy and practically-oriented reference for the harried advocate facing a jury trial date.  The manual’s tone and content conformed to that purpose, alluding only briefly to its intellectual underpinnings.  By contrast, this Article discloses—for the first time—those intellectual underpinnings and explains the weak evidence effect at a level of exposition appropriate for a scholarly journal.