Lecture 2: The 'Rules of Evidence' Impossibility Theorem

Event date
22 November 2017
Event time
Oxford week
Gulbenkian Lecture Theatre
Professor Dan Kahan



Professor Dan Kahan - Elizabeth K. Dollard Professor of Law & Professor of Psychology at Yale Law School 

Listen to Podcast


This series of lectures will use the laws of cognition to cast a critical eye on the cognition of law. Using experimental data, statistical models, and other sources, the lectures will probe how legal decisionmakers perceive facts and law and how the public perceives what legal decisionmakers are doing.  The unifying theme of the lectures will be that simply doing impartial law is insufficient to communicate the law’s impartiality to those who must obey it, and hence insufficient to deliver the assurance of neutrality on which the law’s legitimacy depends.  The lecture series will propose a new science of law the aim of which is to endow law with the resources necessary to bridge this critical gap between professional and lay perspectives.

Dan Kahan is the Elizabeth K. Dollard Professor of Law & Professor of Psychology at Yale Law School. His primary research interests are risk perception and science communication. He is a member of the Cultural Cognition Project, an interdisciplinary team of scholars who use empirical methods to examine the impact of group values on perceptions of risk and related facts. In studies funded by the National Science Foundation, his research has investigated public disagreement over climate change, public reactions to emerging technologies, and conflicting public impressions of scientific consensus. Articles featuring the Project’s studies have appeared in a variety of peer-reviewed scholarly journals including the Journal of Risk Research, Judgment and Decision Making, Nature Climate Change, Science, and Nature. He is a Senior Fellow at the National Center for Science and Civic Engagement and a member of the American Academy of Arts and Sciences.


This lecture will adopt a critical stance toward a position, dominant in the study of evidence law, that I will call the “cognitive fine-tuning” thesis (CFT).  CFT posits that the recurring decisionmaking miscues associated with bounded rationality—such as hindsight bias, the availability effect, probability neglect, representatives bias, etc.—can be managed through judges’ adroit application of evidence and other procedural rules.  Focusing on “coherence based reasoning” (CBT), Professor Kahan will argue that CFT is a conceit.  CBT refers to a form of “rolling confirmation bias” in which exposure to a compelling piece of evidence triggers the motivation to conform evaluations of the strength of all subsequent, independent pieces of evidence to the position that compelling item of proof supports. Grounded in aversion to residual uncertainty, CBT results in overconfident judgments, and also makes outcomes vulnerable to arbitrary influences such as order of proof.  What makes CBT resist CFT is that the triggering mechanism is admittedly valid evidence; indeed, the stronger (more probative) the item of proof is, the more likely it is to trigger the accuracy-distorting confirmation-bias cascade associated with CBT.  Accordingly, to counteract CBT, judges, using “cognitive fine tuning,” would have to exclude the most probative pieces of proof from the case—guaranteeing an outcome that is uniformed by the evidence most essential to an accurate judgment.  Symptomatic of the dilemmas that managing cognitive biases entails, this contradiction exposes the fundamental antagonism between rational truth-seeking and an adversary system that relies on lay factfinders.