Legal scholarship has long treated substantive criminal law and evidence as two separate and distinct fields. The former largely concerns itself with evaluating substantive criminal law rules by reference to various animating theories—most prominently, those of deterrence and retributivism. Scholars, students, and policymakers laud or condemn doctrines based on notions of “just deserts” or ideas about the incentives they create for those disposed to commit a crime. When it comes to the numerous evidentiary and other rules that determine the course of prosecutions and proof, however, the conversation is different. Here, questions of reliability, evidential worth, and accuracy in fact-finding dominate the debate. References to the deeper concerns of deterrence and retributivism, and the significance of various evidentiary and procedural rules toward the program of one or the other, are by and large absent.
Our article “Mediating Rules in Criminal Law” challenges this conventional divide between evidence and substantive criminal law theory. Our claim is that the traditional understanding of evidentiary rules in criminal law as geared overwhelmingly to truth in fact-finding is incomplete. Evidentiary rules, we argue, also perform a deeper, systemic function by mediating latent conflicts between criminal law’s deterrence and retributivist objectives. They do this by skewing errors in the application of the substantive law to favor whichever theory has been disfavored by the substantive rule itself. So, for example, if retributivism dominates the substantive law of insanity, special evidentiary rules governing the presentation and proof of that defense might cabin it in a way that responds to deterrence concerns by making it more difficult to invoke successfully. These “mediating rules” of evidence do this, moreover, without undercutting retributivist objectives as significantly as would redrawing the substantive defense itself. How is this so? In the next few pages, we will sketch the outlines of our theory and offer a brief illustration.