The Blackstone ratio, modified

In his discussion of evidentiary policies, Blackstone famously noted that ‘it is better that ten guilty persons escape than that one innocent suffer’ (Blackstone 1769). The conventional wisdom among lawyers, judges as well as academics holds that accepting this statement as a maxim necessitates the adoption of pro-defendant evidentiary rules. It is also commonly believed that costs associated with false convictions being greater than those associated with failures to punish offenders due to the presence of punishment costs provides a utilitarian rationale for Blackstonian principles. After formalizing Blackstone ratios (either as marginal rates of substitution or, alternatively, as the ratio between quantities of errors), I show these conventional views are incorrect. I then propose a simple modification of the Blackstone ratio, which shifts the focus from aggregate outcomes to consequences for individuals within the criminal justice system. This modification better aligns commonly held views about the Blackstone ratio with its actual implications and justifications.