Justifications, Excuses, and Affirmative Defenses


A defendant who admits to having committed an offense may nevertheless be acquitted if he can provide a legally cognizable justification or excuse for his actions by raising an affirmative defense. This article explains how affirmative defenses generate social benefits in the form of avoided unnecessary punishment. It then asks what kind of evidentiary standards must be used in order to balance these benefits against potential social costs arising from frivolous defense claims. It thereby provides an economic rationale for the uniformity across US jurisdictions in allocating the burden on the prosecution to prove the commission of the offense, as well as the variation across states in the standards of proof they use in determining the validity of affirmative defenses. The analysis also explains why mere assertions of undeterrability should not be considered as affirmative defenses.