After clarifying the distinction between mistakes of fact and mistakes of law, this article explores in detail an important distinction within the category of mistake of law, between mistake about the criminal law itself and mistake about noncriminal law norms that the criminal law makes relevant - for example, about the civil law of property (in a theft prosecution) or of divorce (in a bigamy prosecution). The Model Penal Code seems to endorse the view that mistakes about noncriminal law norms should presumptively be treated as exculpatory in the same way as analogous mistakes about facts. Case law on the matter is more ambiguous.

As a matter of policy, when should mistakes of noncriminal law exculpate? Should they always be treated in the same manner as an analogous mistake of fact? Sometimes? Answering these questions is a complex matter; the article identifies some relevant factors.

Conversely, when should a mistake of noncriminal law inculpate, creating attempt liability? In the parallel scenario of factually impossible attempts, liability is frequently imposed. But I suggest caution before recognizing attempt liability here.

Classifying a mistake as one of criminal or noncriminal law is especially difficult in three scenarios: the incorporation of a civil schedule of prohibited items within a criminal law, the criminalization of acts that violate a civil regulatory prohibition, and criminal law terms whose meaning draws from both criminal law and civil law.

A final section questions the view that we should always give symmetrical treatment to (1) exculpatory mistake and ignorance (precluding liability for the completed crime) and (2) inculpatory mistake and ignorance (producing liability for the attempt). This view is especially implausible when applied to categories of mens rea other than belief or knowledge. Ignorance, for example, will often exculpate, but it will rarely inculpate.


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