When legislatures create new crimes, they sometimes fail to specify what mental state must be proven with respect to each element. These gaps in substantive criminal law give real significance to deafult rules or presumptions that certain levels of mens rea are required in certain circumstances. The Model Penal Code, for instance, specifies that some form of mens rea (purpose, knowledge, recklessness, or negligence) must be proven as “each material element of the offense.”
The Supreme Court has also recognized a mens rea presumption, although in a somewhat more limited form than the MPC. This was made clear, for instance, in Dean v. United States, 556 U.S. 568 (2009), in which the Court held that a ten-year mandatory minimum sentence could be triggered by the entirely accidental discharge of a firearm during a bank robbery. The relevant statute did not specify any mental state, and Dean argued that the government should be required to prove that the discharge was knowing or intentional. However, the Court indicated that its mens rea presumption only applied to elements that made a defendant’s conduct criminal, not to aggravating factors (like the discharge of a weapon) that merely made the defendant’s illegal conduct even worse.