Does mens rea matter to the criminal legal system? Our study addresses this question by performing the first-ever empirical analysis of a culpable mental state's impact on administration of a criminal statute. We focus on the U.S. Supreme Court's 2019 decision in Rehaif v. United States, which applied a culpable knowledge requirement to the federal felon-in-possession statute, 18 U.S.C. § 922(g). Prior to Rehaif, federal courts uniformly treated the critical objective element under 922(g)—whether a firearm or ammunition possessor meets the conditions for one of nine prohibited legal categories—as a question of fact for which an actor could be held strictly liable. Adding a knowledge requirement to this element resulted in a significant decline in the likelihood of a defendant being charged with 922(g), the number of 922(g) charges per defendant, the total number of defendants charged with 922(g), and the total number of 922(g) charges filed each month. We estimate that these charging reductions prevented 2,365.32 convictions and eliminated 8,419.06 years of prison sentences for 922(g) violations during the eight-month period following issuance of the Rehaif opinion. At the same time, prosecutors were just as likely to secure convictions of those they charged with 922(g) after the Rehaif decision as they were before it. All told, our study suggests that adding culpable mental states to criminal statutes can meaningfully constrain prosecutorial discretion, lower convictions, and reduce punishment without bringing criminal administration to a halt.