While other scholars have pointed to imbalances in the design and functioning of the criminal justice system as the source of troubling disparities between the law on the books and the law in action, this Article argues that for crimes that rest on contested notions of social harm, factors external to the justice system are likely to be far more influential than internal dynamics in producing legal change. This insight, called the external evolutionary model of analysis, derives from social science work on the role of interest groups, social norms, and media influences on the law’s creation and development, and the Article discusses these topics in the context of the consensus-conflict debate and the sociology of social problems. It then draws on historical, sociological, and original empirical data about statutory rape enforcement in the United States to demonstrate how political, cultural, and economic forces change the meaning of an unstable criminal law over time. Following this case study, the Article shows how broad statutory language and contested theories of harm largely contribute to the unchecked discretion of law enforcement actors who control the contours of actual criminal cases brought under these types of provisions. Finally, the Article suggests that, while flexibility in criminal prohibitions might be efficient, it ultimately diminishes the criminal law’s legitimacy as an instrument of state power.