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How can law be a mechanism for police reform? The most familiar answer, for legal scholars who work on the regulation of law enforcement, is as a deterrent: the law sets some limit on police behavior and imposes some sanction for violations. But the deterrent model is not the only method through which the law can affect police behaviors. In this article, Stoughton contends that evidentiary considerations have the potential to change both police training and agency culture. Stoughton’s contention is based on the observation that evidentiary considerations have shaped not just police behavior but also the culture of policing itself. Cultural change is a critical component of meaningful police reform; one who seeks to change some aspect of policing must take into account the role of culture in shaping the objectionable behavior. But police culture is neither independently organic nor develops in a vacuum. The development of a police culture depends, in large part, on external factors including the legal rules and social expectations in which the culture develops. Stoughton focuses on three examples of how the culture and practices of law enforcement have been shaped by different evidentiary considerations: the warnings required by Miranda v. Arizona, the perceived need to maintain an unbroken chain of custody for evidence, and the popular enthusiasm for forensic investigations. Stoughton then explores the possibility of using evidentiary rulings to further advance reform, focusing on the use of officers’ opinion testimony. Police testimony often straddles the line between lay and expert testimony. Much of their testimony requires specialized knowledge, experience, or training. Unfortunately, under the existing evidentiary framework, police officer testimony is frequently admitted in a way that blurs the already muddy line that separates lay and expert testimony, leading to three distinct problems. First, an officer may provide what is undeniably lay testimony, but using language that indicates a particular expertise. Second, an officer may provide expert testimony dressed in the guise of a lay opinion. Finally, and most importantly for the ultimate thesis of this article, an officer’s statements, even when he speaks as an expert, may lack the reliability that expert testimony is supposed to demonstrate. By addressing these concerns through the mechanism of evidentiary rulings, Stoughton suggests that police culture and training itself could be improved. While the impact of evidentiary considerations is neither certain nor the appropriate way to address all aspects of policing, it can be a useful addition to the broader conversation about police reform.