This paper examines factors affecting the decision of whether or not to make certain harmful acts illegal. It considers factors related both to the cost of law enforcement and to the crime commission decision. On the enforcement side, illegality is limited by the existence of fixed notice and response costs, which are unrelated to the harm from the act, and also by costs of imposing punishment. In addition, illegality is limited by a finite marginal productivity of detection, one cause of which is legal error. On the commission side, illegality is limited if offenders have strictly positive benefits from committing the act. The paper concludes by examining how the optimal scope of law is affected by its “expressive function”, the idea that some people are deterred by the mere fact that an act is illegal. We specifically ask how the scope of law changes if more people behave in this way. The answer depends on whether “efficient” violations of the law are possible, which in turn depends on whether offenders’ gains are counted in welfare.

本文考察了影响决定是否将某些有害行为定为非法的因素。它考虑了与执法成本和犯罪委员会决定相关的因素。在执法方面，违法性受到与行为危害无关的固定通知和应对成本的存在以及处罚成本的限制。此外，非法性受到有限边际检测生产力的限制，其原因之一是法律错误。在委员会方面，如果违法者从实施该行为中获得了严格的积极利益，则非法性是有限的。该论文最后通过研究法律的最佳范围如何受到其“表达功能”的影响，即某些人被行为是非法的这一事实吓倒的想法。我们特别询问，如果更多人以这种方式行事，法律的范围将如何变化。答案取决于是否有可能“有效”违反法律，而后者又取决于违法者的收益是否计入福利。