Why are COVID-19 mitigation strategies successful in reducing infections in some cases but not in others? Existing studies of individual policies tend to neglect the many interaction effects that arise when multiple policies are enacted simultaneously. Particularly, if a socially undesirable behavior has a close (and equally problematic) substitute, then a prohibition of that behavior will simply cause people to switch to the substitute, resulting in no effect on infections. However, joint policies that prohibit both the targeted behavior and the substitute will create a positive interaction effect, which closes the loophole. Respectively, behaviors that are complements (rather than substitutes) can be discouraged by prohibiting one behavior because this discourages complementary behaviors as well.

We provide a new argument for why COVID-19 policies can fail and why the evaluation of such policies may be incorrect: policies are effective only when they reduce infections as a general equilibrium, accounting not only for the targeted behavior but also for interaction effects.

We illustrate our arguments by applying insights from traditional and behavioral law and economics to several examples. Thereby, we highlight regulators’ challenge when facing interaction effects and factors such as social norms and time preferences.

为什么 COVID-19 缓解策略在某些情况下成功地减少了感染，而在其他情况下却没有？对个别政策的现有研究往往忽略了同时制定多项政策时产生的许多相互作用效应。特别是，如果一种社会不受欢迎的行为有一个接近（同样有问题的）替代品，那么禁止这种行为只会导致人们转向替代品，对感染没有影响。然而，禁止目标行为和替代行为的联合政策将产生积极的互动效应，从而填补漏洞。分别地，可以通过禁止一种行为来阻止互补行为（而不是替代行为），因为这也会阻止互补行为。

我们为 COVID-19 政策为何会失败以及为何对此类政策的评估可能不正确提供了一个新的论据：政策只有在将感染减少为一般均衡时才有效，不仅考虑了目标行为，还考虑了相互作用效应。

我们通过将传统和行为法律和经济学的见解应用于几个例子来说明我们的论点。因此，我们强调了监管机构在面对互动效应和社会规范和时间偏好等因素时所面临的挑战。