Economists are often interested in the mechanisms by which a treatment affects an outcome. We develop tests for the "sharp null of full mediation" that a treatment $D$ affects an outcome $Y$ only through a particular mechanism (or set of mechanisms) $M$. Our approach exploits connections between mediation analysis and the econometric literature on testing instrument validity. We also provide tools for quantifying the magnitude of alternative mechanisms when the sharp null is rejected: we derive sharp lower bounds on the fraction of individuals whose outcome is affected by the treatment despite having the same value of $M$ under both treatments (``always-takers''), as well as sharp bounds on the average effect of the treatment for such always-takers. An advantage of our approach relative to existing tools for mediation analysis is that it does not require stringent assumptions about how $M$ is assigned. We illustrate our methodology in two empirical applications.

翻译：经济学家常常关注处理如何通过特定机制影响结果的机制问题。本文针对"完全中介的尖锐零假设"——即处理$D$仅通过特定机制（或机制集合）$M$影响结果$Y$——构建了检验方法。我们的研究思路通过建立中介分析与计量经济学中工具变量有效性检验的理论联系来实现。当尖锐零假设被拒绝时，我们还提供了量化替代机制影响程度的工具：推导出处理效应不通过$M$传导的个体比例（"始终接受者"）的尖锐下界，以及此类始终接受者平均处理效应的尖锐界限。相较于现有中介分析工具，本方法的优势在于无需对$M$的分配过程施加严格假设。我们通过两项实证应用展示了该方法的具体实践。