Political scientists are increasingly attuned to the promises and pitfalls of establishing causal effects. But the vital question for many is not if a causal effect exists but why and how it exists. Even so, many researchers avoid causal mediation analyses due to the assumptions required, instead opting to explore causal mechanisms through what we call intermediate outcome tests. These tests use the same research design used to estimate the effect of treatment on the outcome to estimate the effect of the treatment on one or more mediators, with authors often concluding that evidence of the latter is evidence of a causal mechanism. We show in this paper that, without further assumptions, this can neither establish nor rule out the existence of a causal mechanism. Instead, such conclusions about the indirect effect of treatment rely on implicit and usually very strong assumptions that are often unmet. Thus, such causal mechanism tests, though very common in political science, should not be viewed as a free lunch but rather should be used judiciously, and researchers should explicitly state and defend the requisite assumptions.

翻译：政治学家日益关注确立因果效应的前景与陷阱。然而对许多研究者而言，关键问题并非因果效应是否存在，而在于其为何及如何存在。即便如此，许多研究者因所需假设而回避因果中介分析，转而通过我们称为"中介结果检验"的方法探索因果机制。此类检验采用与估计处理对结果效应相同的研究设计来估计处理对一个或多个中介变量的效应，作者常将后者的证据视为因果机制的证据。本文证明，若无进一步假设，这种方法既不能确立也不能排除因果机制的存在。相反，此类关于处理间接效应的结论依赖于隐含且通常非常强烈的假设，而这些假设往往无法满足。因此，这类在政治学中极为常见的因果机制检验不应被视为免费午餐，而应审慎使用，研究者需明确陈述并论证必要的假设条件。