  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.
