Mediating variables stand in the middle of the actions and the outcomes that we can observe. They are often tied together and hard to separate from the action and the outcome, making their direct impact hard to pull apart from other factors. They play an important role in determining causal structures, and ultimately in shaping discourse and public policy about good and bad actions.
Judea Pearl writes about mediating variables in The Book of Why. He uses cigarette smoking, tar, and lung cancer as an example of the confounding nature of mediating variables. He writes, “if smoking causes lung cancer only through the formation of tar deposits, then we could eliminate the excess cancer risk by giving smokers tar-free cigarettes, such as e-cigarettes. On the other hand, if smoking causes cancer directly or through a different mediator, then e-cigarettes might not solve the problem.”
The mediator problem of tar still has not been fully disentangled and fully understood, but it is an excellent example of the importance, challenges, and public health consequences of mediating variables. Mediators can contribute directly to the final outcome we observe (lung cancer), but they may not be the only variable at play. In this instance, other aspects of smoking may directly cause lung cancer. An experiment between cigarette and e-cigarette smokers can help us get closer, but we won’t be able to say there isn’t a self-selection effect between traditional and e-cigarette smokers that plays into cancer development. However, closely studying both groups will help us start to better understand the direct role of tar in the causal chain.
Mediating variables like this pop up when we talk about the effectiveness of schools, the role for democratic norms, and the pros or cons of traditional gender roles. Often, mediating variables are driving the concerns we have for larger actions and behaviors. We want all children to go to school, but argue about the many mediating variables within the educational environment that may or may not directly contribute to specific outcomes that we want to see. It is hard to say which specific piece is the most important, because there are so many mediating variables all contributing directly or possibly indirectly to the education outcomes we see and imagine.